4
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1-(tert-butyl)-4-chlorocyclohexane

Why does compound 1 undergo the $\mathrm{S_N1}$ reaction faster than 2 even though both proceed via the same carbocation intermediate?

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    $\begingroup$ You had the two compounds the wrong way round, by the way. The cis diastereomer reacts faster. Also, please no MathJaX in titles. $\endgroup$ – orthocresol Sep 12 '16 at 7:50
  • $\begingroup$ @orthocresol do you know what the relative rates of solvolysis are for these two compounds? $\endgroup$ – ron Sep 12 '16 at 15:50
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    $\begingroup$ @ron I went searching a bit. Can't find anything on the chlorides, but acetolysis of the tosylates proceeds with $k_\mathrm{ax}/k_\mathrm{eq} = 3.24$. JACS 1955, 77, 5562 $\endgroup$ – orthocresol Sep 12 '16 at 16:30
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Conformations

In compund 2, both substituents can be placed in equatorial positions, whereas in 1 the $\ce{Cl}$ group is forced into an axial position since the bulky t-butyl group has to be placed equatorial. This makes compound 1 more unstable (higher energy), leading to a faster rate of formation of the carbocation.

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  • $\begingroup$ I think it's highly unlikely that the cyclohexane will adopt a non-chair conformation. The chlorine will just be stuck in an axial position. $\endgroup$ – orthocresol Sep 12 '16 at 7:34
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    $\begingroup$ Just to add: an alternative way of looking at it is that, with compound 1, upon leaving of the $\ce{Cl-}$, there is relief of 1,3-diaxial interactions. But it is really just a different way of describing the same thing. $\endgroup$ – orthocresol Sep 12 '16 at 7:52

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