101CHAPTER 7 NITROGEN COMPOUNDS
102SUBTOPICS EXPLANATORY NOTES7.1 Naming and classification of aminea) classify and naming amine according to IUPAC nomenclatureb) identify functional group of amine and amide7.2 Formation of Amine compoundsa) show the formation of amine from following reaction:i) reduction of nitro compoundii) reduction of nitrilesiii) reduction of amidesiv) Hofmann degradation of amides 7.3 Reaction of primary aminea) explain the reaction of amine with:i) acyl chlorideii) anhydrideiii) esteriv) benzene sulphonyl chloridev) Nitrous acid; - diazonium compounds7.4 Amino acids: formation of zwitterionsa) Show the structure of 5 amino acids- Glycine, Alanine, Valine, Leucine and Isoluecine.b) define the terms zwitterion and isoelectronic point (pI)7.5 Proteins: identify linkage, structure a) identify the structure of peptides in proteins b) show the formation of polypeptides7.1 a) Introduction of AminesAmines are organic derivatives of ammonia, NH3, in the same way that alcohols and ethers are organic derivatives of water, H2O. Like ammonia, amines contain a nitrogen atom with a lone pair electron, making amines both basic and nucleophilic.b) Classification of AminesAmines are classified as primary, secondary, tertiary or quaternary, depending on how many alkyl groups bonded to nitrogen. Quaternary amines are ionic and usually exist as compounds called quaternary ammonium salts.
103c) Nomenclature of Amines(i) Primary aminesPrimary amines are named in several ways, depending on their structure. For simple amines, the suffix-amine is added to the name of the alkyl substituent (common name). Alternatively, the suffix-amine can be used in place of the final “e” of the alkane name (IUPAC system).Structure Common name IUPAC systemCH3NH2methylamine methanamineCH3CH2NH2ethylamine ethanamineCH3CH(NH2)CH3isopropylamine 2-propanamineNH2CH2CH2NH21,2-diaminoethane Ethane-1,2-diamineNH2cyclopentylamine cyclopentanamineWhen multiple functional groups are present and the amine group –NH2 does not take priority, it is named as an “amino” substituent.Examples O || NH2CH2CH2CH2CCH3 5-amino-2-pentanone CH3CHNH2COOH 2- aminopropanoic acid (ii) Secondary and Tertiary aminesSymmetrical secondary and tertiary amines are named by adding the prefix di- or trito the alkyl group. Unsymmetrically substituted secondary and tertiary amines are named as N-substituted primary amines. The largest alkyl group is chosen as the parent name, and the other alkyl groups are considered N-substituents on the parent (N because they are attached to nitrogen).NH2COOH3-Aminobenzoic acid
104• Secondary aminesStructure Common name IUPAC systemCH3NHCH3dimethylamine N-metylmethanamineCH3NHCH2CH3methylethylamine N-methylethanamineCH3CH2NHCH2CH3diethylamine N-ethylethanamineCHCH2CH3NHCH3CH3CH2 -N-methyl-3-pentanamineHCCH3CH3CHNHCH3CH3 -N,3-dimethyl-2-butanamine• Tertiary aminesStructure Common name IUPAC system(CH3)3Ntrimethylamine N,N-dimetylmethanamine(CH3)2NCH2CH3dimethylethylamine N,N-dimethylethanamineCH3CH2 N CH2CH3CH3diethylmethylamine N-ethyl-N-methylethanamineN....-N-ethyl-N-methyl-4-octanamine• Aromatic amines Aromatic amines are named as derivatives of aniline.NH2NH2NNH2AnilineB BrrBrCH3CH3 CH2CH32,4,6-TribromoanilineN-ethy-methylaniline 4-Methylaniline
105d) Functional Group of Amine and AmideAmines are molecules that contain carbon-nitrogen bonds. The nitrogen atom inan amine has a lone pair of electrons and three bonds to other atoms, either carbonor hydrogen. Various nomenclatures are used to derive names for amines, but allinvolve the class-identifying suffix –ine as illustrated here for a few simple examples:Amides are molecules that contain nitrogen atoms connected to the carbon atom of a carbonyl group. Like amines, various nomenclature rules may be used to name amides, but all include use of the class-specific suffix –amide:Amides can be produced when carboxylic acids react with amines or ammonia in a process called amidation. A water molecule is eliminated from the reaction, and the amide is formed from the remaining pieces of the carboxylic acid and the amine (note the similarity to formation of an ester from a carboxylic acid and an alcohol discussed in the previous section):
