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Question

What is the observed order of the stability of the carbocation -(CH3)3C+>,2CH+,CH3CH2+andCH3+


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Solution

Carbocation

  • A carbocation (formerly called carbonium ion) is a cationic reaction intermediate species with a carbon atom that bears a positive charge and is two electrons short for completing its octet.

Stability order of the carbocation

  • The order of the stability can be explained on the basis of the positive inductive effect and hyperconjugation effect.

[Note: Positive inductive effect is the electron releasing nature of the groups or atoms and is denoted by +I. Hyperconjugation is the delocalization of electrons by the overlap of a σ bond orbital with a πor p-orbital.]

Positive inductive effect

  • According to the positive inductive effect, the alkyl groups show an electron-releasing inductive effect (+I effect) in the direction of the positive charge in a carbocation.
  • Through the +I effect, an alkyl group disperses the positive charge and thus stabilizes the carbocation.
  • The greater the number of alkyl groups attached to the positive carbon, the greater the dispersal of the charge and consequently, the greater the stability of the carbocation.
  • Thus, the t-butyl cation with three methyl groups attached to the positive carbon would be more stable than the isopropyl cation with two methyl groups.
  • Likewise, the isopropyl cation with two methyl groups would be more stable than the ethyl group with only one methyl group.
  • The methyl cation with no alkyl group attached to the positive carbon would be the least stable among them.

Hyperconjugation effect

  • According to the hyperconjugation effect, the greater the number of α-hydrogens attached to ht positive carbon, the greater will be the stabilization of the carbocation.
  • For carbocations, the number of α-hydrogens and the number of hyper conjugative structures decreases in the order:3°>2°>1°>CH3+
  • So the order of stabilization will decrease in the same order.
  • Therefore, the observed order of stability of the carbocation is (CH3)3C+>(CH3)2CH+>CH3CH2+>CH3+

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