[Mike Dacre]

I just got the following question from the MCAT question a day site:

In each molecule below, a carbocation is formed when the alcohol group is removed. Which of the following best describes the stabilities of the carbocations?



A   The carbocation of Alcohol A is more stable than the carbocation of Alcohol B since it is non-resonance stabilized
B   The carbocation of Alcohol A is less stable than the carbocation of Alcohol B since it is resonance de-stabilized
C   The carbocation of Alcohol B is more stable than the carbocation of Alcohol A since it is a resonance stabilized primary carbocation
D   The carbocation of Alcohol B is less stable than the carbocation of Alcohol A since it is a resonance stabilized primary carbocation

To me the cation of alcohol A is more stable because it will produce a secondary carbocation. Both alcohols will be resonance stabilized, the only difference is that the cation of alcohol A will be a secondary carbenium, and so will be more stable. Right? Here are the two resonance structures I think will happen:

Alcohol A:


Alcohol B:


The 'correct' answer to this question was A: "The carbocation of Alcohol A is more stable than the carbocation of Alcohol B since it is non-resonance stabilized", but I think that this is nonsense for two reasons:

• Resonance stabilizes carbocations, it does not destabilize them.
• Both alcohols will have the same resonance stabilization anyway

So I want feedback from actual chemists; am I missing something? Am I wrong about either of my proposed resonance structures? Am I wrong that carbocations are stabilized by resonance?

The original question is here: http://www.mcatquestion.com/evaluateemail2015.php?arg1=2561&arg2=4

Here is a screenshot of that page:

 


Thanks in advance for any help

[mjc123]

I can only think that resonance stabilisation such as you have drawn would break the conjugation between the benzene rings, so you would lose resonance stabilisation as well as gaining it. But what the balance is I couldn't say.