Saved Bookmarks
| 1. |
Give a detailed account on homolytic and heterolytic cleavage . |
|
Answer» Solution :Homolyticcleavage : Homolytic cleavage is the process in which a covalent bonded atoms retains one electron . It is denotedby a half headed arrow (fish hook arrow ) . This type of cleavage occurs under high temperature or in the presence of UV lightin 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 reagents that promote holmolytic cleavage in substrate are called as free radical initiators . For example AZOBISISOBUTYRONITRILE (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 - Cbonds to form alkyl free radicals . Thestability of alkyl free radicals is in the following order ` C (CH_(3))_(3) gt CH(CH_(3))_(2) gt CH_(2) CH_(3) gt CH_(3)` Heterolytic cleavage : 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 formations 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 towardsthe more electronegative atom For example, in tert-butyl bromide, the C-Br bond is polar as bromine is more electronegative then carbon . The bonding electrons of the C-Br bond are attracted more by bromine then 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 -hydrogen from the aldehyde which leads to the formation of the below mentioned carbanion . 
|
|