1067.2 Formation of Amine Compounds(a) Reduction of nitro compoundsBoth aromatic and aliphatic nitro groups are easily reduced to amino groups. R NO2 R NH2reduction [H]reduction reagents that can be used is either1. H2/catalyst (Pt, Pd or Ni) 2. Active metal (Fe, Zn or SnCl2) and H+ 3. LiAlH44. NaBH4Example1.NO2 NH2H2,Pt2.CH3CH2CH2CHCH3(i) SnCl2, HCl(ii) NaOH, H2O CH3CH2CH2CHCH3NH2NO2(b) Reduction of nitrilesNitriles are reduced to primary amines by lithium aluminum hydride or by catalytic hydrogenation.R C NH2, catalyst (Pt, Pd or Ni)or1. LiAlH4,ether2. H2OR CH2NH2ExampleOHCN1. LiAlH42. H3O+OHCH2NH2
107(c) Reduction of amidesReduction of an amide usind LIAlH4 can yield a primary, secondary, or tertiary amine depending on the type of amide used.1. CH3CONH2(i) LiAlH4(ii) H3O+CH3CH2NH2Ethanamide Ethanamine2. CH3CONHCH3(i) LiAlH4(ii) H3O+CH3CH2NHCH3N-Methyletanamide N-Methyletanamine3.CH3CON(CH3)2(i) LiAlH4(ii) H3O+CH3CH2N(CH3)2N,N-Dimethyletanamide N,N-Dimethyletanamine(d) The Hofmann degradation of amidesIn the presence of strong base, primary amides react with chlorine or bromine to form amines with the loss of the carbonyl groups. This reaction is used to synthesize primary alkyl and aryl amines.R CONH2X2, OH-H2OR NH2X2 = Cl2 or Br2+ CO32-ExampleCONH2 NH2+ CO32-Br2, OH-H2O
1087.3 Reaction of Primary Amine I) AcylationAmines are acylated by reaction with acid chlorides, acid anhydrides or esters to yield amides(a) Reaction with acid chloridesR NH2 Cl C +OR' R NH CO+ R' HClCH3CH2 NH2 Cl C +OCH3 CH3CH2 NH CO+ CH3 HClexample:1o(b) Reaction with acid anhydridesRNH2 + R'COCR'O ORNHCR' + R'COOHONH2CH3+ (CH3CO)2O CH3COOHNHCOCH3CH3+example:(c) Reaction with ester R NH2 + R' COOR'' NH COR R' + R''OH ExampleCH3 NH2 + CH3 COOCH2CH3NH COCH3 CH3 + CH3CH2OH methanamine ethyl ethanoate N-methylethanamide ethanol
109(d) Reaction with benzene sulfonyl chlorideThis reaction is used to differentiate between primary, secondary and tertiary amines. Primary and secondary amines react with sulfonyl chlorides to form sulfonamides. Sulfonamide formation is the basis for a chemical test, called the Hinsberg’s test.Primary amine reacts with benzene sulfonyl chloride to form N-substituted benzene sulfonamide. These in turn, undergo acid-base reaction with excess KOH to form water-soluble potassium salts.RNH2 + C6H5 SOOCl C6H5SO2NHRKOHC6H5SO2NR−K+ C6H5SO2NHRH+1o sulfonamide(precipitate)water soluble salt(clear solution)InsolubleOH−(e) Reaction with nitrous acid, HNO2 Nitrous acid (HONO) is a weak, unstable acid. It is always prepared in situ, by treating sodium nitrite (NaNO2) with an aqueous solution of a strong acid: HCl + NaNO2HONO + NaClor H2SO4 + 2NaNO22HONO + Na2SO4Nitrous acid react with all classes of amines. The products obtain from the reaction depend on whether the amine is primary, secondary or tertiary and whether the amine is aliphatic or aromatic.(i) Primary aliphatic aminesPrimary aliphatic amines react with nitrous acid to yield highly unstable aliphatic diazonium salts. The salts decompose by losing nitrogen to form carbocations. The carbocations go on to produce mixtures of alkenes, alcohols, and alkyl halides respectively by removal of a proton, reaction with H2O and reaction with X-
110+ ⎯ ⎯→+ −RCH CH NH NaNO RCH CH N Cl HCl2 2 2 2 2 2 + = +22 RCH CH2 RCH 2 C H N RCH2CH2OH RCH CH Cl 2 2(ii) Primary aromatic aminesPrimary arylamines react with nitrous acid to give arenediazonium salts. NH2N2+Cl−+NaNO2, HClcoldNaCl + H2Oarenediazonium saltThese aryl diazonium ions are stable at 0 C. They are particularly useful in synthesis because the diazonium group can be replaced by almost any nucleophile as indicated below:7.4 Amino Acids: Formation of ZwitterionsIntroductionThe term amino acid, as its name implies, are difunctional containing both an amino group, NH2, and carboxyl group, -COOH. Their value as biological blocks stems from the fact that amino acids can join together into long chains by forming amide bonds between the –NH2 of one amino acid and the –COOH of another. For classification purposes, chains with fewer than 50 amino acids are often called peptides, while the term protein is reserved for longer chains.Amino acids commonly found in proteins are -amino acid, meaning that the amino group in each is a substituent on the carbon atom – the one next to the carbonyl group.
