What is the explanation for the relative stabilities of methyl, primary, secondary, and tertiary carbocations?

1 Answer
Oct 19, 2015

Inductive Effect.


Carbocations become more stable when the electron-deficient carbon becomes less deficient. In other words, it is when the electron density around that carbon atom becomes higher.

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Image source: Organic Chemistry-Janice Gorzynski Smith 3rd Ed

If a carbocation is connected to electron-donating groups, these groups will pump electrons to this carbon atom to compensate for the loss of the electron density around it.

Alkyl groups in general are electron-donating groups by inductive effect. Inductive effect by definition is the transfer of electrons through #sigma# bonds.

Therefore, the higher the number of alkyl groups (or in this case methyl groups) connected to the carbocation, the more stable it becomes, therefore, we can rank the stability as follows:

#(CH_3)_3C^+ > (CH_3)_2CH^+ > CH_3CH_2^+ > CH_3^+#