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Does alcoholic $\ce{KOH}$ only show $E_2$ mechanism with alkyhalides?

I have studied with two organic chemistry teachers, one told me that there is always $E_2$ mechanism, except in the case of tertiary alkylhalide and the other told me it can be any depending on the substrate with $\alpha$ hydrogens more than 4 or if it has resonance it will show $E_1$.

Which one is true?

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    $\begingroup$ This seems like a homework question. We ‎have a policy which states that you should show your thoughts and/or efforts into solving the ‎problem. It'll make us certain that we aren't doing your homework for you. Otherwise, this ‎question may get closed. Also, please put some more effort into your formatting and spelling. It doesn't have to be perfect but I'm sure you can do better than that. $\endgroup$ – bon Sep 11 '15 at 18:24
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    $\begingroup$ A "homework question" is any question whose value lies in helping you understand the method by which the question can be solved, rather than getting the answer itself. This includes not just questions from actual homework assignments, but also self-study problems, puzzles, etc. $\endgroup$ – bon Sep 11 '15 at 19:28
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    $\begingroup$ We are not being abusive in any way. We are enacting the policies of the site in a civil manner. And trust me, there are many people on this site who know the answer to this question and many more, which is why it will profit you to show some thought towards you question. $\endgroup$ – bon Sep 11 '15 at 21:21
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    $\begingroup$ If you don't like the policies of our site then feel free to go somewhere else. If you stay here though, you must play by our rules. If you edit your thoughts into the question and tell us what is confusing you then the question will be reopened and you will get an answer. $\endgroup$ – bon Sep 11 '15 at 21:39
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    $\begingroup$ @Abhishek - I can see you have edited in your thoughts - I have voted for reopening. Having said that, it would have been done faster if you had followed the rules of the site, as per bon's advice, without the attitude. $\endgroup$ – user15489 Sep 11 '15 at 21:50
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With a strong base $E2$ will always predominate over $E1$.

Both elimination mechanisms have a preference for tertiary substrates over secondary substrates over primary substrates. This behavior is on contrast with the substitution reactions, where steric factors are important for $S_N2$. Steric factors are less important for the proton transfer necessary for $E2$.

Since $E2$ and $E1$ have the same substrate preference order, you may feel that you cannot decide which is more likely. However, $E2$ is usually faster because $E2$ is bimolecular. Consider the rate laws:

  • $\mathrm{rate}_{E2}=k\mathrm{[substrate][base]}$
  • $\mathrm{rate}_{E1}=k\mathrm{[substrate]}$

If we make the assumption that the rate constants are of similar magnitude, then increasing the concentration of the base increases the rate of $E2$.

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  • $\begingroup$ E2 does have a stereochemical requirement though. The leaving group must be antiperiplanar to the hydrogen. $\endgroup$ – bon Sep 12 '15 at 15:20
  • $\begingroup$ Means it will mainly show E2 mechanism.... $\endgroup$ – Abhishek Sep 12 '15 at 21:27

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