I haven't been able to find anything concrete or consistent on this topic. For the following reactions, which product would be favored, assuming that they both have to go through a carbocation intermediate?

What I've gathered so far: The electron withdrawing group (-NO2) would destabilize the benzylic carbocation (through loss of electron density for stabilization), and would at least help push the reaction in the direction of the tertiary carbocation.

Thanks for any and all help.Reactions


In the first instance, bromine will attach itself on the benzylic carbon as the benzylic carbocation formed in this case would be more stable than than the tertiary carbocation due to the mesomeric effect.

However in the second case, the benzylic carbonation formed would be destabilized due to the -M effect of the nitro group, like you pointed out. So bromine would favor the the tertiary carbocation which in this case would be more stable due to hyperconjugation and inductive effects.

  • $\begingroup$ First off, thanks so much for the answer! As a follow-up question: as a general rule, are secondary benzylic carbocations just more stable than regular tertiary carbocations? $\endgroup$ – Bob Apr 16 '18 at 2:46
  • $\begingroup$ Unless there is an electron withdrawing group causing instability, yes. However exceptions are always there - the tertiary butly carbonation is even more stable than benzylic carbonation. $\endgroup$ – Siddharth Porwal Apr 16 '18 at 2:49

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