111i) General structure of -amino acid H2N C CR1 OOHHcarboxyl group carbonamino group• The hydrogen atom and R group are attached to carbon.• The differences of R group in each amino acid give the unique characteristic of the amino acids.• There are 20 different amino acids commonly found incorporated into proteins.Classification of 5 Amino Acids in Proteinii) Name iii)StructureGlycineGly H CHNH2COOHAlanineAla H3C CHNH2COOHValineVal HC CHNH2COOHCH3CH3LeucineLeu CHCH2CH3CH3CHNH2COOH
112IsoleucineIle CHNH2CH3CH2CH COOHCH3B) Physical Properties ZwitterionAmino acids can undergo an internal acid-base reaction, in which a proton is transferred from the carboxy to the amino group to form dipolar ion called zwitterion. O O C –O-H C-O¯ H2N C H H3+N C H R R uncharged structure dipolar ion or zwitterionThe dipolar nature of amino acids give to their unique properties:-(i) Amino acids have high melting points, generally over 2000C.(ii) Amino acids are more soluble in water.(iii) Amino acids are amphoteric or ampholite ( amphoteric electrolyte ) which can react as acid or base depending on the pH of the solution:-In acidic solutions all amino acids are present primarily as cations (amino acids are base).In basic solutions they are present as anions (amino acids are acid)...
113Isoelectric point (pI)In general, zwitterion is electrically neutral and exists at specific pH. This particular pH is called isoelectric points. Each amino acid has it’s specific isoelectric point. For example, isoelectric point, pI, of alanine is at pH 6.02. At this pH, alanine exists as zwitterions.7.5 ProteinsProtein is a polymer of amino acids.Structure of proteins can be divided into 4 major types:• The primary structure of a protein is the covalently bonded structure of the molecule e.g: amino acid sequence.• The secondary structure of a protein describes how segments of the peptide backbone orient into a regular pattern (arrangement of hydrogen bonds and SS linkage) e.g. -helix and pleated sheet.• The tertiary structure describes how the entire protein molecule coils into an overall three-dimensional shape e.g hydrogen bonding, ionic interaction.• The quaternary structure describes how individual protein molecules come together to yield large aggregate structures e.g: haemoglobin.Peptides are amino acid polymers in which the individual amino acid units, are linked together by amide bonds, or peptide bonds. ✓ 2 amino acids form dipeptide✓ 3 amino acids form tripeptide✓ 15 – 30 amino acids oligopeptide> 30 amino acids polypeptideAmino group of a substance reacts with carboxyl group of another amino acid to form a peptide bond by elimination of water.
