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  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?

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  • $\begingroup$ 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

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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. :)

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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.

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    $\begingroup$ 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
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    $\begingroup$ 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

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