CHEM 9 6 2 I / S 3 TRY S T P M T e r m 3 Written by: Darren yong
ORGANIC CHEMISTRY TABLE OF CONTENTS 1 INTRODUCTION TO ORGANIC CHEMISTRY 5 HYDROCARBONS 19 HALOALKANES 24 HYDROXY COMPOUNDS 29 CARBONYL COMPOUNDS 33 CARBOXYLIC ACIDS AND THEIR DERIVATIVES 38 AMINES, AMINO ACIDS AND PROTEINS 42 POLYMERS COPYRIGHT © 2022 DARREN YONG YUNG HIEN ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR UTILISED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPYING, RECORDING OR BY ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, OR USED IN ANOTHER BOOK, WITHOUT SPECIFIC WRITTEN PERMISSION FROM THE AUTHOR. Darren Yong
Chapter 14 Introduction to Organic Chemistry Carbon Compound Homologous series Alkane Alkene Arene Alkyne Hydrocarbon Methane, CH4 Ethene, C2H4 Benzene, C6H6 Ethyne, C2H2 Structure Shape Tetrahedral Trigonal planar Hexagonal planar Linear Hybridisation / hybrid orbitals sp3 sp2 sp Orbital overlapping diagram sp3 hybridised sp2 hybridised sp hybridised Exam tips: Lone pairs (↿⇂) Bonds ( / ) Orbitals (1s / 2p) Hybridisation (sp / sp2 / sp3 ) Bond angle 109.5° 120° 120° 180° Type of C − C bond bonds bonds and bonds bonds and delocalised bonds bonds and bonds C − C C − C C = C C C C ≡ C Empirical Formula and Molecular Formula Empirical formula Simplest whole number ratio between different types of atoms (or elements) in a molecule of the compound CH, CH2 Molecular formula Actual number of different types of atoms (or elements) in a molecule of a compound. C6H6 (benzene) Example Combustion of hydrocarbon X in excess oxygen produces 0.66 g of carbon dioxide and 0.27 g of water. At room temperature and pressure, X is a gas with density 1.75 g dm-3. What could the molecular formula of X be? [The relative atomic mass: C=12.0, H=1.0; 1 mol of gas 24 dm-3 at room temperature and pressure] Mass of C = ଵଶ ସସ × 0.66 = 0.18 Mass of H = = ଶ ଵ଼ × 0.27 = 0.03 Element C H Mass (g) 0.18 0.03 Number of moles .ଵ଼ ଵଶ = 0.015 .ଷ ଵ = 0.003 Simplest ratio .ଵହ .ଵହ = 1 .ଷ .ଵହ = 2 The empirical formula is CH2. (statement must be stated) Molar volume of (CH2)n = 1.75 g dm-3 x 24 dm-3 mol-1 = 42 g mol-1 Relative molecular mass, n(12+2) = 42 14n = 42 n = 3 The molecular formula is C3H6. (statement must be stated) Labelling ↿⇂ ↿⇂ ↿⇂ C ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ C C 1s 2p ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ ↿⇂ If percentage is given, use mass in 100 g 1Introduction to Organic Chemistry
Structural formula Displayed formula Shows all the bonds in the molecule as individual lines Each line represents a pair of shared electrons Butane Ethanol Condensed formula Show simple structure Branches in the carbon chains are indicated in brackets Skeletal formula Standard notation for more complex organic molecules Carbon atoms are represented as points and not shown Hydrogen atoms only shown in functional group Functional Group, Classification and Nomenclature Class of organic compound Functional group Prefix / Suffix General formula Example Hydrocarbon Alkane C − C (carbon-carbon single bond) -ane CnH2n+2 Ethane Alkene C = C (carbon-carbon double bond) -ene CnH2n Ethene Alkyne C ≡ C (carbon-carbon triple bond) -yne Ethyne Arene (benzene) -benzene phenyl- (C6H5) Methylbenzene Phenylpropene Halogen-substituted derivative Haloalkane / Alkyl halides R−X fluorochloro bromoiodoCnH2n+1X Bromomethane Haloarene / Aryl halides Bromobenzene Oxygen containing compound Alcohol −OH (hydroxyl) -ol CnH2n+1OH Ethanol Phenol −OH (phenolic) Phenol Ether −OR alkoxy Methoxymethane Carbonyl compound Aldehyde (carbonyl) -al CnH2nO Propanal Ketone -one Butanone 3Introduction to Organic Chemistry Chemistry | STPM Term 3
Oxygen containing compound Carboxylic acid −COOH (carboxyl) -oic acid CnH2n+1COOH Ethanoic acid Acyl chloride -oyl chloride CnH2n+1COCl Ethanoyl chloride Ester (carboxylate) -oate CnH2nO2 Ethyl propanoate Acid anhydride -oic anhydride Ethanoic anhydride Amide (amide) -amide CnH2n+1CONH2 Ethamide Nitrogen containing compound Amine −NH2 (amino) -amine CnH2n+1NH2 Ethylamine Nitrile −C ≡ N -nitrile Ethanenitrile Nitro −NO2 attached directly to benzene ring NitroNitrobenzene Carbon Atom Identification Carbon atom identification Primary (°) carbon atom is bonded to one other carbon atom Secondary (°) carbon atom is bonded to two other carbon atoms Tertiary (°) carbon atom is bonded to three other carbon atoms Isomerism Isomerism is the existence of two or more compounds that have the same molecular formula but different arrangement of atoms Structural Isomerism Structural isomerism occurs when compounds have the same molecular formula but different structural formula Chain isomerism Same functional group Same chemical properties but different physical properties Involve straight or branched chains 3 Introduction to Organic ChemistryChemistry| STPM Term 3
Structural Isomerism Positional isomerism Different in the position of the functional group in the molecule Same chemical properties but different physical properties but-2-ene but-1-ene Functional Group Same molecular formula but different functional group/homologous series Different chemical and physical properties Alkenes and cycloalkanes Hexene / Cyclohexane (C6H12) Hexene Cyclohexane Alcohols and ethers Ethanol / Methoxymethane (C2H6O) Ethanol Methoxymethane Aldehydes and ketones Propanal / Propanone (C3H6O) Propanal Propanone Carboxylic acids and esters Propanoic acid / Methyl ethanoate / Ethyl ethanoate (C3H6O2) Propanoic acid Methyl ethanoate Ethyl methanoate Stereoisomerism Stereoisomerism occurs when compounds have the same molecular and structural formulae but different arrangement of atoms in space. Geometrical (cis-trans) isomerism Trans-but-2-ene Cis-but-2-ene No of isomers = 2 , n = number of C = C bond Presence of double bond which restrict rotation Carbon atoms with double bond are bonded to two different groups/atoms Similar chemical properties Different physical properties (boiling point/ melting point/ density) Cis isomer o Higher boiling point (more polar) o Lower melting point (less symmetrical, less closely packed in lattice structure) Optical isomerism Solid line ( ) – bond in the plane Wedge ( ) – bond coming out of the plane Dashed line (− − − −) – bond behind the plane Contain at least one chiral carbon (carbon atom which is attached to four different groups/atoms) Optically active and able to rotate plane polarised light Mirror images are not superimposed one on the other A chiral molecule and its mirror image are called enantiomers Same chemical and physical properties Nucleophiles and Electrophiles Nucleophiles Electrophiles electron-rich species that can donate an electron pair to an electron-deficient species Lewis base Eg. OH- , CN- , Cl- , Br- ,C- , NH3, H2O electron-deficient species that can accept an electron pair from an electron-rich species Lewis acid Eg. H + , NO2 + , Cl+ , Br+ , C+ , AlCl3, FeBr3 * * 4 Introduction to Organic Chemistry Chemistry | STPM Term 3
Chapter 15 Hydrocarbons Alkanes Alkanes General formula: CnH2n+2 Saturated aliphatic hydrocarbons (sp3 hybridised) Cycloalkanes General formula: CnH2n Ring(cyclic) structures Isomerism: geometrical isomerism Nomenclature Physical properties Alkanes | Straight chain Solubility Insoluble in water (cannot form hydrogen bonding with water molecules) Soluble in organic solvent Melting point and boiling point Low boiling point and melting point (weak van der Waals forces between molecules) Boiling point The boiling point of branched chain isomers is lower than straight chain isomers o Branched chain isomers are more compact (spherical) o Lower surface area o Weaker van der Waals forces between molecules o Less energy is required to overcome the weak van der Waals forces between molecules Melting point The melting point of branched chain isomers is higher than the straight chain isomers o Branched chain isomers are more compact (spherical) | more symmetrical o More closely packed in a solid lattice structure 5Hydrocarbon