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115CHAPTER 8CARBONYL COMPOUNDS
116UBTOPICS EXPLANATORY NOTES8.1 Naming and physical properties of aldehyde and ketone.a) Describe the general formula of aldehydes and ketonesb) Draw the structure and name the compound according to IUPAC nomenclaturec) Explain the boiling point of carbonyl compounds.8.2 Chemical Reaction Explain the chemical properties with reference to these reactions.a) oxidation with KMnO4, K2Cr2O7, Tollens Testand Fehling Testb) reduction to alcohol using LiAlH4, H2/Pt, Zn/H+c) nucleophilic addition with i) HCN/KCN @ NaCN ii) Grignard reagentd) reaction with 2,4-dinitrophenyl hydrazine (Brady’s Test) e) Iodoform Test
1178.1 INTRODUCTION • Aldehydes and ketones contain a carbonyl group, aldehyde ketone• The carbonyl compounds have the general formula of CnH2nO• Aldehydes and ketones are isomeric.• Aldehydes: at least one hydrogen atom is attached to the carbonyl carbon atom.• Ketones: There are no hydrogen atoms attached to the carbonyl carbon atom.8.1.1 NOMENCLATURE• The number of carbon atom in longest continuous chain that contains the -CHO group provides the base name for aldehydes. The -e ending of the corresponding alkane name is replaced by –al, and substituents are specified in the usual way. • With ketones, the –e ending of an alkane is replaced by –one in the longest continuous chain containing the carbonyl group. The chain is numbered in the direction that provides the lower number for this group.• Example:CH3CH2CH2CHO ButanalCH3CHCH2CHCHCH3 CH2CH3O2-ethyl-4-methylpentanalCH3CCHCH2CHCH3OCH3 CH3 3,5-dimethyl-2-hexanone CompoundStructure formula IUPAC nomenclatureCommon nameAldehyde H2C=OCH3CH=OCH3CH2CH=OCH3CH2CH2CH=OMethanalEthanalPropanalButanalFormaldehydeAcetaldehydePropionaldehydeButyraldehydeKetone CH3COCH3CH3CH2COCH3CH3CH2CH2COCH3CH3CH2COCH2CH3Propanone2-butanone2-pentanone3-pentanoneAcetoneMethyl ethyl ketonen–propyl methyl ketoneDiethyl ketoneIUPAC nomenclature and common names for aldehyde and ketone compoundsCO
118• Aldehyde and ketone are functional group since both have the same general formula.• Example:CH3CH2CHCH2C OCH3 H3- methylpentanalC6H12OCH3CH2CCH2CH2CH3O3-hexanoneC6H12O8.1.2 BOILING POINT • Due to highly polarized C=O bond, aldehyde and ketone have HIGHER boiling point than halogen compounds, alkanes and other nonpolar compounds of similar molar mass.• But LOWER than COOH and OH because carbonyl cannot form intermolecular hydrogen bonding with their own molecules.8.2 CHEMICAL PROPERTIES8.2.1 Oxidation of carbonyl compoundsA) Oxidation by acidified KMnO4 or acidified K2Cr2O7• Aldehydes are much more easily oxidized than ketones. Strong oxidizing agents such as potassium permanganate, potassium dichromate, readily oxidize aldehydes and they are also oxidized by such mild oxidizing agents as silver oxide.• Oxidation of aldehydes produces acids with the same number of carbon atoms.• Vigorous oxidation of ketone produces acids with lesser number of carbon atoms. Ketones are oxidized by refluxing for a few hours with alkaline potassium manganate (VII) or concentrated nitric acid.• Example:CH3(CH2)4 C HO+ K2Cr2O7H+CH3(CH2)4 C OHO hexanal hexanoic acidB) Oxidation with Tollen’s reagents (Tollen’s Test) • Tollen’s reagent is used to differentiate aldehydes and ketones. Tollen’s reagent is prepared by mixing aqueous silver nitrate with aqueous ammonia. The reagent contains the silver amine complex ion, [Ag(NH3)2]+. • Although this ion is a very weak oxidizing agent, it oxidizes aldehydes to carboxylate anions. As it does this, silver is reduced from the +1 oxidation state of [Ag(NH3)2]+to metallic silver. If the rate of reaction is slow and the walls of the vessel are clean, metallic silver deposits on the walls of the test tube as a mirror; if not, it deposits as a gray to black precipitate. Tollen’s reagent gives a negative result with all ketones except alpha-hydroxy ketones.
