Number of carbocations from the following that are not stabilized by hyperconjugation is . [figure]
Answer
5
Solution
Hyperconjugation needs C–H σ bonds on the carbon attached to C+ (α-H on an sp³ carbon).
1st: (CH3)2 +C-CH2CH3 has 8 α-H ⇒ stabilized
2nd: both α-carbons of the tert-butyl groups are quaternary (no α-H) ⇒ not
3rd: +CH3 has no α-carbon ⇒ not
4th: ring α-carbons are sp² (their C–H lie in the ring plane) ⇒ not
5th: +CH2-OCH3 – α-atom is O (resonance, not hyperconjugation) ⇒ not
6th: (CH3)3C+ has 9 α-H ⇒ stabilized
7th: (CH3)2N- +CH2 – α-atom is N (resonance) ⇒ not
∴ not stabilized by hyperconjugation = 5
Answer: 5