Understanding Benzylic and Allylic Carbocations: Can You Help?

[chaoseverlasting]

Our teacher was very curt with us in class, and I am not sure if the Benzylic carbocation is more stable than the allylic one or not. Can anyone help me?

 

[chaoseverlasting]

I think the benzylic one is more stable due to the resonance of the positive charge present on it, but the allylic one also undergoes resonance.

The + charge on the benzylic one is delocalised over the benzene ring where as the + charge on the allylic c+ is only delocalised over 3 carbon atoms. So I think the benzylic one is more stable, but I am not sure... Any help would be greatly appreciated...

 

[Gokul43201]

That's a pretty good guess!

Since hypeconjugation is not in the picture, and the inductive contribution to carbocation stability is typically weaker than the resonance contribution, we can limit ourselves to this last aspect. How many resonance structures contribute to the hybrid in say, the benzyl carbocation (ph-CH2(+)) as opposed to the allyl carbocation (CH2=CH-CH2(+))? As a rule of thumb, the greater the number of resonance contributions, the greater the stability of the carbocation.

Note, however, that the following statement is not correct:

The + charge on the benzylic one is delocalised over the benzene ring where as the + charge on the allylic c+ is only delocalised over 3 carbon atoms.

You will see what's wrong when you draw the resonance structures. For instance, is there a structure where the positive charge occupies the meta position in the benzylic carbocation?

 

[chemisttree]

Don't forget that in addition to the delocalization of the positive charge over the carbons in this aromatic benzylic system, there is the potential that the benzylic cation can expand to the aromatic seven member ring "tropylium" ion. As a tropylium ion, the delocalization can actually be rationalized over all seven carbons! Destroy the symmetry by substituting something on the ring and all bets are off...

The Chaoseverlasting question is, "What is more reactive, benzyl cation or allyl cation?"

Allyl be awatitin for your answer...

 

[Gokul43201]

Destroy the symmetry by substituting something on the ring and all bets are off...

Nice. If I may, allyl be asking you a quickie. Where would you put your money, if you had a tertiary benzylic carbocation?

 

[chaoseverlasting]

I drew 4 canonical structures for the ph-CH2+ carbocation and only 2 for CH2=CH-CH2+, and the meta position does not get the positive charge, I overlooked that. Only the 2 ortho positions and the para position get the positive charge on them, so the +ive charge is delocalised over 4 carbon atoms (2 ortho, 1 para and the CH2+).

On the CH2=CH-CH2+, there were only two structures but they were both equivalent... It only exists in one form and undergoes resonance... Does that make it more stable?

 

[chaoseverlasting]

In a tertiary benzylic c+, there would be 6 alpha hydrogens through which hyperconjugation would be possible, in that case, the benzylic c+ would be more stable as there is no hyperconjugation in the allylic c+.

 

[Gokul43201]

I drew 4 canonical structures for the ph-CH2+ carbocation and only 2 for CH2=CH-CH2+, and the meta position does not get the positive charge, I overlooked that. Only the 2 ortho positions and the para position get the positive charge on them, so the +ive charge is delocalised over 4 carbon atoms (2 ortho, 1 para and the CH2+).

On the CH2=CH-CH2+, there were only two structures but they were both equivalent... It only exists in one form and undergoes resonance... Does that make it more stable?

As a rule of thumb, the greater the number of resonance contributions, the greater the stability of the carbocation.

Alternatively, you can use the argument you used before about delocalization of the positive charge. The greater the number of positions over which you can spread the positive charge, the greater the stability.

 

[chaoseverlasting]

Thank you, that would mean that the benzylic one is more stable, right?