LECTURE NOTE SK025 (ORGANIC) KMKt

Example 1: C4H10O (General Formula : CnH2n+2O)

CH3CH2CHCH3 CH3⎯CH2⎯O⎯CH2CH3
OH

Hydroxyl Alkoxy
(alcohol) (ether)

Example 2: C5H10O (General Formula : CnH2nO)

O O
CH3CH2CH2CH2C CH3CH2C

H CH2CH3

Carbonyl Carbonyl
(aldehyde) (ketone)

Example 3: C6H12 (Gene51ral Formula : CnH2n)

H2C CH CH2 CH2 CH2 CH3

Carbon carbon double bond
(alkene)

HH

HC H

HC CH

HC CH or

H CH

HH

(Cycloalkane)

STEREOISOMERISM

Definition

The existence of chemical compounds which have their
atoms connected in same order but differ in
arrangement in space

TYPES OF STEREOISOMERISM

stereoisomerism

enantiomers diastereomers
(mirror image) (non–mirror image)

optical isomers cis–trans isomer
(geometric isomer)

C═C cyclic

(ring) structure

HOW TO IDENTIFY CIS–TRANS ISOMERS

Restricted rotation of carbon–carbon bonds
due to:

C═C
cyclic (ring) structure

Two different atoms or group of atoms
attached to each of C atoms which form
double bond or cyclic structure

Example CH3CH═CHCH3
2–butene

H3C H H3C CH3
CC C C

H CH3 H H

trans–2–butene cis–2–butene

Two groups on the same side
 cis

Two groups on opposite side
 trans

Example

CH3 CH3
H H 1,2–dimethylcyclopentane

CH3 CH3 cis–1,2–dimethylcyclopentane

CH3 H
trans–1,2–dimethylcyclopentane

H CH3

Exercise 1

Draw the structure of cis–trans isomer of
1,2–cyclobutanediol.

HO OH
1,2–cyclobutanediol

Enantiomers

❑ Stereoisomers that
nonsuperimposable mirror images of
one another.

❑ occur only with compounds whose
molecules are chiral molecule

CHIRALITY CENTER

Definition

sp3 hybridized carbon bonded to four
different atoms or groups.

Also called chiral carbon, asymetric center,
stereogenic center
Labeled as “❋”

Example CH3 HH
H C OH
CH3—C—C—CH2CH3

COOH OH OH
lactic acid  chiral

molecule is not superimposable with its mirror image

CH3 CH3

C CH2CH3 CH
H CH3CH2 OH
HO

a pair of stereoisomers with structures that are mirror-
images of each other but nonsuperimposable.

3-D REPRESENTATION

How to show the 3-D
structure on paper?

Wedge–line:
 bond in front

Wedge–dashed:
 bond behind

Solid–line:
 bond lies in the plane

DRAWING A PAIR OF ENANTIOMERS IN 3D

Example This formula is called
3–D formula or
H wedge–dashed–wedge–line
CH3CH2 C CH3
formula
OH
2–butanol

CH3 CH3

C CH2CH3 CH
H CH3CH2 OH
HO

Example : 1

2-butanol, 

CH3 CH2 CH CH3

OH

CH3 CH3

H C OH C H

CH3CH2 HO

2-butanol CH2CH3

2-butanol

enantiomers

Example : 2 64



2-bromobutane, CH3 CH2 CH CH3

CH3 Br

CH3

C H C
Br H
CH3CH2 CH2CH3
Br

a pair of enantiomers

Example 3

Label(“”) at the chiral carbons (chirality center)
in each of the following molecules:

a) CH3CH2CHOHCH3
b) CH3CHOHCHOHCH2CH3
c) CH3CHFCH3
d) CH3CH(CH3)CH2CH3

Answer b) CH3CHOHCHOHCH2CH3

a) CH3CH2CHOHCH3 HH

H CH3—C—C—CH2CH3

CH3CH2—C—CH3 OH OH
d) CH3CH(CH3)CH2CH3
OH
c) CH3CHFCH3 H
CH3—C—CH2CH3
H
CH3—C—CH3 CH3
no chirality center
F
no chirality center

Example 4

Label (“”) at the chiral carbons (if any) in each
of the following molecules:

a) Cl b) H OH
CH–CH3

Cl

c)

H
CH3

Answer b) H OH

a) Cl  CH–CH3

 no chiral carbon
Cl

c)

H

CH3

Optical activity

 Molecules that are optically active is the molecules
that have the ability to rotate the plane polarized
light.

