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Grade: 12th pass
        Why carbocation is more stable than carbanion? Generally in alkenes undergoes electrophilic addition reaction but not nucleophilic addition, why?
one year ago

Answers : (1)

dolly bhatia
200 Points
							Carbocation is more stable than carbanions. Let us take for example tert-butyl carbocation (CH3)3-C+. This charged ion is highly stabilized due to presence of the three donor methyl groups that donate electrons and hence largely stabilizes positive charge. The tert-butyl carbanion (CH3)3-C- would be less stabilized and more reactive than its positive cation since negatively charged carbon atom in carbanion is sp3 hybridized. Thus, it has a tetrahedral structure/ three of the four sp3 hybridized orbital’s form 3s-bonds with hydrogen or carbon atom of alkyl group. Fourth sp3 hybrid orbital contains the lone pair of electrons. This would cause to bring reactivity to this ion and makes it act as a free radical. When it comes to carbocations, they are presented with sp2 hybridized carbons, having a planar geometry. Their stability can be explained through hyperconjugation effects, where alkyl substituents act as weak electron donors, lowering positive charge at the carbon. The lower the positive charge, the more favourable the structure and consequently, larger the number of methyl groups makes the carbocation more stable. The same way of thinking could be applied to carbanions too. Electron donation increases the charge at the carbon, which makes the structure less favourable, hence more substituents lead to a less stable structure. But, there is also another way of thought that could be applied, taking into account the geometry.Carbanions are structures where a carbon atom has three substituents and one unshared electron pair. In terms of hybrid orbitals, it can be assumed that carbon is sp3 hybridized. This corresponds to tetrahedral geometry. The effective volume of substituents causes deviations from ideal tetrahedral structure. An unshared electron pair is ‘situated’ close to nucleus since it only belongs to one atom. When a bond is formed, the electrons occupy a sigma orbital and highest electron density is between the two nuclei, that is – farther from carbon atom compared to unshared electron pair. Since lowest energy structure will be the one with least repulsion, it means that bringing sigma bonds closer to each other would be more favourable than doing the same to bonding and unshared electrons.Angle alpha between unshared electron pair and sigma bonds is higher than angle beta between two sigma bonds.Now, what happens when substituent is different? A hydrogen atom is smallest one possible. But, say, a methyl group is bulkier. If geometry of carbanion was to be the same, there would be unfavourable interaction between methyl groups.  Electrophilic addition to alkenes starts with the pi electrons attacking an electrophile, forming a carbocation on most stable carbon. A nucleophile then attacks the carbocation to form product. Electrophilic addition adds to give the Markovnikov Product, with nucleophile added to the more highly substituted carbon. This is because carbocation intermediate is significantly stabilized by alkyl substituents.During electrophilic addition to alkenes, formation of carbocation on most highly substituted carbon takes place followed by attack of chloride on the carbocation to give addition product.
						
one year ago
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