119 C) Oxidation with Fehling Test• Fehling’s solution conceive of Cu (II) ion, as a complex ion in a basic solution, It oxidizes aldehydes to carboxylic acids as the Cu2+ is reduced to Cu+, which forms a brick- red (red-brown) precipitated, Cu2O. Fehling’s solution has the characteristic blue color of Cu2+ which fades as the red precipitated of Cu2O forms. Because Fehling’s solution is basic, the carboxylic acid product is formed as its conjugate base.RCOH + Cu2+ (tartarat) RCOO- + 2Cu2O + 3H2O Fehling’s solution copper (I) oxide (blue) (brick red precipitate)• Fehling’s solution-confirmatory test for aldehydes.• Benzaldehyde and ketone give a negative reaction for both tests.8.2.2 Reduction• Aldehydes are reduced to primary alcohols and ketones to secondary alcohols by a variety of reducing agents. R CHOH+R CHOHH aldehyde alcohol 1°R CR'OH+R CR'OHH ketone alcohol 2° • The common reducing agents used are:(a) i) Lithium aluminium hydride, LiAlH4 ii) H3O+ (b) Zn/H+, heat (c) H2(g)/Ni @ Pt, heatExampleH3C CHOH+ H3C CHOHHLiAlH4
1208.2.3 Nucleophilic Addition A) Addition of Grignard Reagent• The special value of Grignard Reagents provides excellent ways to form new carboncarbon bonds. A carbanion is a good nucleophile and adds to the carbonyl group of an aldehyde or ketone to form a tetrahedral carbonyl addition compound. • The driving force for these reactions is the attraction of the partial negative charge on the carbon of the organometallic compound to the partial positive charge of carbonyl carbon.Reaction: C O + R MgX R C O H + Mg(OH)XetherH3O+ExampleH3C C OH+ CH3CH2CHCH3MgBrCH3C OCHHH3C CH2CH3Mg Brethanal sec-butylmagnesiumbromidaBromomagnesium alkoxideH3O+CH3C OHCHHH3C CH2CH33-methyl-2-penthanol+ Mg(OH)BrB) Addition of HCN, NaCN and KCN• Reagents: HCN, NaCN @ KCN followed by dilute H2SO4
1218.2.4 Reaction with 2,4- dinitrophenylhydrazine (2,4-DNPH) @ Brady’s Test• NH3 & its derivatives react as nucleophile and react with carbonyl compounds.• Involved addition reaction followed by dehydration (elimination of H2O molecule to form C=N)8.2.5 Haloform formation @ Iodoform Test• When methyl ketones or ethanal react with halogens in the presence of base to form halogenated methyl group. Hydroxide ion attack carbonyl group to give haloform and carboxylic ion. C OH3CH+ 3X2base (OH-)C OH3CH+ 3HX + 3H2OC OH3CH+ OH- CHX3 + HCOOtriiodomethane(iodoform)where X = iodine
122C OH3CR+ 3I2base (OH-)C O-OR+ketoneCHI3yellow precipitate(iodoform) CH OHH3CR+ 3I2base (OH-)C O-OR+alcoholCHI3yellow precipitate(iodoform)Example:CH3CH2CCH3O+ 3I2butanonebase (OH-)CH3CH2COO- + CHI3yellow precipitate(iodoform)
123CHAPTER 9CARBOXYLIC ACIDS AND DERIVATIVES
124SUBTOPICS EXPLANATORY NOTES9.1 Nomenclature Name aliphatic and aromatic acids according to IUPAC nomenclature and their common names for C1 to C6.9.2 Strength of acid Compare the acidity of carboxylic acids with electron donating groups and electron withdrawing groups9.3 Uses of carboxylic acids in industryExplain the various uses of carboxylic acids in food, perfumes, polymers and other industries.9.1.1 NAMING, PHYSICAL PROPERTIES AND ACIDITYSTRUCTURE• Carboxylic acids are compounds containing the carboxyl group (because it contains a carbonyl group and a hydroxyl group). Sometime it simply referred to as a acid group.OCR OH• Summarized ways the carboxylic acid functional group is shown. or• Monoprotic carboxylic acid have the general formula CnH2nO2 or CnH2n+1COOH or RCOOHNAMING ACIDS• Find the parent : The parent compound (or chain) must have the carboxyl carbon in the chain. Parent compound should have the longest carbon atom.• Number the parent : Start with the carbonyl carbon. The carboxyl carbon is always No. 1• Name the parent : Name the compound as you normally would in alkane then drop the –e and add “ –oic acid”.CO2H COOHWhere R = H, alkyl or aryl
