Answer to Question #84393 in Organic Chemistry for anuj kumar

Question

Explain why a tertiary carbocation is more stable than secondary carbocation which in turn, is more stable than a primary carbocation

Answer 1

All carbocations carry a positive charge on a carbon atom.

In a primary (1) carbocation, the carbon which carries the positive charge is only attached to one other alkyl group R - CH₂⁺

In a secondary (2) carbocation, the carbon with the positive charge is attached to two other alkyl groups which may be the same or different R - ⁺CH - R'

In a tertiary (3) carbocation, the positive carbon atom is attached to three alkyl groups, which may be any combination of same or different R(R^')C⁺- R''

Alkyl groups have positive inductive effect (they "push" electrons away). This means that the alkyl group becomes slightly positive (δ⁺)and the carbon they are attached to becomes negative (δ^-):

CH₃^δ+ - > C^δ-

The more charge is spread out over several atoms the more stable the ion.

primary carbocation CH₃^δ+ - ₊C^δ-

sacondary carbocation CH₃^δ+ - ₊C^2δ- - CH₃ ^δ+

tertiary carbocation CH₃^δ+(CH₃^δ+) ₊C^3δ- - CH₃ ^δ+

The positive inductive effect of the alkyl group is placing more and more negative charge on the positive carbon as we go from primary to secondary to tertiary carbocations. The effect of this is to cut down that positive charge. At the same time, the region around various alkyl groups is becoming positive. The net effect is that the positive charge is being spread out over more and more atoms as we go from primary to secondary to tertairy ion, what makes carbocations more stable: tertairy carbocation is more stable that secondary carbocation, secondary carbocation is more stable than primary carbocation.