Kaplan Online Test - Ochem #74 ..help please

[STACM]

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In this following question below, why isnt the asnwer (D), where the carbocation is resonance stabilized versus being on a tertiary carbon? I thought resonance stabilization is always more stable than even the most substituted carbon?

Which of the following carbonium ions will be the most stable?

(A) Ph3C+
(B) CH3CH2+
(C) (CH3)2CH+
(D) CH2=CHCH2+
(E) +CH2Cl

 

[Simiam]

What does it say the answer is? A?

 

[DentalDeac]

I have seen this question like 6 times this week. A tertiary phenyl carbocation is always the most stable.

 

[STACM]

well even if a tertiary carbocation is more stable, wouldnt the conjugation at least be more stable than the secondary carbocation? answer says seconday wins over conjugation too...so i dont get it, i could have sworn conjugation was more stable than something

 

[klutzy1987]

D is just an allylic position. Choice A is 3 allylic positions. (really they are benzylic but thats basically the same thing. To boot it is tertiary. A tertiary carbocation that is in a tri benzylic position is way more stable than any of the others.

 

[STACM]

D is just an allylic position. Choice A is 3 allylic positions. (really they are benzylic but thats basically the same thing. To boot it is tertiary. A tertiary carbocation that is in a tri benzylic position is way more stable than any of the others.

oh ok that makes sense, what about C vs. D