11.4) Reactions of Organic Compounds - Part 1

11.4 REACTIONS OF ORGANIC COMPOUNDS

At the end of PART 1, student should be able to:

a) explain covalent bond cleavage

i. homolytic cleavage

ii. heterolytic cleavage

b) differentiate homolytic cleavage and heterolytic

cleavage.

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11.4 REACTIONS OF ORGANIC COMPOUNDS

➢ Reactions of organic compounds involve the forming
and breaking of covalent bonds.

➢ A covalent bond may break in two fundamentally
different ways:

a) Homolytic cleavage

b) Heterolytic cleavage

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a) Homolytic Cleavage

• Occurs in a non-polar bond of two atoms of

similar electronegativity. unpaired
electron

XX X +X

free radicals

• A single bond breaks symmetrically into two
equal parts; leaving each atom with one
unpaired electron, known as a *free radical as
the product.

• Shown by a single headed arrow,

*free radical : an uncharged chemical species that contains
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an unpaired / a single electron

Example 1(a):
Homolytic cleavage in some molecules:

i) Cl Cl uv Cl + Cl

chlorine radicals

HH HH

ii) H C C H HC+ HC

HH H methyl H
H radicals

iii) H

HCH HC +H

H methyl hydrogen
radical H radical 5

b) Heterolytic Cleavage

• Occurs in a polar bond of two atoms with
different electronegativities.

• A single bond breaks unsymmetrically in which
both bonding pair electrons are transferred to
the more electronegative atom thus form a
cation and an anion.

bonding pair e

δ - δ+ A- + B+

AB

anion cation

• Shown by a double headed arrow,

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If the heterolytic cleavage involves carbon atom, either
a carbocation (+ve charge on the carbon atom)
or

a carbanion (–ve charged carbon atom) is formed.

• A molecule in which a carbon atom has a positive

charge and three bonds. CH3

EXAMPLE: H3C C+

CH3

• Formerly known as carbonium ion.
• It can be defined as any even-electron cation that

possesses a significant positive charge on the

carbon atom.

• A carbocation can be formed through heterolytic

cleavage CH3 H3C CH3 Cl-
EXAMPLE:
δ+ δ - C+ +
H3C
C Cl

CH3 CH3

carbocation anion

✓ chlorine is more electronegative than carbon thus the C—Cl
bond is polar.

✓ both bonding pair electrons are transferred to Cl atom.

✓ a carbocation (CH3)3C+ and a chloride, Cl- ion is formed.

• can be defined as an anion in which carbon has an

unshared pair of electrons and bears a negative charge

usually with three substituents for a total of eight valence

electrons H3C CH3 +

CH3 C- + Li
 - +

C ELXAi MPLE:

CH3 CH3

• A carbanion can be formed through heterolytic
cleavage

EXAMPLE: CH3 H3C CH3 Li+
 - +
H3C - cation
C Li
C+
CH3
CH3

carbanion

✓ carbon is more electronegative than Li thus the
C—Li bond is polar.

✓ both bonding pair electrons are transferred to
carbon atom.

✓ a carbanion (CH3)3C- and a lithium, Li+ ion is

formed.

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The differences between homolytic cleavage
and heterolytic cleavage

homolytic cleavage heterolytic cleavage

Occurs in a non-polar bond Occurs in a polar bond of
of two atoms of similar two atoms with different
electronegativity. electronegativities.

A single bond breaks A single bond breaks
symmetrically into two equal unsymmetrically in which
parts; leaving each atom with both bonding pair electrons
one unpaired electron, known are transferred to the more
electronegative atom thus
as a free radical as the form a cation and anion.
product.