# Elimination reaction with 1,2-dibromo-4-methylcyclohexane

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

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?

• I think they should have mentioned heat in the question. 1,2-diiodo when heated undergo elimination to form alkenes. – FreakyLearner Apr 14 '18 at 7:15
• Iodide is an even better leaving group than bromide. So if anything is to happen, it has to be more complicated than a simple substitution. – Karl Jul 8 at 19:11

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):
• 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 >). – andselisk May 21 at 7:48