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I know that conc $\ce{H2SO4}$ is dehydrating agent. And the most easiest mechanism example would be this, mechanism

But ,I want to know how to find theextent of dehydration of it. I came across this question question

And answer is d

my thinking- (a) $O$ is EN and will pull $-OH$ towards itself inductively, thus decreasing the electron density on $-OH$ ,and decreasing its power to pull $H$ from conc $\ce{H2SO4}$.

Similarly for (b) and (c) and I think due inductivity (c) will have more chances than (b)

As for the (d) I think $-CH3$ group is a pusher so it should increase the electron density, and hence reducing the effect on $-OH$ group .

1)but my doubt is ,is this the only reason for the extent of dehydration by conc $\ce{H2SO4}$ on (d) to be more than (c)

2) I don't think there would be much difference if it tautomerises to enol form ?

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  • $\begingroup$ The extent of dehydration would relate to how much hydrate the starting material could potentialy form with the sulfuric acid? $\endgroup$ – Technetium Oct 28 '15 at 13:53
  • $\begingroup$ Could you please tell in more detail? $\endgroup$ – shaistha Oct 28 '15 at 14:40
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    $\begingroup$ You should think about the relative stabilities of the products. Which has the most stable dehydration product and why? $\endgroup$ – bon Oct 28 '15 at 15:13
  • $\begingroup$ I'll give you two more hints. (1) You must think in 3D. The ring on the molecule isn't flat. (2) a,b and c have OH moved relative to C=O group. The OH would have the most effect "meta" or "ortho" to the C=O, so b is very likely out off the bat. $\endgroup$ – MaxW Oct 28 '15 at 19:03
  • $\begingroup$ @MaxW thank you, it kind of helped. But I want to know why d over c? Is the reason what I have already written or something else. If I'm wrong, then please tell me where I'm wrong or I'm missing here. $\endgroup$ – shaistha Oct 28 '15 at 21:24
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The most stable Alkene is the most highly substituted Alkene. (d) gives a tri-substituted alkene, all others give the less disubstituted alkenes. Most stable Alkene (d) therefore more of it is formed! (Edited thanks to Bon for highlighting my error)

Also a factor to keep in mind is conjugation. (a), (b) and (d) will have conjugated double-bonds, while (c) will not. This conjugation also adds stability to the system. But the major stability contributor is the higher substituted alkene in (d).

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  • $\begingroup$ @bon only saw your comment after posting my answer...credit to you... $\endgroup$ – Leeser Oct 29 '15 at 8:50
  • $\begingroup$ No, credit goes to you because you posted an answer ;). However, you meant tri-substituted, didn’t you? $\endgroup$ – Jan Oct 29 '15 at 10:14
  • $\begingroup$ indeed I did..tri substituted it is, others are disubstituted.. $\endgroup$ – Leeser Oct 29 '15 at 11:24

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