Consider the compounds, (I) - 1,2-diphenylethene and (II) - 1-phenylpropene. Compare their rate of hydrohalogenation (HX addition)

enter image description here

I feel that the answer should be (II) > (I), but to my surprise it's given to be (I) > (II).

My thoughts:

  1. In (I), phenyl group consists of sp2 carbons, operates -I effect and destabilises the carbocation intermediate.

  2. In (II), protonation should occur at the carbon directly attached to the phenyl group. The methyl group (instead of phenyl, as in (I)) operates +I effect and stabilises the carbocation.

  3. In both cases, the carbocation formed enjoys resonance stabilisation from a phenyl group - so it's not a factor we need to compare.

So, what is the real order of rates of $\ce{HX}$ addition? Is my reasoning correct, or have I missed something?


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.