Chemical properties Combustion C௫H௬ + ቀ + 4 ቁ Oଶ → COଶ + 2 HଶO complete combustion when oxygen is in excess Free radical substitution CHଷCHଶCHଷ + 2Clଶ → CHଷCHClCHଶCl + 2HCl MECHANISM Initiation: Sunlight provides energy for homolytic fission to produce chlorine free radicals. Cl-Cl bond is preferably break during the initiation step rather than C-H and C-C bond. This is because the photons (pockets of light energy) can provide sufficient energy to break the bonds, forming Cl• free radicals Propagation: Termination: uv white fumes 6 Hydrocarbon Chemistry | STPM Term 3
What are the conditions for free radical substitution of alkanes with different halogens of different reactivity? Reactivity of halogens with alkanes increases in the order (reverse of the X–X bond energies): I2 < Br2 < Cl2 < F2 F2 : reaction proceeds explosively even at r.t.p. (not carried out in laboratory) Cl2 / Br2 : reaction occurs at 250 – 400 ℃ or in UV light I2 : least reactive and reaction is so slow that it is not carried out Down group 17 elements, the energy released in the formation of C−X and H−X bonds (X: halogen) in the propagation step decreases as the size of halogen atom increases. This leads to a decreasing ease of substitution. Cracking Process whereby big molecules of alkanes are broken up to smaller molecules (more useful as fuel/ raw materials in the synthesis of other organic compound) Produces branched-chain alkanes and reforming straight-chain alkanes into ring molecules (arenes / cycloalkanes) Thermal cracking (pyrolysis) [600℃-900℃] C10H22 ⟶ C5H10 + C5H12 C10H22 ⟶ C4H8 + C3H6 + C3H8 Catalytic cracking (400℃-500℃) [SiO2 / Al2O3] o Produce alkane and alkene with higher octane number (more branching) Sources of hydrocarbons Sources: Natural gas (methane,CH4) Crude oil (petroleum) Crude oil is a thick, viscous liquid mixture of hydrocarbons. Separate into different fractions through fractional distillations. Catalytic converter Catalyst: Platinum, Rhodium, Palladium Pollutant gases are oxidised or reduced to harmless gases At high temperature, CO is oxidised to CO2. The reaction is catalysed by platinum/palladium 2CO + O2 → 2CO2 At lower temperature, oxides of nitrogen are reduced to nitrogen gas and oxygen gas 2NO → N2 + O2 Overall reaction: 2CO + 2NO → 2CO2 + N2 7 HydrocarbonChemistry| STPM Term 3
Effects of hydrocarbons on the environment Air Pollution Carbon dioxide Cause greenhouse effect which leads to global warming Carbon monoxide Produce by incomplete combustion Excessive inhalation may cause death Unburnt hydrocarbons Produce by incomplete combustion of fuels. Cause liver damage/ cancer In strong sunlight, combine with nitrogen oxides to form photochemical smog. Oxides of nitrogen Sulphur dioxide Soluble in rainwater to produce acid rain (damages trees, aquatic life and buildings) 2SO2 + O2 → 2SO3 SO2 + H2O → H2SO3 SO3 + H2O → H2SO4 Carbon and lead particulates Found in smoke Lead particles cause brain damage Water Pollution Oil spillage Oil insoluble in water and less dense than water which forms oil slick (thin film of oil on the surface of water) Damages aquatic life Alkenes General formula: CnH2n Unsaturated aliphatic hydrocarbons (sp2 hybridised) Functional group: carbon-carbon double bond Isomerism: structural isomerism and geometrical isomerism(cis-trans isomerism) Ethene and propene do not exhibit isomerism Physical properties Solubility Insoluble in water (cannot form hydrogen bonding with water molecules) Soluble in organic solvent Melting point and boiling point Simple covalent molecules Low boiling and melting point Weak van der Waals force between molecules Cis isomer has higher boiling point than trans isomer Trans isomer has higher melting point than cis isomer o Trans isomer more symmetrical o More closely packed o Stronger intermolecular van der Waals forces Preparation of alkenes Elimination Dehydrohalogenation Dehydration of alcohol 8 Hydrocarbon Chemistry | STPM Term 3
Chemical properties Electrophilic Addition Catalytic hydrogenation (reduction) Convert unsaturated fats (vegetable oil) to saturated fats (margarine) Halogenation Observation: decolourisation of reddish-brown bromine MECHANISM Step 1: Step 2: Trans attack (due to steric hindrance) MECHANISM Step 1: Step 2: 9HydrocarbonChemistry| STPM Term 3
Electrophilic Addition MECHANISM Step 1: Step 2: (i) (ii) Addition of hydrogen halide MECHANISM Step 1: Step 2: Catalyst: AlCl3 / FeCl3 is used for the reaction between alkenes and HCl. Bond strength: HF > HCl > HBr > HI Hydrogen bromide and hydrogen iodide reacts readily in room temperature where HF reacts only with difficulty. Markovnikov’s rule When an unsymmetrical reagent is added to an unsymmetrical alkene, the more electropositive atom or group becomes attached to the carbon atom (of the double bond) which has the larger number of hydrogen atoms. 10 Hydrocarbon Chemistry | STPM Term 3
Electrophilic Addition Direct hydration In the industry, a mixture of ethene and steam is passed over phosphoric(v) acid to produce ethanol. Indirect Hydration Stage 1: Stage 2: Oxidation Combustion Combustion of alkenes produces a more smoky and luminous flame compared to the corresponding alkanes with same number of carbon atoms Higher percentage by mass of carbon in alkenes Catalytic oxidation (Epoxidation) Stage 1: Stage 2: Mild oxidation Observation: decolourisation of purple acidified KMnO4 solution Vigorous oxidation “heating under reflux” is not required for distinguishing test, just state “heat”. Distinguishing tests are done in test-tubes and it is impossible to do reflux in test-tubes. Observation: decolourisation of purple acidified KMnO4 solution Carbon-carbon double bond undergoes cleavage to form two carbonyl compounds(ketone / aldehyde) o Ketone (cannot further oxidised) o Aldehyde (can further oxidised) o Methanoic acid (oxidised to become CO2 and H2O) 11HydrocarbonChemistry| STPM Term 3
Oxidation Ozonolysis Stage 1: Stage 2: Similar to the oxidative cleavage of C = C by hot acidified KMnO4, ozonolysis can also be used to determine the position of = in an alkenes by the types of carbonyl compounds formed. Ozone, O3 cleaves C = C to gives carbonyl compounds. However, ozonolysis is milder and both ketones and aldehydes can be recovered without further oxidation. Comparatively, KMnO4 will further oxidise aldehyde to form carboxylic acid. Polymerisation Addition polymerisation Use of alkenes Ethene is used to make Poly(ethene), a useful plastic Chloroethene, monomer to make the polymer PVC Epoxyethane, which is converted to 1,2-ethanediol, one of the monomers used to make polyester such as Terylene 12 Hydrocarbon Chemistry | STPM Term 3
Arenes Aromatic compounds are compounds which contain benzene ring (C:H ≈ 1:1) Benzene, C6H6 Resonance structure of benzene are as shown below. Delocalised electrons give benzene additional aromatic stability Nomenclature: Chemical reactions of arenes Electrophilic Aromatic Substitution Reaction 1 Nitration Observation: yellow oil is formed Nitrating mixture: equimolar of conc. HNO3 and conc. H2SO4 MECHANISM Step 1: Production of electrophile, NO2 + Step 2: Formation of arenium carbocation intermediate Step 3: Expulsion of H+ from the intermediate 2 Halogenation Catalyst: AlCl3 (anhydrous)/ FeCl3 / AlBr3 / FeBr3 Acts as halogen carrier Acts as Lewis acid to accept lone pair of electrons ≡ 13 HydrocarbonChemistry| STPM Term 3
MECHANISM Step 1: Production of electrophile, Cl+ Step 2: Formation of arenium carbocation intermediate Step 3: Expulsion of H+ from the intermediate 3 Friedel-Crafts reactions a) Friedel-Craft alkylation MECHANISM Step 1: Production of electrophile, CH3 + Step 2: Formation of arenium carbocation intermediate Step 3: Expulsion of H+ from the intermediate b) Friedel-Craft acylation MECHANISM Step 1: Production of electrophile Step 2: Formation of arenium carbocation intermediate Step 3: Expulsion of H+ from the intermediate 14 Hydrocarbon Chemistry | STPM Term 3
