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Explain heterolytic cleavage of organic compound (b) Carbanions by examples |
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Answer» Solution :Definition: When a covalent bond breaks in such a way that both the electrons of the covalent bond (i.e. shared pair) are taken away by one the bonded atoms, the mode of bond cleave is called heterolytic fission. After heterolysis, one atom has a sextet electronic structure and a positive charge and the other, a valence octet with at least one lone pair and a negative charge. Heterolytic fission producing carbocation: e.g. Heterolytic CLEAVAGE of bromoethane will give `overset(+)(C )H_(3)` (positive ion) and `B overset(-)(r )` (negative ion).  Both electron of C-Br bond are transfer on Br. So, the non-bonding electron pair, octet and negative charge on Br. A species having carbon atom POSSESSING sextet of electron and +1 positive charge is called a carbocation. This `overset(+)(C )H_(3)` ion is known as a methyl cation or methyl carbonium ion. Positively charged carbon of `overset(+)(C )H_(3)`possessing `sp^(2)` hybridisation  These carbocations have trigonal planar shape because it derived from the overlap of three equivalent C `(sp^(2))` hybridised orbitals with is orbital of each of the three hydrogen atoms. Each bond may be represented as `C (sp^(2))`, H(IS) sigma bond. The REMAINING carbon orbital is perpendicular to the molecular PLANE and contains on electron. (b) Heterolytic fission producing carbanion: The heterolytic cleavage can also give species in which carbon gets the shared pari of electron and obtain negative charge containing species. e.g., When group Z attached to the carbon leaves without electron pair, the methyl anion `(H_(3)C^(-))` is formed  Carbon species carrying a negative charge on carbon atom is called carbanion. On carbanion carbon the octet and non-bonding `e^(-)` pair present and it possessing `sp^(2)` hydridisation, so its shape is tetrahedral  Carbanions are also unstable and reactive species. (c ) Heterolytic cleavage and reaction mechanism: The organic reactions which proceed through heterolytic bond cleavage are called ionic or heteropolar or just polar reactions |
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