125 Example 1) Name the parent 2) Number the parent3) Name the parent4) Name the side groups 2,4-dimethylpentanoic acidNames for carboxylic acidsCH3CHCH2CHCOOHCH3 CH3CH3CHCH2CHCOOHCH3 CH35 4 3 2 1CH3CHCH2CHCOOHCH3 CH35 4 3 2 1-CO2H-COOHorcarboxylic acidpentanoic acidC H3CHC H2C HC OOHC H3 C H35 4 3 2 1methyl methylParent: pentanoic acidPentanoic acid Valeric acid CH3CH2CH2CH2COOHButanoic acid Butyric acid CH3CH2CH2COOHPropanoic acid Propionic acid CH3CH2COOHEthanoic acid Acetic acid CH3COOHMethanoic acid Formic acid HCOOHIUPAC Name Common Name Formula Structure
126Nomenclature• If the carboxyl group is attached to a ring, the parent name is : the name of the ring by using prefix + “-carboxylic acid”• Carbon number 1 in the ring is the carbon with the carboxyl attachedCOOHCH2CH3123452-ethylcyclopentanecarboxylic acidBOILING POINTCarboxylic acids have much higher boiling points than hydrocarbons, alcohols, ethers, aldehydes, or ketones of similar molecular weight. Even the simplest carboxylic acid, formic acid, boils at 101 °C, which is considerably higher than the boiling point of ethanol (ethyl alcohol), C2H5OH, which boils at 78.5 °C, although the two have nearly identical molecular weights. The difference is that two molecules of a carboxylic acid form two hydrogen bonds with each other (two alcohol molecules can only form one). Thus, carboxylic acids exist as dimers (pairs of molecules), not only in the liquid state but even to some extent in the gaseousstate.Dimerization effect on carboxylic acidTherefore, boiling a carboxylic acid requires the addition of more heat than boiling the corresponding alcohol, because (1) if the dimer persists in the gaseous state, the molecular weight is in effect doubled; and, (2) if the dimer is broken upon boiling, extra energy is required to break the two hydrogen bonds. Carboxylic acids with higher molecular weights are solids at room temperature (e.g., benzoic and palmitic acids). Virtually all salts of carboxylic acids are solids at room temperature, as can be expected for ionic compounds.
127ACIDITY OF CARBOXYLIC ACID Comparison acidity of carbocylic acid, phenol and alcohol• Carboxylic acids are much more acidic than phenol and alcohol due to the resonance stabilization of the resulting carboxylate ion.• The electrons in carboxylate ion are delocalized between two oxygen atoms, whereas in phenoxide ion and alkoxide ion, the electrons are localized at only one oxygen atom.•RCOOH , C6H5OH , ROH Decreasing Acidity • Carboxylic acidR COOH+ H2O R COO-R COO-+ H3O+Carboxylate ion: identical resonance structure• Phenol OH + H2O O + H3O+phenoxide ion hydronium ion• AlcoholROH + H2O RO + H3O+alkoxide ion (no resonance structure)Substituent effects on acidity of carboxylic acidity.• The dissociation of a carboxylic acid is an equilibrium process.OR C OH + H2OOR C O + H3O+• An electron-withdrawing group (EWG) that attached to a carboxylate ion will delocalize the negative charge, thereby stabilizes carboxylate ion and increasing acidity.