4 Sulphonation Addition Reaction of Benzene 1 Reduction 2 Reaction of benzene with halogens Chemical reactions of alkylbenzene 1 Halogenation MECHANISM: Free radical substitution Initiation: Propagation: 15 HydrocarbonChemistry| STPM Term 3
Termination: MECHANISM: Electrophilic aromatic substitution Step 1: Production of electrophile Step 2: Formation of arenium carbocation intermediate Step 3: Expulsion of H+ from the intermediate 2 Friedel-Craft Alkylation 3 Nitration Alkyl group is ring activating group. Thus, methylbenzene is more reactive than benzene. Milder condition for nitration to take place. 16 Hydrocarbon Chemistry | STPM Term 3
4 Oxidation Observation: decolourisation of purple acidified KMnO4 solution white precipitate is formed when cooled 5 Reduction Effects of substituent Activating Groups Deactivating Groups Substituents that donate electron to the benzene ring Increase the electron density of the ring and activate the ring Make the ring more susceptible (more reactive) to electrophilic attack Increase the rate of electrophilic substitution Requires a milder condition for reaction. Substituents that withdraw electron from the benzene ring Decrease the electron density of the ring and deactivate the ring Make the ring less susceptible (less reactive) to electrophilic attack Decrease the rate of electrophilic substitution Requires a harsher condition for reaction ortho / para directing meta directing Strongly activating Weakly activating Weakly deactivating Deactivating Amino group −NH2, −NHR Hydroxyl or alkoxy group −OH, −OR Alkyl group −CH3 Halogen group −Cl, −Br Nitro group −NO2 Carbonyl group −CHO, −COR Carboxyl group −COOH Cyano group −CN Uses of arenes Benzene and methylbenzene are useful solvents for organic compounds. Benzene is a starting material for the manufacture of many other organic compound such as phenol, propanone , polymers, aniline and azo dyes. Methylbenzene is used as raw material to produce explosive, trinitrotoluene(TNT) However, benzene is carcinogens (cause cancer) Exposure of benzene can lead to various blood disorders and leukaemia. 17HydrocarbonChemistry| STPM Term 3no benzylic hydrogen
Br2, FeBr3 Halogenation conc. HNO3, conc. H2SO4, 55 ℃ − 60 ℃ Nitration SO3, H2SO4 (fuming) Sulphonation 3Cl2, uv Addition CH3Cl, AlCl3 Alkylation KMnO4/H+ Cl2, uv , heat under reflux Halogenation conc. HNO3, conc. H2SO4, 30 ℃ Nitration Halogenation Cl2, FeCl3 Electrophilic substitution Addition Free radical substitution Oxidation 18Hydrocarbon Chemistry | STPM Term 3
Chapter 16 Haloalkanes Structure and nomenclature General formula CnH2n+1X, R − X Primary(1°) haloalkanes Secondary(2°) haloalkanes Tertiary(3°) haloalkanes The halogen is bonded to a primary carbon atom The halogen is bonded to a secondary carbon atom The halogen is bonded to a tertiary carbon atom Isomerism Eg. C4H9Cl Structural isomerism Optical isomerism Physical properties Boiling point Higher boiling point than alkanes of comparable relative molecular mass (polar molecules) For corresponding alkyl groups, the boiling points increase in the order fluoro < chloro < bromo < iodo For isomeric haloalkanes, the boiling points increase in the order tertiary < secondary < primary Solubility Insoluble in water (not able to form hydrogen bonding with water molecules) Soluble in organic solvent Substitution reactions of haloalkanes 1 Hydrolysis For haloalkane with the same halogen atom, the rate of hydrolysis increases in the order tertiary < secondary < primary i. ii. 19 Haloalkanes
2 Formation of nitriles a) Reduction b) Acid hydrolysis c) Alkaline hydrolysis 3 Formation of amines The hydrogen halide liberated reacts with excess ammonia to form ammonium halide Overall equation: If excess haloalkanes is used, further substitution takes places and a mixture of secondary amines, tertiary amines and quaternary ammonium salts are formed. iii. iv. 20Haloalkanes Chemistry | STPM Term 3
Elimination reactions of haloalkanes | dehydrohalogenation Saytzeff’s rule In an elimination reaction, the more substituted alkene is more stable and therefore is the major product. Mechanism of nucleophilic substitution (SN1 and SN2) Mechanism Unimolecular nucleophilic substitution (SN1) Bimolecular nucleophilic substitution (SN2) Classification Tertiary haloalkane Primary haloalkane Example 2-bromo-2-methylpropane, (CH3)3CBr bromoethane, CH3CH2Br Equation Steps in mechanism Step 1: Formation of carbonium ion Step 2: Attack of nucleophile to carbonium ion Rate equation rate = k [(CH3)3CBr] First order reaction rate = k [CH3CH2Br] [OH- ] Second order reaction Energy profile Effect of structure of haloalkane on the rate of SN1 and SN2 reactions Explanation Relative reactivity: 1° haloalkanes < 2° haloalkanes < 3° haloalkanes Reaction involve the formation of carbocation Stability of carbonium ion 1° < 2° < 3° Relative reactivity: 3° haloalkanes < 2° haloalkanes < 1° haloalkanes 3° haloalkanes will experience steric hindrance during nucleophilic attack, therefore it does not favor SN2. 21HaloalkanesChemistry| STPM Term 3
Chemical test of haloalkanes Reagent: ethanolic AgNO3 (aq) Condition: heat Haloalkane Observation Identity of precipitate Solubility in NH3 Dilute Concentrated R − Cl White precipitate AgCl Soluble R − Br Cream precipitate AgBr Insoluble Soluble R − I Yellow precipitate AgI Insoluble Reactivity Haloalkane Observation Primary Cloudiness appears after several minutes Secondary Cloudiness appears after one minute Tertiary Cloudiness appears after a few seconds Aryl halide Resists nucleophilic substitution Strength of carbon-halogen bond in aryl halides o Delocalisation of lone pair of electrons of halogen atom into electron system of benzene ring produces resonance structures o Carbon-halogen bond becomes shorter and stronger o Carbon-halogen becomes less polar o Less susceptible towards nucleophilic attack High electron density o The electron-rich aromatic nucleus tends to repel nucleophiles Chlorobenzene undergoes electrophilic substitution similar to benzene such as nitration, sulphonation, halogenation and FriedelCrafts alkylation to form ortho- and para-substituted compounds. *Pg 13 | Chapter 15 Hydrocarbons [Chemical reactions of benzene] (Chloromethyl)benzene The halogen atom in (chloromethyl)benzene is not bonded directly to the benzene ring. Thus, it considered a substituted alkyl halide. (Chloromethyl)benzene undergoes nucleophilic substitution similar to the haloalkanes. Due to the presence of aromatic ring, it can also undergo electrophilic substitution reactions. Reaction with chlorine Free radical substitution Electrophilic substitution Oxidation Observation: decolourisation of purple acidified KMnO4.solution 22Haloalkanes Chemistry | STPM Term 3
Organometallic compounds Grignard reagents Organolithium Tetraethyllead(IV) Reactions of Grignard reagents Synthesis of alcohol Synthesis of carboxylic acid Reaction with water Grignard reagents can be hydrolysed easily by water to form alkanes. Thus, reactions involving Grignard reagents must be carried out in dry conditions. Chlorofluoroalkanes and uses of haloalkanes Aerosol Propellants | Coolants Dicholorodifluoromethane, CF2Cl2 (Freon-12) used as aerosol propellant Low boiling point, easier to be liquified Widely used as coolant Non-toxic and non-corrosive Liquefied under pressure and then vaporised by sudden expansion. The expansion process draws heat from the surroundings and produces the cooling effect. Solvents Freon-13, CFCl2CF2Cl is used to dissolve non-polar solutes Good solvents to remove grease in engineering equipment and electric circuits Used for ‘dry’ cleaning Fire extinguishers Halons, organic compounds obtained by replacing all the hydrogen atom by bromine and other halogens atoms. bromochlorodifluoromethane(CBrClF2), dibromochlorofluoromethane(CBr2ClF), bromotrifluoromethane(CBrF3) Chemically unreactive and non-toxic Denser than air Dichlorodiphenyltrichloroethane, DDT As insecticide, kills malaria-carrying mosquitoes and kills lice that cause typhus Stable fat-soluble compound, remains and accumulates in human body causing harmful effects to the central nervous system CFCs and ozone depletion Initiation: Ultraviolet radiation causes homolytic fission of the − bond to form chlorine free radical Propagation: Overall equations: In recent years, the more expensive hydrochlorofluorocarbons(HCFCs) are being used to as they will not release the damaging chlorine free radicals into the atmosphere Furthermore, the C − H bonds will break down at lower altitudes before they reach the stratosphere 23HaloalkanesChemistry| STPM Term 3
