Fission of a covalent bond
All organic molecules contain covalent bonds which are formed by the mutual sharing of electrons between atoms. These covalent bonds break in two different ways, namely homolytic cleavage (symmetrical splitting) and heterolytic cleavage (unsymmetrical splitting). The cleavage of a bond in the substrate is influenced by the nature of the reagent (attacking agent).
Homolytic cleavage is the process in which a covalent bond breaks symmetrically in such way that each of the bonded atoms retains one electron. It is denoted by a half headed arrow (fish hook arrow). This type of cleavage occurs under high temperature or in the presence of UV light in a compound containing non polar covalent bond formed between atoms of similar electronegativity. In such molecules, the cleavage of bonds results into free radicals. They are short lived and are highly reactive. The type of reagents that promote holmolytic cleavage in substrate are called as free radical initiators. For example Azobisisobutyronitile (AIBN) and peroxides such as benzoyl peroxide are used as free radical initiators in polymerisation reactions.
As a free radical with an unpaired electron is neutral and unstable, it has a tendency to gain an electron to attain stability. Organic reactions involve homolytic fission of C- C bonds to form alkyl free radicals. The stability of alkyl free radicals is in the following order
˙C(CH3)3 > ˙CH(CH3)2>˙CH2CH3 >˙CH3
Heterolytic cleavage is the process in which a covalent bond breaks unsymmetrically such that one of the bonded atoms retains the bond pair of electrons. It results in the formation of a cation and an anion. Of the two bonded atoms, the most electronegative atom becomes the anion and the other atom becomes the cation. The cleavage is denoted by a curved arrow pointing towards the more electronegative atom.
For example, in tert-butyl bromide, the C-Br bond is polar as bromine is more electronegative than carbon. The bonding electrons of the C-Br bond are attracted more by bromine than carbon. Hence, the C-Br undergoes heterolytic cleavage to form a tert-butyl cation during hydrolysis.
Let us consider the cleavage in a carbon-hydrogen (C-H) bond of aldehydes or ketones We know that the carbon is more electronegative than hydrogen and hence the heterolytic cleavage of C-H bonds results in the formation of carbanion (carbon bears a negative charge). For example in aldol condensation the OH- ion abstracts a α-hydrogen from the aldehyde, which leads to the formation of the below mentioned carbanion.
In a carbocation, the carbon bearing positive charge is sp2 hybridised and hence it has a planar structure. In the reaction involving such a carbocation, the attack of a negatively charged species (nucleophiles) take place on either side of the carbocation as shown below.
The carbanions are generally pyramidal in shape and the lone pair occupies one of the sp3 hybridised orbitals. An alkyl free radical may be either pyramidal or planar.
The relative stability of the alkyl carbocations and carbanions are given below.
Relative stability carbocations.
+C(CH3)3 > +CH(CH3)2 > +CH2CH3 > +CH3
relative stability of carbanions
–C(CH3)3 < –CH(CH3)2 < –CH2CH3 < –CH3
The energy required to bring about homolytic splitting is greater than that of heterolytic splitting.
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