2
$\begingroup$
  1. (1s,4s)-1-bromo-4-tert-butylcyclohexane (1) and (1r,4r)-1-bromo-4-tert-butylcyclohexane (2) are heated (E2) with $\ce{NaOCH3}/\ce{CH3OH}.$ Which isomer will react faster?
    1: (1s,4s)-1-bromo-4-tert-butylcyclohexane; 2: (1r,4r)-1-bromo-4-tert-butylcyclohexane
  2. What will change when $\ce{CH3OH}$ without the strong base is used?

I drew chair conformation of both with $\ce{Br}$ in axial position, so it can undergo E2. In 1 I got $\ce{^tBu}$ in axial position, in 2 I got $\ce{^tBu}$ in equatorial position.

Would it be correct to say that because 2 is more stable due to bulky group in equatorial position, it would react slower in E2?

For the second part of the question I suggested SN1. But then how would I know which would be faster in this case, since the $\ce{Br}$ does not have to be axial?

$\endgroup$
1
  • $\begingroup$ Which group has the greater A-value? Is it equatorial or axial? What are the geometrical requirements for an E2 elimination? Now you have the answer. $\endgroup$
    – user55119
    May 1, 2022 at 22:53

2 Answers 2

1
$\begingroup$

You are right on the part of E2, 2 will react slower. The reactivity of SN1 follows the same order - the carbocations are same but in 1 due to $\ce{Br}$ being on the axial position, the $\ce{C-Br}$ bond will dissociate easily to form the carbocation and so 1 reacts faster than 2.

Hope this helps. :)

$\endgroup$
-1
$\begingroup$

These are geometric isomers, cis-trans compounds so the RS nomenclature is not appropriate. If they were mirror images they would have identical reaction rates in an achiral system

The general idea is that a bulky group such as t-butyl is almost always equatorial. The 1,4-trans has both groups equatorial so is most likely the lower energy reactant [higher transition E] by a SN1-E1 mechanism since the cis and trans carbonium ion are the same. It is a bit different for E2 since the cis is set up for the lower energy anti transition state and is the higher energy molecule. What I am having trouble visualizing is the reaction of the carbonium ion in an SN1 reaction and the cis-trans product ratio in the ether. Models might help here.

$\endgroup$
3
  • 1
    $\begingroup$ The CIP rules assign lower case r/s descriptors to these two achiral, stereoisomers. See examples here. $\endgroup$
    – user55119
    May 2, 2022 at 19:58
  • $\begingroup$ I didn't know this and after checking your link am thoroughly confused. I learned this years ago when the rules were in their infancy and obviously didn't learn enough. I fail to see why this rates a down vote perhaps a comment. I think my answer should help the questioner evaluate the energetics of the reaction of the two isomers. $\endgroup$
    – jimchmst
    May 3, 2022 at 21:56
  • 1
    $\begingroup$ It wasn't my down vote. CIP rules have evolved over time. I think the link is quite explicit as to how the assignments are made. This link goes through the CIP rules step-by-step. Your explanation for the E2 elimination, cis faster than trans, is fine. $\endgroup$
    – user55119
    May 3, 2022 at 23:10

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.