128COEWG ONote: EWG withdraw electrons, reduces the negative charge, stabilizes the carboxylate ion and thus increases acidity. • An electron-donating group (EDG) will destabilize the carboxylate anion and decrease acidity. COEDG ONote: EDG releases electrons, increases the negative charge, destabilizes the carboxylate ion and hence decreases acidity.Substituent effects on acidity of p - substituted benzoic acids.Y COOHi) The inductive effect can be quite large if one or more strongly electron-withdrawing groups are present on the carbon atom. ii)Example:iii) The inductive effects are dependent on distance. The effect of electron- withdrawing groups decreases as the substituent moves farther from the carboxyl.Carboxylic acid pKaCH3COOHBrCH2COOHClCH2COOHCl2CHCOOHCl3COOH4.742.682.861.260.64
129Example:9.1.2 PREPARATION OF CARBOXYLIC ACIDOXIDATION OF ALKENE, ALKYL BENZENE AND PRIMARY ALCOHOLSa) Oxidation of AlkenesAlkenes may be converted into carboxylic acid through oxidative cleavage of the double bond with neutral or acid permanganate, for instance. However, the alkene must contain at least one hydrogen located at the double bond, otherwise only ketones are formed.b) Oxidation of Alkyl BenzenesAromatic carboxylic acid preparation is possible through the oxidation of alkyl benzenes. Vigorous oxidation of alkyl benzene compound with acidic or alkaline potassium permanganate or chromic acid can lead to the formation of aromatic carboxylic acid compounds. The oxidation of complete side chain of the carboxyl group takes place regardless of the side chain length. The resulting side products of the reaction vary depending on the primary or secondary alkyl groups. However, the tertiary alkyl group is not affected. Moreover, properly substituted alkenes can also undergo oxidation process to form carboxylic acids with the help of these oxidizing agents. Alkyl groups that contain benzylic hydrogens—hydrogen(s) on a carbon α to a benzene ring—undergo oxidation to acids with strong oxidizing agents. Refer to the example below for the reactions under this preparation technique.2.864.054.52Carboxylic acid pKa
130c) Oxidation of Primary Alcohols The oxidation of primary alcohols leads to the formation of aldehydes that undergo further oxidation to yield carboxylic acids. All strong oxidizing agents (potassium permanganate and potassium dichromate) can easily oxidize the aldehydes to form carboxylic acids.d) Hydrolysis of NitrilesThe hydrolysis of nitriles, which are organic molecules containing a cyano group, leads to carboxylic acid formation. These hydrolysis reactions can take place in either acidic or basic solutions.
1319.1.3 REDUCTIONCarboxylic acids can be converted to 1o alcohols using Lithium aluminum hydride (LiAlH4). An aldehyde is produced as an intermediate during this reaction, but it cannot be isolated because it is more reactive than the original carboxylic acid.General Reaction:Example:9.2 ACYL CHLORIDE9.2.1 FORMATION OF ACYL CHLORIDE FROM CARBOXYLIC ACID.Carboxylic acids react with Thionyl Chloride (SOCl2) to form acid chlorides. During the reaction the hydroxyl group of the carboxylic acid is converted to a chlorosulfite intermediate making it a better leaving group. The chloride anion produced during the reaction acts a nucleophile.General Reaction :
132Example9.2.2 REACTION WITH ALCOHOL, PHENOL AND PRIMARY AMINEThe reaction with alcoholsGeneral case of any alcohol reacting with ethanoyl chloride. The equation would be:The organic product this time is an ester. For example, with ethanol you would get the ester ethyl ethanoate:. . . or, more commonly:This is an easy way of producing an ester from an alcohol because it happens at room temperature, and is irreversible. Making an ester from an alcohol and a carboxylic acid (the usual alternative method) needs heat, a catalyst and is reversible - so that it is difficult to get a 100% conversion.The reaction with phenolsPhenols have an -OH group attached directly to a benzene ring. In the substance normally called \"phenol\", there isn't anything else attached to the ring as well. We'll look at that first.The reaction between phenol and ethanoyl chloride isn't quite as vigorous as that between alcohols and ethanoyl chloride. The reactivity of the -OH group is modified by the benzene ring.That apart, the reaction is just the same as with an alcohol.
133The reaction with primary aminesIn a primary amine, it is attached to an alkyl group (shown by \"R\" in the diagram below) or a benzene ring. The initial equation would be:The organic product this time is called an N-substituted amide.If you compare the structure with the amide produced in the reaction with ammonia, the only difference is that one of the hydrogens on the nitrogen has been substituted for a methyl group.This particular compound is N-methylethanamide. The \"N\" simply shows that the substitution is on the nitrogen atom, and not elsewhere in the molecule.The equation would normally be written:You can think of primary amines as just being modified ammonia. If ammonia is basic and forms a salt with the hydrogen chloride, excess methylamine will do exactly the same thing.The salt is called methylammonium chloride. It is just like ammonium chloride, except that one of the hydrogens has been replaced by a methyl group.You would usually combine these equations into one overall equation for the reaction:Phenylamine is the simplest primary amine where the -NH2 group is attached directly to a benzene ring. Its old name is aniline.In phenylamine, there isn't anything else attached to the ring as well. You can write the formula of phenylamine as C6H5NH2.There is no essential difference between this reaction and the reaction with methylamine, except that the phenylamine is a brownish liquid, and the solid products tend to be stained brownish.
