Question

How to find the most easily dehydrated compound in organic chemistry?

Answer 1

Rate of dehydration when given compounds are treated with conc. H2SO4HX2SOX4 is



According to me the answer should be R>Q>S>PR>Q>S>P but my textbook solution gives the answer as R > Q > P > S (Sadly, there is no explanation).

Dehydration of secondary & tertiary alcohols in the presence of conc. H2SO4HX2SOX4 proceed through E1 mechanism whereas most primary alcohols proceed through E2 mechanism.

In the molecular R, the carbocation formed after the alcohol group leaves the molecule after protonation is a tertiary carbocation and not just that, it is resonance stabilized as well.

In the molecule Q, you get a secondary carbocation and this is resonance stabilized as well.

In molecule S, you get a secondary carbocation but there is no resonance stabilization.

In molecule P, the dehydration goes via E2 mechanism and there is a bulky cyclohexene group attached to the αα

If the question did not have molecule P, then its pretty much obvious that the answer is R > Q > S but with molecule P, I am not quite sure where to place it.

I thought that P will be least reactive since it proceeds via E2 mechanism and there is one bulky group attached to the carbon.

So how do I come up with a concrete answer to justify why molecule P should react faster than molecule S but slower than molecule Q.