Chapter 17 Hydroxy Compounds Alcohols General formula: CnH2n+1OH Functional group: hydroxyl group (−OH) Primary alcohols Secondary alcohols Tertiary alcohols Physical properties Boiling point Higher than other organic compounds with equivalent relative molecular mass o Intermolecular hydrogen bonding 3° alcohols < 2° alcohols < 1° alcohols o Due to steric hindrance, alkyl group hinders the formation of intermolecular hydrogen bonding Solubility Lower numbers of alcohols are soluble in water o Can form hydrogen bonding with water molecules via -OH group As hydrocarbon chain get longer, solubility decreases triol > diol > monohydric alcohol o More hydroxyl group present in a molecule, the more hydrogen bonding can be formed with water molecules 3° alcohols < 2° alcohols < 1° alcohols o Due to steric hindrance, alkyl group hinders the formation of hydrogen bonding with water molecules Isomerism Alcohols and ether exist as functional group isomerism Alcohols with four or more carbons exist optical isomerism (contain chiral carbon) Reactions of hydroxy compounds Cleavage of O−H bond Formation of alkoxides 1 mol of alcohol produce mol of hydrogen gas Observation: effervescence occurs Esterification Observation: sweet fruity smell Observation: sweet fruity smell and white fumes (HCl) is liberated Oxidation ** If methanoic acid is produced, further oxidation to form CO2 and H2O Catalytic dehydrogenation Chemical test Differentiate between primary and tertiary alcohols Differentiate between secondary and tertiary alcohols Alcohols Observations KMnO4/H+ K2Cr2O7/H+ (chromic acid) Primary Decolourisation of purple acidified KMnO4 solution Orange acidified K2Cr2O7 solution Secondary turns green Tertiary No observable change 1 2 3 24 Hydroxy Compounds
Cleavage of C−OH bond Halogenation **PBr3 produced in situ ( Br2 + ROH + P4) **PI3 produced in situ ( I2 + ROH + P4) **HBr produced in situ ( concentrated H2SO4 + NaBr) Lucas test Lucas reagent: solution of ZnCl2 in concentrated HCl Alcohols Observations Primary alcohol No cloudiness at room temperature Secondary alcohol Cloudiness after 5 minutes Tertiary alcohol Immediate cloudiness Elimination Saytzeff’s rule The alkene with the greatest number of alkyl substituents is the major product. Iodoform Test Test for Preparation of alcohols Hydration *Pg 11 | Chapter 15 Hydrocarbons [Chemical reactions of alkene] Grignard reagent *Pg 23 | Chapter 16 Haloalkanes [Reactions of Grignard reagents] Fermentation Reduction of carbonyl compound 25 Hydroxy CompoundsChemistry| STPM Term 3
Uses of alcohols Alcohol Uses Methanol, CH3OH To prepare methylated spirit (mixture of CH3OH + C2H5OH) To prepare formalin Used as solvent Used as fuel Ethanol, C2H5OH Used as solvent in the preparation of medicine and cosmetic Used as an antiseptic Used as fuel (gasohol is 90% petrol + 10% C2H5OH) Isopropanol, CH3CHOHCH3 Use as an antiseptic Glycerol, Used in the preparation of pharmaceutical and cosmetics products such as hand lotion and creams Phenols Phenols are aromatic alcohols with one or more –OH groups bonded directly to the benzene ring. Acidity The p-orbital of oxygen overlaps with the electron cloud of the benzene ring. This enables negative charge on the oxygen to be delocalised into the benzene ring. Charge on the phenoxide ion is dispersed through resonance. o electron density across benzene ring increases o electron density between the oxygen atoms and hydrogen atoms of the hydroxyl group is reduced In addition, benzene is an electron withdrawing group(negative inductive effect), it weakens the O − H bond, hence easing the dissociation of H + ion from the − group in phenol Alkyl group is electron donating group with positive inductive effect, increases the electron density between oxygen atoms and hydrogen atoms of the − group, reduces the ease of dissociation of H+ ions. Phenoxide ion is thus more stable than alkoxide ion or hydroxide and equilibrium favours right as compared to the case in aliphatic alcohol o greater acidity of phenol 26 Hydroxy Compounds Chemistry | STPM Term 3
Comparison Reagent − Na Observation: effervescence occurs NaOH (aq) Na2CO3 (aq) KMnO4/H+ Observation: decolourisation of purple acidified KMnO4 Lucas reagent (conc. HCl + ZnCl2) PCl3 / PCl5 / SOCl2 excess conc. H2SO4 I2 + NaOH (aq) Observation: sweet fruity smell Observation: sweet fruity smell and white fumes (HCl) neutral FeCl3 (aq) Br2(aq) Electrophilic aromatic substitution Observation: decolourisation of reddish-brown bromine dilute HNO3 conc. HNO3 + conc. H2SO4 H2 27 Hydroxy CompoundsChemistry| STPM Term 3
Manufacture of phenols Cumene process Cumene(1-methylethylbenzene) Step 1: Friedel-Crafts alkylation Step 2: Oxidation of cumene Step 3: Rearrangement Uses of phenols Phenol is used to prepare cyclohexanol by reduction. Oxidation of cyclohexanol to produce 1,6-hexanedioic acid which is used as starting material to make nylon. Phenol is used in the manufacture of epoxy resin, bakelite aspirin and dye Phenols and substituted phenols are used as active ingredients in antiseptics such as Dettol. 28 Hydroxy Compounds Chemistry | STPM Term 3
Chapter 18 Carbonyl Compounds Introduction Aldehydes and ketones contain the carbonyl (C=O) functional group. Aldehydes and ketones are isomers of each other. General formula: CnH2nO The carbonyl carbon atom is sp2 hybridised forming a trigonal planar. Aldehydes Ketones Physical properties Boiling points Molecules of both aldehydes and ketones are polar due to the presence of the carbon-oxygen double bond. Generally, boiling points of aldehydes and ketones are higher than those hydrocarbons of similar Mr due to intermolecular permanent dipole-dipole interactions. Aldehydes and ketones lack hydrogen atoms bonded directly to N, O or F to form intermolecular hydrogen bond. Therefore, they have lower boiling points than alcohols of similar relative molecular masses. Solubility Lower members of aldehydes and ketones can form hydrogen bonding with water molecules and are considerably soluble in water. As the length of carbon chain increases, solubility in water decreases. Ketones such as propanone are good solvents because they can dissolve both aqueous and organic compounds. Preparation of carbonyl compounds Oxidation of alcohols *Pg 24 | Chapter 17 Hydroxy Compounds [Reactions of Hydroxy compounds] Reactions of carbonyl compounds Reactions of Carbonyl Compounds Nucleophilic addition Reactions with HCN Condensation Reactions with 2,4-dinitrophenylhydrazine (chemical test for the presence of carbonyl compound) Oxidation of aldehyde Acidified KMnO4/ K2Cr2O7, heat Tollens’ reagent ([Ag(NH3)2] + ), warm Fehling’s solution, warm Reduction Aldehydes reduced to primary alcohols Ketones reduced to secondary alcohols Triiodomethane Reaction (Iodoform Test) Warm with aqueous alkaline iodine (a distinguishing test for methyl carbonyl group) 29 Carbonyl Compounds
