Which of the following carbocation is most stable? A. (CH3)3C.C+H2  B. (CH3)3C+ C. CH3CH2C+H2 D. CH3C+HCH2CH3

Stability of a Carbocation

The number of alkyl groups (also known as alkyl substituents) that are joined to the positively charged carbon atom in a carbocation is an important indicator of its stability. The concept is known as the "alkyl group effect" or "hyperconjugation effect." The carbocation becomes more stable as more alkyl groups are attached to it.

Solution

Which of the following carbocation is most stable?

1. (CH₃)₃C.C⁺H₂
2. (CH₃)₃C⁺
3. CH₃CH₂C⁺H₂
4. CH₃C⁺HCH₂CH₃

The most stable carbocation is (CH₃)₃C⁺. The stability of a carbocation depends on the number of alkyl groups attached to the positively charged carbon atom. The greater the number of alkyl groups, the more stable the carbocation.

• In option (A), there are three methyl (CH₃) groups attached to the carbocation carbon with a total of three alkyl groups.
• In option (B), there are three methyl (CH₃) groups attached to the carbocation carbon, just like in option (A). However, option (B) lacks an additional hydrogen atom (H₂). Since hydrogen is less bulky than alkyl groups, its absence leads to greater stability.
• In option (C), there is only one methyl group attached to the carbocation carbon, making it less stable than options (A) and (B).
• In option (D), there is only one methyl group attached to the carbocation carbon, similar to option (C), and an ethyl (CH₂CH₃) group. Although an ethyl group is larger than a methyl group, it still provides less stability compared to the three methyl groups in options (A) and (B).

Summary

Which of the following carbocation is most stable? A. (CH₃)₃C.C⁺H₂ B. (CH₃)₃C⁺C. CH₃CH₂C⁺H₂D. CH₃C⁺HCH₂CH₃

The most stable carbocation is (CH₃)₃C⁺due to hyperconjugation. The electrons from a nearby σ-bond delocalize into the vacant p orbital of the carbocation, which results in hyperconjugation. The stability of the carbocation is raised due to the stabilising effects of this electron delocalization on the positive charge. Carbocations that have more carbon atoms adjacent to one another tend to be more stable.

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