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I was doing a question from a practice exercise on the topic of elimination reactions and alkene synthesis. It is shown below:

enter image description here

I predicted that the product would be 1,2-diiodo-4-methylcyclohexane because I thought the reaction conditions favoured substition with iodide via the $\ce {S_N2}$ mechanism. Firstly, iodide is a rather good nucleophile and secondly, the substrate is a secondary alkyl halide. Even if it was not $\ce {S_N2}$, I would expect perhaps $\ce {S_N1}$ since iodide is a rather weak base and the conditions do not seem to favour elimination. However, the answer provided is very interestingly, 4-methylcyclohexene. Would someone kindly enlighten me on where I went wrong?

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  • $\begingroup$ I think they should have mentioned heat in the question. 1,2-diiodo when heated undergo elimination to form alkenes. $\endgroup$ – FreakyLearner Apr 14 '18 at 7:15
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Dehalogenation of vicinal dihalides by iodide ion occurs stereospecifically and concertedly via an $\mathrm{E2}$ mechanism. An anti-coplanar arrangement of both bromine atoms is required (see diagram below):

diagram

Since, the given compound is a 1,2-trans-dibromocyclohexane, two bromo groups can arrange in 1,2-anti-diaxial positions in one of chair confirmations, fulfilling the requirement. It won't be possible if it'd have been a 1,2-cis-dibromocyclohexane.

Reference: Structure & Synthesis of Alkenes (Chapter 7): Leroy G. Wade, Organic Chemistry; 8th Edn, Pearson Education Limited: Harlow, Essex, UK, 2014.

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  • $\begingroup$ Here it would also good to redraw important parts yourself. $\endgroup$ – Mithoron Apr 14 '18 at 16:35
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Conformation of 1,2 dibromo,4-tertbutyl cyclohexane

As you can see from above mechanism the dibromo needs to be in an axial positions for elimination

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    $\begingroup$ I see a set of structures and a textual description posted as an image, but not the mechanism itself. Please post text as text, not a screenshot, whenever possible. Also, if you are using a third-party source, also add a proper reference and format the copied part accordingly (e.g. as a block qoute with >). $\endgroup$ – andselisk May 21 at 7:48

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