1 Nucleophilic addition Hydrogen cyanide(cyanohydrin) is very volatile and extremely poisonous. Therefore, the reaction is carried out at low temperature. Prepared in situ by using a solution of KCN or NaCN in dilute sulphuric acid. NaOH acts as base to produce a higher concentration of -CN. This in turn increases the rate of reaction. Acid hydrolysis Reduction MECHANISM: Nucleophilic addition Step 1: Nucleophilic attack by -CN ion (rate determining step) Step 2: Reaction with HCN REACTIVITY OF ALDEHYDES AND KETONES TOWARDS NUCLEOPHILIC ADDITION The susceptibility of the carbonyl carbon to nucleophilic attack is affected by: Steric factor The carbonyl carbon in ketones has two alkyl groups attached while that of aldehydes has only one. Hence there is less steric hindrance about the carbonyl carbon in aldehydes to hinder the approach of the attacking nucleophile. Electronic factor There is only one electron-donating alkyl group to reduce the partial positive charge on the carbonyl carbon in aldehydes compared to the two electrondonating alkyl groups in ketones, the carbonyl carbon in aldehyde is more electron deficient. Benzaldehyde is less reactive than aliphatic aldehydes or ketones due to, Resonance effect electrons in the carbonyl group delocalised into the benzene ring, stabilising benzaldehyde and causing it to be less reactive The electron-rich aromatic nucleus also repels the approaching nucleophile Steric hindrance Reactivity decreases in the order: 2 Condensation Chemical reaction in which two molecules combine to form a larger molecule with the elimination of a smaller molecule such as water, ammonia or hydrogen chloride. Brady’s reagent – solution of 2,4-dinitrophenylhydrazine in methanol and sulphuric acid 30 Carbonyl Compounds Chemistry | STPM Term 3
3 Oxidation a) Acidic oxidation b) Alkaline oxidation Benzaldehyde is oxidised to form benzoic acid, which is soluble in hot water and form white precipitate when cooled. Cyclohexanone can be oxidised by nitric acid to form hexanedioic acid, monomer in the synthesis of nylon 6,6 Reaction with Tollens’ Reagent Contains [Ag(NH3)2] + ion Aldehyde and benzaldehyde give positive test with Tollens’ reagent, while ketones cannot be oxidised by Tollens’ reagent. Reaction with Fehling’s Solution Prepared by adding excess sodium hydroxide solution to a mixture of copper(II) sulphate and sodium potassium tartrate (deep blue solution) Ketones and benzaldehyde do not give positive test with Fehling’s solution. 4 Reduction Reducing agent H2 / Ni, 180 ℃ 1. LiAlH4 / dry ether 2. H3O+ / heat Zn / dilute HCl Ethanol / Na 1. NaBH4 / methanol 2. H2O 5 Triiodomethane Reaction Test the presence of SUMMARY OF DISTINGUISHING TESTS Reagents and Conditions Observations for positive test Species Present Aliphatic aldehydes Benzaldehydes Ketones 2,4-dinitrophenylhydrazine Orange precipitate formed Acidified KMnO4, heat Purple acidified KMnO4 decolourised Acidified K2Cr2O7, heat Orange acidified K2Cr2O7 turns green Tollens’ reagent [Ag(NH3)]+ , warm Silver mirror formed Fehling’s solution (alkaline solution of Cu2+ complex), warm Brick-red precipitate formed Mild reducing agent Cannot reduce 31 Carbonyl CompoundsChemistry| STPM Term 3
Uses of carbonyl compounds Methanal (formaldehyde) (gas which soluble in water) 40% aqueous solution of methanal (formalin) is used to preserve biological specimens Aqueous of methanal can be used as disinfectant as it kills most bacterial and fungi Methanal solutions is applied to dry skin, such as in the treatment of warts **methanal is rarely found in its original state because it is decomposed by light to form a toxic substance. The major toxic exposure occurs at eye, nose and throat. If ingested, methanal corrodes the alimentary canal. Propanone (sweet-smelling colourless liquid) Solvent (nail polish remover) Solvent for plastic Solvent base in many organic reactions ‘drying agent’ for laboratory equipment [solubility in water, volatile] Benzaldehyde (colourless, volatile liquid with strong smell of almond) Solvent Food flavouring agent (almond flavour) Sugar and carbohydrates Naturally occurring compounds like glucose, sucrose and other carbohydrates have carbonyl group. Carbohydrates are organic compounds containing carbon, hydrogen and oxygen only, with the hydrogen and oxygen atoms in the molar ratio of 2:1. Hence, carbohydrates have the general formula of CnH2mOm or Cn(H2O)m. Carbohydrates are polyhydroxy aldehydes or ketones as they have hydroxyl (−OH) and carbonyl (C=O) functional groups. Carbohydrates are formed from monosaccharide units. The class of carbohydrates depends on the number of monosaccharides units in their structures. They include the monosaccharides, disaccharides, oligosaccharides and polysaccharides. Monosaccharides Disaccharides Monosaccharides (simplest sugars) are the simplest carbohydrates They cannot be broken down into smaller molecules by hydrolysis. General formula: CnH2nOn Very soluble in water (there are many hydroxyl groups in their molecule) Source of energy for many cells Example, glucose and fructose Glucose in produce by photosynthesis in green plants. A glucose molecule is usually represented in the form of a flat planar hexagonal ring. The ring form of glucose can be open up to the linear structure which shows the present of hydroxyl group and aldehyde group. Hence, glucose is known as aldohexose sugar. Glucose is optically active as it contains chiral carbons. Another hexose sugar is fructose, a sugar found in honey and fruits. The linear structure shows that fructose has hydroxyl and ketone functional groups. Hence, it is called a ketohexose sugar. When two monosaccharides units combine by a condensation process, a disaccharide is formed. The two monosaccharides units are joined by a glycosidic bond, C−O−C The most common examples of disaccharides are shown below. Disaccharide Monosaccharide units Sucrose (cane sugar) Glucose + Fructose Maltose (malt sugar) Glucose + Glucose Lactose (milk sugar) Glucose + Galactose Sucrose is the most abundant disaccharide in the biological world. It is obtained principally from the juice of sugar cane and sugar beet. The formation of sucrose by condensation is as shown below. Maltose is obtained from malt, the juice extracted from barley and other cereal grains. Lactose is principal sugar present in milk. When disaccharides are hydrolysed, the monosaccharides units will be reformed. Disaccharides are soluble in water. They are sweet-tasting and can be crystallised. Reducing and non-reducing sugars A monosaccharide or a disaccharide that can donate electrons to other molecules and therefore can act as a reducing agent is called a reducing sugar. A monosaccharide or a disaccharide that cannot donate electrons to other molecules and therefore cannot act as a reducing agent is called a non-reducing sugar. In solution, the ring forms of monosaccharides are in equilibrium with the linear forms, which bear the aldehyde or ketone groups. Carbonyl groups in aldehydes act as a reducing agent. Fehling’s solution is usually used to test for reducing sugars. (brick-red precipitate of copper(I) oxide, Cu2O is formed) Fructose is a reducing sugar although it has a ketone group. This is because the linear form with the ketone group can be converted to the aldehyde group. 32 Carbonyl Compounds Chemistry | STPM Term 3
Chapter 19 Carboxylic Acids and Their Derivatives Introduction Functional group: Carboxyl group General formula: CnH2n+1COOH Isomerism occurs in carboxylic acid with at least four carbon atoms. Physical properties Melting and boiling points Carboxylic acids have higher boiling points than alcohols with comparable relative molecular mass. The O–H bond in carboxylic acids is more polarised due to the presence of electron withdrawing carbonyl group (C=O). Thus, more energy is required to overcome the stronger hydrogen bonding between carboxylic molecules. Solubility Solubility of carboxylic acids in polar solvents (such as water, ethanol) decreases down the homologous series. Carboxylic acids with short hydrocarbon chain are completely miscible with water as it can form hydrogen bonding with water molecules. Benzoic acid is a white solid that is only slightly soluble in cold water but is soluble in hot water. Dimerisation Carboxylic acid molecules (e.g. CH3COOH) dimerise in non-polar solvents (e.g. hexane), forming two hydrogen bonds in each pair of molecules. Acidity Alcohols < Phenols < Carboxylic acids Alcohols Phenols Carboxylic Acids R − OH + HଶO ⇌ RO ି + HଷO ା R− is electron donating group