[dhruv8890]

Hi,

I'm a Georgia Tech student in Orgo 2. I have to take the ACS Organic Chemistry test and I'm studying from the ACS study guide.

There's a question on it that goes:

Which carbocation is the most stable?

A)   CH3---Ph---+CH--CH3

B)   CH3---Ph---CH2--+CH2

C)   +CH2---Ph---CH2---CH3

D)   CH3---+C6H3---CH2---CH3

where Ph = Phenyl group

*In the last one, the positive charge is on one of the phenyl carbons.

I thought it should be D, because the phenyl carbon is a tertiary carbon with 3 carbons attached to it, but the book tells me its A, a secondary cabon with only 2 carbons attached, and a tertiary carbon will stabilize a positive charge better, right?

Is there something special about the phenyl carbon that destabilizes it to a much greater degree if a Hydrogen is taken off?

Thank you,
Dhruv

[orgoclear]

If a positive charge is on the phenyl group itself, then will the carbocation be stabilised. (will resonance be possible) . Further, have you read about vinylic carbocations. This one is also a type of vinylic carbocation which makes it exceptionally less stable

(a) though it is secondary, it is a benzylic carbocation which is stabilized by resonance effect of the benzene ring. This confers it a lot more stability.

(c) it is primary as well as benzylic.

So obviously (a) is most stable

[dhruv8890]

Thank you.. I'll read up on vinylic carbocations.. Resonance isn't possible and that's why the positive charge on the phenyl group destabilizes the molecule more.. got it..

thanks..