134The overall equation for the reaction is:The products are N-phenylethanamide and phenylammonium chloride.This reaction can sometimes look confusing if the phenylamine is drawn showing the benzene ring, and especially if the reaction is looked at from the point of view of the phenylamine.9.3 ESTERESTERIFICATION PROCESS, HYDROLYSIS AND REDUCTION WITH LiAlH4a) Show The Formation of Esters Through EsterificationEsters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. We shall just be looking at cases where it is replaced by an alkyl group, but it could equally well be an aryl group (one based on a benzene ring).Esters are produced when carboxylic acids are heated with alcohol in the presence of an acid catalyst. The catalyst is usually concentrated sulphuric acid. Dry hydrogen chloride gas is used in some cases, but these tend to involve aromatic esters (ones containing a benzene ring). The esterification reaction is both slow and reversible. The equation for the reaction between an acid RCOOH and an alcohol R'OH (where R and R' can be the same or different) is:So, for example, if you were making ethyl ethanoate from ethanoic acid and ethanol, the equation would be:
135b) Predict The Product Formed From(i) Hydrolysis of EsterTechnically, hydrolysis is a reaction with water. That is exactly what happens when esters are hydrolysed by water or by dilute acids such as dilute hydrochloric acid.The alkaline hydrolysis of esters actually involves reaction with hydroxide ions, but the overall result is so similar that it is lumped together with the other two.Hydrolysis using water or dilute acidThe reaction with pure water is so slow that it is never used. The reaction is catalysed by dilute acid, and so the ester is heated under reflux with a dilute acid like dilute hydrochloric acid or dilute sulphuric acid.Here are two simple examples of hydrolysis using an acid catalyst.First, hydrolysing ethyl ethanoate:. . . and then hydrolysing methyl propanoate:Notice that the reactions are reversible. To make the hydrolysis as complete as possible, you would have to use an excess of water. The water comes from the dilute acid, and so you would mix the ester with an excess of dilute acid.Hydrolysis using dilute alkaliThis is the usual way of hydrolysing esters. The ester is heated under reflux with a dilute alkali like sodium hydroxide solution.There are two big advantages of doing this rather than using a dilute acid. The reactions are one-way rather than reversible, and the products are easier to separate.
136Taking the same esters as above, but using sodium hydroxide solution ratherthan a dilute acid:First, hydrolysing ethyl ethanoate using sodium hydroxide solution:This mixture is relatively easy to separate. Provided you use an excess of sodium hydroxide solution, there won't be any ester left - so you don't have to worry about that.The alcohol formed can be distilled off. That's easy!If you want the acid rather than its salt, all you have to do is to add an excess of a strong acid like dilute hydrochloric acid or dilute sulphuric acid to the solution left after the first distillation.If you do this, the mixture is flooded with hydrogen ions. These are picked up by the ethanoate ions (or propanoate ions or whatever) present in the salts to make ethanoic acid (or propanoic acid, etc). Because these are weak acids, once they combine with the hydrogen ions, they tend to stay combined.The carboxylic acid can now be distilled off.Hydrolysing complicated esters to make soap.This next bit deals with the alkaline hydrolysis (using sodium hydroxidesolution) of the big esters found in animal and vegetable fats and oils.If the large esters present in animal or vegetable fats and oils are heated with concentrated sodium hydroxide solution the same reaction happens as with the simple esters.A salt of a carboxylic acid is formed - in this case, the sodium salt of a big acid such as octadecanoic acid (stearic acid). These salts are the important ingredients of soap - the ones that do the cleaning.An alcohol is also produced - in this case, the more complicated alcohol, propane-1,2,3-triol (glycerol).
137Because of its relationship with soap making, the alkaline hydrolysis of esters is sometimes known as saponification.(ii) Reduction with LiAlH4Esters can be converted to 1o alcohols using LiAlH4, while sodium borohydride (NaBH4) is not a strong enough reducing agent to perform this reaction.General ReactionEXAMPLE9.4 USES OF CARBOXYLIC ACID• Benzoic acid is used as a food preservative.• Ethanoic acid is found in vinegar• Long chain carboxylic acids (fatty acids) are used to produce soaps.• 1,6-hexanedioic acid is a monomer for nylon 6,6.• Methanoic acid (formic acid) is used to coagulate latex.• 1,4-benzenedicarboxylic acid is used to make Terylene, a polyester used for fabric making.
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