which result in positive inductive effect increases the negative charge density on the oxygen atom. Destabilised ROି This strengthens the O−H bond and it is more difficult for the hydrogen atom to dissociate from the O−H bond. Lone pair of electrons in the oxygen atom overlap with the delocalised electrons in the benzene ring. The delocalisation results in the stabilisation of phenoxide ion. (phenol can be ionised easily) Benzene ring is electron withdrawing group which result in negative inductive effect. This weakened the O−H bond where hydrogen atom is easily dissociated out when dissolved in water. RCOOH + HଶO ⇌ RCOOି + HଷO ା Inductive effect The strong electron withdrawing effect of the carbonyl oxygen atoms causes the displacement of the electrons and the carbon atom of the carbonyl group bears a large positive charge. The O−H bond is weakened and hydrogen atom can be dissociated easily. Resonance effect Resonance effect stabilised the carboxylate anion (RCOO- ) Strength of acidity 1 [size of alkyl group] CH3CH2COOH < CH3COOH < HCOOH o −CH3 is electron donating group which result in positive inductive effect which destabilised the CH3COO- ion. Hence, CH3COOH is a weaker acid than HCOOH. o Size of −CH2CH3 is bigger than −CH3, stronger electron donating group and destabilised the CH3CH2COO- ion more than CH3COOion. 2 [position of electron withdrawing group] CH3CH2COOH < CH2ClCH2COOH < CH3CHClCOOH o Cl atom is electron withdrawing group which results in negative inductive effect. CH2ClCH2COOH is stronger acid than CH3CH2COOH o The Cl atom in CH3CHClCOOH is nearer to COO- , therefore the electron withdrawing effect is stronger. The CH3CHClCOO- is more stable. 3 [type of substituent] CH2BrCOOH < CH2ClCOOH < CH2FCOOH o Electronegativity increases from Br to Cl to F. o Stability: CH2BrCOO- < CH2ClCOO- < CH2FCOO4 [number of electrons withdrawing group] CH2ClCOOH < CHCl2COOH < CCl3COOH o Cl atom is electron withdrawing group. Number of electrons withdrawing group increases. o Stability of conjugate base increases. 5 o Phenyl is electron withdrawing group which result in negative inductive effect. o is more stabilised. 33 Carboxylic Acids and Their Derivatives
Preparation of carboxylic acid Chemical reactions of carboxylic acids 1 Reaction with electropositive metal Observation: effervescence occurs 2 Neutralisation 3 Reaction with carbonates and hydrogen carbonates Observation: effervescence occurs 4 Reactions with halogens (free radical substitution) 5 Reduction 34Carboxylic Acids and Their Derivatives Chemistry | STPM Term 3
6 Neutral iron(III) chloride Observation: dark red colouration Observation: buff coloured precipitate 7 Esterification of benzoic acid Methanoic acid, HCOOH Ethanedioic acid, HOOC−COOH Reducing agent (undergoes oxidation) [contains aldehyde group] PCl5 HCOOH + PClହ CO + 2HCl + PClଷ Oxidation Acidified KMnO4 Observation: decolourisation of purple acidified KMnO4 Tollens’ reagent Observation: silver mirror formed Mercury(II) chloride, HgCl2 Dehydration Comparisons between hydroxy compounds and carboxylic acids Alcohols Phenols Carboxylic acid Remarks Na (redox) Effervescence of H2 Effervescence of H2 (Not used as a test for carboxylic acid as this reaction is explosive) Na, a reactive metal, is used to test for the presence of −OH group in organic compounds. NaOH(aq) (acid-base) Forms a soluble salt NaOH, being a strong base, will not react with alcohols as alcohols are neutral in aqueous medium. NaHCO3(aq) (acid-base) Forms a soluble salt. Effervescence of CO2 Both NaHCO3 and Na2CO3 are used to test for the presence of carboxyl group. Na2CO3 (aq) (acid-base) Uses of carboxylic acids Food preservatives A dilute solution of ethanoic acid is used as home vinegar. Benzoic acid and sodium benzoate are used as food preservatives, especially in cordials, tomato sauce and chilli sauce. Manufacture of perfumes Carboxylic acids react with alcohol to form fragrant compounds called esters. Esters are used to make perfumes. Coagulation of latex Methanoic acid and ethanoic acid are used in industrial coagulation of latex. Manufacture of polymers Ethanoic acid reacts with ethene and oxygen gas at high temperature to form ester, vinyl acetate. Hexanedioic acid is used in the manufacture of nylon 6,6. 35 Carboxylic Acids and Their DerivativesChemistry| STPM Term 3
Acyl chloride General formula: CnH2n+1COCl Physical properties Most acyl chlorides are colourless, fuming liquids at room temperature. Hydrolysed easily in moist air to liberate white acidic fumes (HCl). React vigorously with water to form acidic solution. Acyl chlorides cannot form hydrogen bonding between molecules. Thus, it has lower boiling points than corresponding carboxylic acid. Reactions of acyl chlorides 1 Hydrolysis 2 Formation of amides 3 Formation of ester Relative Reactivity of Hydrolysis [Electronic factor] The presence of highly electronegative O atom and Cl atom attached to the carboxyl C atom makes it highly electrondeficient, hence most susceptible to nucleophilic attack. [Steric factor] Nucleophiles are also able to approach more easily from both faces of the planar acyl carbon compared to the tetrahedral carbon in alkyl chlorides and geometrically impossible SN2 attack in aryl chlorides. Chlorobenzene is inert because of the partial double bond character of C–Cl bond due to the overlapping of the p orbital of Cl with the electron cloud of the benzene ring. In addition, the high electron density on the aromatic ring tends to repel the approaching nucleophile. Esters General formula: CnH2nO2 Identified by their sweet fruity smell. Physical properties Solubility Esters have the C=O that can form hydrogen bond with water molecules, which makes them water soluble. However, only the lower ester such as methyl methanoate and ethyl methanoate are soluble in water. Boiling point Esters do not have −OH group that can form hydrogen bonds between molecules. Esters are more volatile than alcohols and carboxylic acid with comparable relative molecular mass. The lower esters are fragrant, volatile liquids. Chemical reactions of esters 1 Hydrolysis Acidic hydrolysis Alkaline hydrolysis 2 Reduction 36 Carboxylic Acids and Their Derivatives Chemistry | STPM Term 3
Uses of esters Flavourings Esters are used as taste enhancers. The fragrant smell of fruits is due to the presence of esters. Volatile ester with pleasant, fruity smells is used to make artificial fruit flavourings. Preservatives Ester of 4-hydroxybenzoic acid also known as parabens are food preservatives. They are most effective against yeast and moulds. Many beverages contain paraben. Paraben also used in jam, salads, cheese, meats and margarine. Solvent Ester are excellent solvents for many organic compounds. Pentyl ethanoate is used in nail polish remover Polystyrene cement is made by dissolving polystyrene in ethyl ethanoate. When the cement is applied and the ester evaporates, a solid plastic is left behind which binds together with surfaces. Amides General formula: RCONH2 Physical properties Solubility The lower amides are soluble in water because they can form hydrogen bonds with water molecules. They also soluble in alcohol and ether. Solubility decreases as the length of hydrocarbon chain increases Boiling point Methanamide is a colourless liquid at room temperature while other amides are colourless crystalline solids. Amides have higher boiling points than acyl chloride and esters as they can form hydrogen bonding between molecules. Chemical reactions of amides 1 Hydrolysis Acid hydrolysis Alkaline hydrolysis 2 Reduction Relative reactivity of carboxylic acid derivatives Due to the electron-donating effect of −OR and −NH2 groups by resonance, the carboxyl carbon in ester and amide are less electrondeficient than that of acyl chloride. Comparing esters and amides, O is more electronegative than N, hence the partial positive charge of the carboxyl carbon in ester is higher than the carboxyl carbon in amide. Hence ester is more susceptible to hydrolysis than amide. 37Carboxylic Acids and Their DerivativesChemistry| STPM Term 3
