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
    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? $\endgroup$ May 23, 2021 at 16:08
  • $\begingroup$ I suspect that ring expansion is also possible. $\endgroup$ 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
    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.


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