 Polarimeter is used to determine the optical activity
of a compound.

polarimeter

Optical activity
The requirements for optical active compounds :-

 molecule contains a chirality centre /

stereogenic centre / asymmetric carbon(*).

 molecule is not superimposable with

its mirror image.

Exercise

Some of the molecules listed here have a
stereogenic (chiral) carbon; some do not.
Write the three dimensional formula for both
enantiomers of those molecules that do have a
chiral carbon.

a) CH3CHClCH2CH3 2–chlorobutane
b) CH3CH2CH(CH3)CH2OH 2–methyl–1–butanol

4.0: INTRODUCTION TO
ORGANIC CHEMISTRY

4.4 Reactions in Organic
Compounds

LEARNING OUTCOMES

4.4 Reactions of Organic Compounds

At the end of the lesson, student should be able to:

a) explain covalent bond cleavage : homolytic & heterolytic. (C2)
b) Differentiate between homolytic cleavage and heterolytic cleavage.

(C3)
c) state the relative stabilities of 1o,2o,3o free radicals,

carbocations and carbanions. (C1)
d) compare the stabilities of carbocations and carbanions

by using the inductive effect of alkyl group. (C4)
e) define: (C1)

i) electrophile and nucleophile
ii) Lewis acid and Lewis base.
f) explain the types of organic reactions : (C2)
addition, substitution, elimination, rearrangement.
g) predict the type of organic reaction given a reaction equation. (C3)

REACTIONS IN ORGANIC COMPOUNDS

Covalent Bond Homolytic
Cleavage Heterolytic

Relative stabilities Free radicals Inductive
Carbocations effect

Carbanions

Reactions in Electrophile Define Lewis acid
Organic Nucleophile Types Cation
Electron deficient
Compound Define
Types site

Lewis base
Anion
Electron rich site

General type of Addition
Organic Reactions
Elimination
Substitution
Rearrangement

HOMOLYTIC CLEAVAGE

Occurs in a non-polar bond involving two
atoms of similar electronegativity.
A single bond breaks symmetrically into
two equal parts, leaving each atom with
one unpaired electron.
Formed free radicals.

HETEROLYTIC CLEAVAGE

Occurs in a polar bond involving unequal
sharing of electron pair between two atoms of
different electronegativities.

A single bond breaks unsymmetrically.

Both the bonding electrons are transferred to
the more electronegative atom.

Formed cation and anion.

Example of homolytic cleavage :

uv

Cl—Cl → Cl• + Cl•
chlorine

uv

H3C—CH3 → 2H3C•
ethane

Example of heterolytic cleavage :

H—Br → H+ + Br–
hydrogen bromide

(CH3)3C—Cl → (CH3)3C+ + Cl–
2–chloro–2–methylpropane

Example 1

Use half–headed curved arrows to show the
movement of electrons in each reaction:

a) •CH3 + •CH3 CH3–CH3
b) HO–OH 2HO•

H H
c) H C H + •C•• ••l•• H C• + H—C•• ••l••

H H

Answer CH3–CH3

a) •CH3 + •CH3

b) HO–OH 2HO•

H H
c) H C H + •C•• ••l•• H C• + H—C•• ••l••

H H

Example 2

Use full–headed curved arrows to
show the movement of electron in each equation:

CH3 CH3 Br–
a) CH3 C Br CH3 C+ +

CH3 CH3

CH3 CH3 X+
b) CH3 C X CH3 C- +

H H

Answer CH3 Br–
CH3 C+ +
CH3
a) CH3 C Br CH3

CH3 CH3 X+
CH3 C- +
CH3
b) CH3 C X H

H

Arrow types in chemical reactions:
 reaction (reactant → product)
 equilibrium
 resonance structures
 movement of an electron pair
 movement of a single electron

