Well I was doing some questions and I have a bit of confusion on dehydration reaction. For instance, consider following reaction:

Acid dehydration of an alcohol

How do I decide which adjacent hydrogen gets attracted by $\ce{HSO4-}$?

Possible mechanism-1

now we have 3 possibilities:

Possible mechanism-2

either hydrogen 1,2, or 3 can leave resulting in 3 different products. So my question is how do I identify the major product in this case?

  • $\begingroup$ By analysing the stability of each product and comparing them by using comparative methods, such as comparison through hyperconjugation in the final products. $\endgroup$
    – Rishi
    Commented May 23, 2021 at 16:05
  • $\begingroup$ Heating gives you the thermodynamically most stable product as the major product. Which is the most stable among three you are proposing? $\endgroup$ Commented May 23, 2021 at 16:08
  • $\begingroup$ I suspect that ring expansion is also possible. $\endgroup$ Commented May 23, 2021 at 16:10
  • $\begingroup$ I think it's exercise in futility - go ahead and try it in lab, or find a paper - pretty sure at least two or three products could be the major depending on how you conduct the reaction. $\endgroup$
    – Mithoron
    Commented May 23, 2021 at 18:51

1 Answer 1


The mechanism of dehydration of 2° alcohol at high temperature (around 170°C-180°C) happens via $E_1$ mechanism. This means that the major product will be decided by the $\mathbf {Zaitsev}$ $\mathbf{rule}$, which says that major product will be the stables one. If you compare the three alkenes, you'll see that the middle compound you've drawn has the most hyperconjugative structures, and hence is the stablest. Therefore, that will be the major product.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.