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In the following reaction, treating the trans-1-bromo-2-phenylcyclopentane with ethanolic KOH gives an alkene. But in this case, the double bond is on the left side (less substituted) instead of being the more substituted one. enter image description here

But why does this happen? Zaitsev's rule favours the highly substituted alkene. Even if a carbocation is formed, a hydride shift would have landed it on the carbon with the phenyl group, as the carbocation would be stabilised by resonance. Does the trans configuration forbid such reaction?

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There are three common elimination mechanisms: $\mathrm{E1, E2}$ and $\mathrm{E1_{cb}}$. $\mathrm{E1}$ and $\mathrm{E1_{cb}}$ both imply that an ionic intermediate is formed first; $\mathrm{E1}$ implies the same carbocationic intermediate as in an $\mathrm{S_N1}$ reaction. The $\mathrm{cb}$ in $\mathrm{E1_{cb}}$ means conjugate base, i.e. the first step is deprotonation of the compound to give an carbanionic intermediate.

$\mathrm{E1}$ reactions require good leaving groups and a moderately stabilised carbocation; this is not the case in your reactant.

$\mathrm{E1_{cb}}$ requires a very strong base and an at least mildly stabilised anion; this is not the case with your reagent and reactant.

Thus, the mechanism must be according to $\mathrm{E2}$, where the abstraction of a proton and the displacement of the leaving group happen at the same time. $\mathrm{E2}$ mechanisms require a $180^\circ$ anti-periplanar configuration of the $\ce{Br-C-C-H}$ bonds for easier attack and favour those double bonds where the hydrogen is less stericly hindered. Additionally, your reactant is trans-configured, thus meaning that there is no available proton on the higher-substituted carbon that can participate in elimination. Altogether, this leads to $\mathrm{E2}$ eliminations generally following Hofmann’s rule rather than Zaitsev’s rule. Hofmann initially observed what later became the Hofmann rule in the Hofmann elimination, which follows an $\mathrm{E2}$ mechanism.

As the ethanolic $\ce{KOH}$ is not strong enough to isomerise the double bond, the Hofmann product will be observed.

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  • $\begingroup$ The carbocation formed by releasing $\ce{Br^-}$ ion will be a secondary carbocation. So how can we say that it will not be a stable one, as the relative order of stability is tertiary>secondary>primary (ignoring resonance effects, if any, of the benzene ring)? $\endgroup$ – Shoubhik Raj Maiti Dec 3 '16 at 13:01
  • $\begingroup$ @ShoubhikRajMaiti Assume only tertiary and benzylic (maybe allylic) carbocations to be stable enough to be formed. Everything else would immediately undergo Wagner-Meerwein rearrangements. $\endgroup$ – Jan Dec 3 '16 at 13:02
  • $\begingroup$ Well, if there is a carbocation formed, and there is a rearrangement, then, the charge will be shifted to the attaching point of the benzene and cyclopentane ring, as it gives maximum stability(tertiary and also resonance). Then the products will also change. $\endgroup$ – Shoubhik Raj Maiti Dec 3 '16 at 13:07
  • $\begingroup$ @ShoubhikRajMaiti But that is not observed since the reaction is much slower than the one by E2. $\endgroup$ – Jan Dec 3 '16 at 13:08
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The product structure is wrong, the Br should be gone.

There's no carbokation here, this is a reaction with E2-mechanism, which works best if the abstracted hydrogen is at a dihedral angle of 180° to the leaving group.

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  • $\begingroup$ Then the choice of product between 3-Phenylcyclopentene and 1-Phenylcyclopentene would depend upon the diastereomer, right? I was thinking about an E1-mechanism here... $\endgroup$ – Eashaan Godbole Nov 13 '16 at 17:24
  • $\begingroup$ Yes, if you use the cis compound you will get the double bond on the other position. $\endgroup$ – DSVA Nov 13 '16 at 17:41
  • $\begingroup$ The update in the figure in the question clears all doubts. $\endgroup$ – Eashaan Godbole Nov 13 '16 at 17:42
  • $\begingroup$ @DSVA A more elaborate answer would have been helpful. $\endgroup$ – Shoubhik Raj Maiti Dec 3 '16 at 13:02

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