EFFECT OF ALKYL SUBSTITUTION ON
FREE RADICAL

H R RR

H C● < H C● < R C● < R C●
H
H HR
methyl primary
secondary tertiary

Radical carbon is electron deficient

Has an unfilled 2p obital

Stabilized by substituents such as
alkyl groups; CH3–, CH3CH2–, etc

Alkyl group  electron releasing

number of alkyl groups ↑ stability of radical ↑

Example 3

List the following radicals in order of
decreasing stability:

● ● CH3 ●

CH3 CH2

Answer

● ● ● CH3
3o radical
CH2 CH3

1o radical 2o radical

Increasing stability

CARBOCATION AND CARBANION

Heterolysis of a bond to C atom:

d+ d– C+ + ●●Z –
CZ Z+
carbocation

d– d+ C●● – +
CZ
carbanion

Carbocations and carbanions can be
intermediates in polar reactions

RELATIVE STABILITIES OF
CARBOCATION

H R RR

H C+ < H C+ < R C+ < R C+
H H
HR
methyl primary
secondary tertiary

more stable

Example: + +
++ (CH3)2CH < (CH3)3C
CH3 < CH3CH2 <

increasing alkyl substitution
increasing carbocation stability

EFFECT OF ALKYL SUBSTITUTION
ON CARBOCATION

H R R R
R C+
H C+ < H C+ < R C+ <
H H R
H tertiary
methyl primary
secondary d+
d+
Carbocation is electron deficient

Has an empty 2p obital

Stabilized by substituents such as
alkyl groups; CH3–, CH3CH2–, etc.

Alkyl group  electron releasing

number of alkyl groups ↑ stability of carbocation ↑

Example 4

Rank the following carbocations
in order of decreasing stability.
Classify each as primary, secondary , or
tertiary.

+

a) (CH3)2CHCH2

+

b) CH3CHCH(CH3)2

+

c) CH3C(CH3)CH2CH3

Answer

c) b) a)
+
CH3C(CH3)CH2CH3 + +
tertiary CH3CHCH(CH3)2 (CH3)2CHCH2
secondary primary

decreasing stability of carbocation

RELATIVE STABILITIES OF
CARBANION

R R R H
H C–
R C– < R C– < < H C–
H
R H primary H
tertiary secondary methyl

more stable

Example:

– – ––
CH3 > CH3CH2 > (CH3)2CH > (CH3)3C

less stable

increasing alkyl substitution

decreasing carbanion stability

EFFECT OF ALKYLSUBSTITUTION
ON CARBANION

R R R H
R C– H C– H C–
< R C– < <
R H H
tertiary H primary methyl
secondary

Carbanion is electron–rich
High electron density

Destabilized by substituents such as
alkyl groups; CH3–, CH3CH2–, etc.

Alkyl group  electron releasing

number of alkyl groups ↑ stability of carbanion ↓

REAGENTS AND SITES OF
ORGANIC REACTIONS

A) Electrophile

➢ Definition : A species that can accept an
electron pair from a nucleophile

➢ Means ‘electron loving’.

➢ Can be either neutral or positively
charged.

Examples of electrophiles :

Lewis acids such as AlCl3, FeCl3, BF3 ,
cations and carbocations.

Oxidizing agents such as Cl2, Br2 , etc.

Electrophilic sites are molecules with
low electron density around a polar
bond.

Examples:

d+ d- d+ d- d+ d-

CO CX C OH

(carbonyl) (haloalkanes) (hydroxyl
compounds)

Electrophilic sites

B. Nucleophile

➢ Definition : An electron-rich species that
can donate a pair of electrons to form a
bond

➢ means ‘nucleus loving’.

➢ A nucleophile can be either neutral or

negatively charged. +

Examples of nucleophiles :

Lewis base such as
Anions (OH-, RO-, Cl-, CN- ,etc)
Carbanions (species with a negative

charge on carbon atoms).

EXAMPLE: Nucleophilic Site

H3N●● ●● HO●●↑●●●●– ●●C●●↑●●l●●–
↑
H2O●●
↑

CH2═↑CH2

i)Electrophile ii) Nucleophile

❑ An electron-deficient ❑ An electron-rich species
species
❑ An electron-pair donor
❑ An electron-pair acceptor
❑ Either neutral or negatively
❑ Either neutral or positively charged species
charged species.
❑ Greek word means ‘nucleus
❑ Greek word means loving’.
‘electron loving’.