carbocation stability

[sweetpea2]

Ok, ive asked this a few times, but different practice tests keep saying different things. Im looking at the free kaplan DAT test and the question is which carbonium ion will be most stable.

Answer says Ph3C+ which is stable but......

Another choice is Ch2=CHCh2+ Thought this should be the answer due to special stability of allylic carbocations.

-Isnt stability in this order Benzylic carbocation, allylic, tert, secondary, etc. And phenyl groups are better at stabilizing than methyl.

Anyone? Thanks

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[DentalDeac]

Ok, ive asked this a few times, but different practice tests keep saying different things. Im looking at the free kaplan DAT test and the question is which carbonium ion will be most stable.

Answer says Ph3C+ which is stable but......

Another choice is Ch2=CHCh2+ Thought this should be the answer due to special stability of allylic carbocations.

-Isnt stability in this order Benzylic carbocation, allylic, tert, secondary, etc. And phenyl groups are better at stabilizing than methyl.

Anyone? Thanks

In the first choice, the carbocation is attached to 3 phenyl groups, making it a tertiary carbocation, which is preferrable. In the second choice, the carbocation is only directly attached to one other carbon, making it primary, and not very stable.

 

[drgreen]

Ph3C+ is extremely stable because it is a tertiary c+ but mainly because adjacent to the c+ are 3 phenyl groups (which stabilize the c+ through resonance of their double bonds). as you mentioned benzylic carbocations are the most stable, here we have 3.

 

[sweetpea2]

But isnt allylic still even more stable than the Ph3C+

 

[Mstoothlady2012]

But isnt allylic still even more stable than the Ph3C+

no just like you stated the order...benyzllic > allylic.

In the question there are 3 phenyl groups attach, which makes it the most stable carbocation.

 

[harrygt]

Ok, ive asked this a few times, but different practice tests keep saying different things. Im looking at the free kaplan DAT test and the question is which carbonium ion will be most stable.

Answer says Ph3C+ which is stable but......

Another choice is Ch2=CHCh2+ Thought this should be the answer due to special stability of allylic carbocations.

-Isnt stability in this order Benzylic carbocation, allylic, tert, secondary, etc. And phenyl groups are better at stabilizing than methyl.

Anyone? Thanks

carbocation stability is Benzylic > tert > allylic > sec > prim
radical stability is Benzylic > allylic > tert > sec > prim

I hope this helps.