Chapter 20 Amines, Amino Acids and Proteins Introduction General formula: CnH2n+1NH2 Functional group: amino group Classification Type General formula Examples Primary amine RNH2 CH3CH2NH2 Ethanamine Secondary amine R2NH (CH3)2NH N-methylmethanamine Tertiary amine R3N N-methyl-N-phenylmethanamine Quaternary ammonium ion R4N + (CH3)4N +ClTetramethylammonium chloride Physical properties 1 The lower numbers of aliphatic amines are soluble in water as it can form hydrogen bonding with water molecules via −NH2 and −COOH. a) The solubility of amines in water decreases with the increase of molecular mass of amines, in accordance with the increase in size of non-polar alkyl group. b) The solubility in water for the 3 classes of amines increases in the order: 3° amines < 2° amines < 1° amines This is due to the increases of hydrogen bonding with water molecules. 2 Th boiling points of amines are higher than hydrocarbons of similar molecular mass due to the formation of intermolecular hydrogens bonds. However the N−H bond is less polar than O−H bond. Hence, the boiling point of amines is lower than that of corresponding alcohols. a) The boiling point increases as the molecular mass of the amines increases. b) The boiling points of the 3 classes of amines increases in the order: 3° amines < 2° amines < 1° amines Tertiary amines have no −N−H group to engage in hydrogen bonding. 3 Amines have a characteristic odour of rotting fish. Preparation of amines 1 Reduction of nitriles, R−C≡N *Pg 20 | Chapter 16 Haloalkanes [substitution reactions of haloalkanes] 2 Reduction of amides, R−CONH2 *Pg 37 | Chapter 19 Carboxylic acids and their derivatives [chemical reactions of amides] 3 Nucleophilic substitution of haloalkanes, R−X with ethanolic ammonia *Pg 20 | Chapter 16 Haloalkanes [substitution reactions of haloalkanes] 4 Reduction of nitro compounds Basicity of amines Amines can act as proton (H+ ) acceptor (Bronsted-Lowry base) due to the availability of the lone pair of electrons on the nitrogen atom to form a dative covalent bond with a H+ . [inductive effect of the alkyl group] The positive inductive effect of the alkyl groups increases the availability of the lone pair of electrons on the N atom. This enhances the N atom to form dative covalent bond with proton resulting in a stronger base. Availability of lone pair of electrons: 3° amines > 2° amines > 1° amines [solvation effect of the water molecules] Solvation stabilises the cation by spreading the charge over the water molecules. Due to steric effect, tertiary amines hinder the approach of water molecules. Stability of conjugate acid: 3° amines < 2° amines < 1° amines Thus, basicity : 2° amines > 1° amines > 3° amines > NH3 38 Amines, Amino Acids and Proteins
Phenylamine is a much weaker base than ammonia and alkyl amines. The electron pair on the nitrogen atom interacts with the system of the benzene ring and becomes delocalised. This stabilises the molecule, thus the electron pair is less available to a proton. Chemical properties of amines 1 Amines as bases Observation: loses its fishy smell 2 Reaction with nitrous acid, HNO2 Primary aliphatic amines Observation: effervescence occurs Secondary aliphatic amines Tertiary aliphatic amines Aromatic amines Benzenediazonium ion is stable at low temperature because the positive charge on the nitrogen atom can be delocalised into the benzene ring. Observation: effervescence occurs 3 Reaction with acyl chloride *Pg 36 | Chapter 19 Carboxylic acids and their derivatives [chemical reactions of acyl chlorides] 4 Reaction with bromine water Observation: decolourisation of reddish-brown bromine and white precipitate is formed 5 Coupling reaction (benzenediazonium ion) Amino acids Amino acids are basic building block of protein. Contains two major functional groups, carboxyl group and amino group. There is a total of 20 naturally occurring -amino acids which made up all proteins. IUPAC name. Eg 39 Amines, Amino Acids and ProteinsChemistry| STPM Term 3
LIST OF TWENTY NATURALLY OCCURRING α−AMINO ACIDS **The most common class of amino acids in nature is the α-amino acids, which are all chiral except for aminoethanoic acid (glycine). Acid-base properties Since amino acids contain both an acidic and a basic group, they undergo an intramolecular acid-base reaction and exist primarily as a dipolar ion or zwitterion. A zwitterion contains both a basic carboxylate ion (COO− ) and an acidic ammonium ion (NH3 + ) in the same molecule. Therefore, an amino acid is said to be amphoteric. Depending on the pH of the solution that they are dissolved in, the predominant form of the amino acids may be as cationic, zwitterionic or anionic. The isoelectric point (pI) is the pH at which the amino acid exists primarily as the neutral zwitterion and it will not migrate under the effect of an electric field. Each amino acid has its own characteristic isoelectric point (may not be pH 7), depending on the R group present in the side chain. 40Amines, Amino Acids and Proteins Chemistry | STPM Term 3
Physical properties Melting points Amino acids are crystalline solids with comparatively higher melting points than amines or carboxylic acids of similar relative molecular mass. The relative high melting points are due to the stronger ionic interactions existing between the zwitterions. Solubility Amino acids are soluble in water but insoluble in organic solvents. The solubility of amino acids is due to the polar interaction between the polar water and the zwitterions. The lack of solubility of amino acids in organic solvents indicates that amino acids exist as zwitterions, do not form any interactions with covalent molecules. Chemical properties Condensation Amino acids are joined together by bond formed between the amino group of each amino acid and the carboxyl group of the neighbouring amino acid. These bonds between the two amino acids are known as peptide bonds/linkages. The process of forming the peptide bond is known as condensation, as water molecules are eliminated. Chemical test for amino acid Aminoethanoic acid with copper(II) solution Proteins Proteins are diverse groups of polypeptides (long chains of polymer made of up amino acids monomer linked up by peptide bonds). Hydrolysis of proteins Condition to carry out hydrolysis: In the laboratory, reflux the proteins with dilute acids (HCl / H2SO4) or alkalis (NaOH) In living organisms, hydrolysis of proteins is catalysed by enzymes 41Amines, Amino Acids and ProteinsChemistry| STPM Term 3
Chapter 21 Polymers Introduction Monomer: small molecule that can be used in polymer synthesis. Polymer: a long chain macromolecule formed by many smaller molecules called monomer. Repeating unit: an elementary unit which repeats itself to form a polymer chain. Natural polymers Carbohydrates, starch, cellulose Glucose Protein, silk, DNA Amino acid Natural rubber Isoprene (2-methylbuta-1,3-diene) Synthetic polymers Poly(ethene) Polychloroethene / polyvinyl chloride (PVC) Poly(tetrafluoroethene) or teflon Homopolymer: polymer formed from one type of monomer Copolymer: polymer formed from two or more different monomers Type of polymerisation Type of polymerisation Addition polymerisation Condensation polymerisation Definition The addition reaction in which monomers with double bonds are joined together by covalent bonds to form large molecule without the loss of a small molecule. Monomers react to form a large molecule (polymer) and during the process eliminate a small molecule such as water, ammonia, methanol or hydrogen chloride. Monomers With double bond (alkenes or alkene derivatives) Has at least two functional groups on both ends Empirical formula Same empirical formula as monomer Different empirical formula as monomer Example Poly(propene) Terylene (polyester) Poly(chloroethene) Nylon 6,6 (polyamide) 42 Polymers
