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?

  • $\begingroup$ I think they should have mentioned heat in the question. 1,2-diiodo when heated undergo elimination to form alkenes. $\endgroup$ Apr 14, 2018 at 7:15
  • $\begingroup$ 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. $\endgroup$
    – Karl
    Jul 8, 2019 at 19:11

2 Answers 2


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):


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.

  • $\begingroup$ Here it would also good to redraw important parts yourself. $\endgroup$
    – Mithoron
    Apr 14, 2018 at 16:35

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

  • 2
    $\begingroup$ 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, 2019 at 7:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.