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Which of the following represents the correct order of stability of carbocations?

1. CH3-CH2+ < CH3-CH+-CH3 < (CH3)3C+
2. CH3-CH2+ > CH3-CH+-CH3 > (CH3)3C+
3. CH3-CH2+ < (CH3)3C+ < CH3-CH+-CH3
4. CH3-CH+-CH3 < CH3-CH2+ < (CH3)3C+

Correct answer: CH3-CH2+ < CH3-CH+-CH3 < (CH3)3C+

Solution

CH3-CH2+ is primary (1 alkyl group), CH3-CH+-CH3 is secondary (2 alkyl groups), (CH3)3C+ is tertiary (3 alkyl groups). Alkyl groups donate electron density and stabilise the positive charge, so stability increases: 1° < 2° < 3°.

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