Role of Ziegler-Natta catalyst Isotactic isomer All the –CH3 groups of poly(propene) are situated on the same side of the polymer chain Stronger and harder compare to atactic Coordination polymerisation mechanism Syndiotactic isomer −CH3 groups regularly alternate on opposite sides of the polymer chain Stronger and harder compare to atactic Coordination polymerisation mechanism Atactic isomer −CH3 groups are randomly oriented Produced by free radical polymerisation mechanism. Ziegler-Natta catalysts consists of a mixture of titanium(VI) chloride, TiCl4 and triethylaluminium, (C2H5)3Al. Advantages of using Ziegler Natta for addition polymerisation: o Enable the production of isotactic and syndiotactic polymers. o Polymer produced greater tensile strength, stiffness and resistance to heat and cracking. o Addition process carried out at lower temperature and pressure. o Polymer produced linear and less branched, therefore higher density and higher melting points Example of addition polymers Low Density poly(ethene), LDPE High Density poly(ethene), HDPE Branched chained polymer with lower density Lower melting point, softer, amorphous and is atactic Linear chained polymer with higher density Higher melting point, crystalline, greater tensile strength, resistance to heat, harder and is isotactic Classification of polymers Type of polymer Structure Properties Thermoplastics No cross-linkages (linear) Can be softened by heating and be remoulded Thermosetting Extensive cross-linkages Hard, rigid, moulded only once Elastomer Small degree of cross-linkages Elastic Type of polymer Description Fibres Polymer that can be drawn into threads and then spun and woven into fabrics. eg. Nylon and Terylene Plastics A synthetic polymer that can flow (become mobile) when heated. Two types: thermoplastics and thermosetting plastics Thermoplastics: Can be moulded and remoulded. Linear polymer with no strong bonds between the chain. eg. PE, PP, PVC Thermosetting plastics / resins: highly cross linked, sets into hard, insoluble mass, cannot be remoulded. Individual chain linked by strong covalent bonds. eg. bakelite and epoxy resin Resins Solids or semi-solids which are incapable of being remoulded because they do not soften on heating. They can only be moulded at the manufacturing stage. Elastomer Polymers that can stretch and then revert to the original shape and size when released. eg. Natural rubber and synthetic rubber. Natural rubber Known as polyisoprene. Monomer: 2-methylbuta-1,3-diene Due to presence of C=C which restricted rotation, there are two isomers of rubber: cis and trans 43PolymersChemistry| STPM Term 3
cis-polyisoprene trans-polyisoprene more elastic because the polymer chains are not arranged in order obtained from Hevea brasilensis rubber tree. natural rubber inelastic because the polymer chains can be arranged orderly due to zigzag pattern. obtained from Palaquium gutta tree, also known as gutta percha rubber. Hevea brasilensis Palaquium gutta Vulcanised rubber Natural rubber is elastic due to the flexibility of the long-chain molecules. It is also easily oxidised, has relatively low melting point and low tensile strength Vulcanisation adds 8% of sulphur. The sulphur atoms form covalent bonds with the chains of the rubber molecules, sulphur linkage. When rubber is vulcanised, the rubber becomes: o More elastic. The polymer chains do not easily slip over each other because they are held by sulphur bridges. o Higher boiling point and more heat resistance. Sulphur added increases molecular mass and hence the strength of the van der Waals force. o Stronger o Less easily oxidised. Formation of sulphur bridges reduces the number of double bonds in the polymer, making it less reactive. Uses of polymer Polyethene Making of plastic bags, plastic bottles, cling films(low density) Polypropene Making of plastic furniture, crates, buckets and toys PVC Water pipes, artificial leather for footwear, bags, seat cover, insulator for electric cables, shower curtains Polystyrene Packaging materials, disposable cups, plates and food containers Nylon 6,6 Making of stockings , tights ropes, carpets, tents, parachutes, fishing lines Terylene Synthetic fibres for non-crease clothing, soft drink bottles, adhesive tapes and video tapes SBR Tyres, toys Natural rubber Tyres, surgical gloves, shoes soles, foam mattresses, elastic bands Bakelite Handles of pots, 3-pin plugs and switches Perspex Alternating to glass, objects which are shatter-resistant Teflon Coating for non-stick pans, electrical insulators Difficulty in the disposal of polymers The main useful property of polymers is that they are chemically inert and non-biodegradable. This led to the difficulty in the disposal of polymers. Landfill Synthetic polymers are non-biodegradable and inert to chemical attack, hence landfill sites are quickly filled up Incineration (combustion of organic substances in waste treatment process) CO2 (greenhouse gas) is produced which adds to global warming. Toxic gases are also produced. Recycling The different types of polymers must be separated before recycling which can be difficult and expensive. Dumping in rivers and seas Block drainage systems, causing flash floods. Method to overcome disposal problems Reduce, reuse recycle Reduce the amount of waste Reuse items before replacing them Recycle items wherever possible Degradable plastic (Biodegradable plastic or photodegradable plastic) Biodegradable plastics are plastics that can be decomposed by the action of living organisms, usually microbes, into water, carbon dioxide and biomass Photodegradable plastics are usually made of oil-based polymers, just like ordinary plastics, which absorbs light and then attacks the polymer and breaks some of the bonds Pyrolysis Pyrolysis is the thermal decomposition of materials at elevated temperatures (above 430 ℃) and under pressure, in an inert atmosphere 44 Polymers Chemistry | STPM Term 3
CHEMICAL TEST FOR FUNCTIONAL GROUP Compound Functional group Chemical test / observation Alkene Add Br2 in CCl4 at room temperature. Reddish-brown bromine is decolourised Add bromine water at room temperature. Reddish-brown bromine is decolourised. Add acidified KMnO4 at room temperature. Purple acidified KMnO4 is decolourised. Benzene Add concentrated HNO3 and concentrated H2SO4 at 55 ℃. Yellow oil of nitrobenzene is observed. Methylbenzene Add hot acidified KMnO4. Purple KMnO4 is decolourised. Haloalkane R−X Add ethanolic silver nitrate solution and warmed. Haloalkane Observation Identity of precipitate R − Cl White precipitate AgCl R − Br Cream precipitate AgBr R − I Yellow precipitate AgI Alcohol −OH Add carboxylic acid with few drops of concentrated sulphuric acid and heat. Sweet fruity smell of ester is produced. Lucas test Add solution of ZnCl2 in concentrated HCl. Cloudiness is observed. Alcohols Observations Primary No cloudiness at room temperature Secondary Cloudiness after 5 minutes Tertiary Immediate cloudiness Phenol Add bromine water at room temperature. Reddish-brown bromine decolourised and white precipitate of 2,4,6-tribromophenol is formed. Add iron(III) chloride solution at room temperature . Purple colouration is observed. Carbonyl compound Add 2,4-dinitrophenylhydrazine at room temperature. Orange precipitate is formed. Aldehyde and benzaldehyde Add Tollens’ reagent and warmed. Silver mirror is formed. Aldehyde Add Fehling’s solution and warmed. Brick-red precipitate is formed. Carboxylic acid Add Na2CO3 solution at room temperature. Effervescence occurs and carbon dioxide is released. Add neutral iron(III) chloride solution at room temperature. Dark red colouration is observed. Add neutral iron(III) chloride solution at room temperature. Buff precipitate is observed. Amide Add aqueous NaOH, then heat. Pungent smell gas is released. Primary amine −NH2 Add sodium nitrite(NaNO2) then followed by dilute hydrochloric acid. Effervescence occurs and nitrogen is released Aromatic amines Add bromine water at room temperature. Reddish-brown bromine decolourised and white precipitate is formed. Chemical Test