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10+ Year Member
Dec 21, 2010
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Fudge sicles- ^disregard the title, I meant to say SN1, brain fart.

Question guys, so I came across a question asking to choose which molecule would undergo SN1 most readily. unfortunately this was way harder than expected.

Molecule one was the standard tertiary alkyl halide, second one was a tertiary alkyl halide with additional metyl groups to the adjacent carbons sooo it would be like
(CH3)3C-C-Br with the two | being the same as the |

my question is, does sterics also affect the rate? And if so, what boundary can be drawn between highly substituted (thus carbocation highly stable and thus reaction rate faster) and sterics (highly substituted also can mean removing leaving group is hard or for nucleophile to attack)

come to think of it, if the rate determining step is the formation of the carbocation, I guess it would mean a super highly substituted tertiary carbon is faster than one less highly substituted yet also tertiary.

^kind of confusing but let me know if you can help.

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I would tend to err towards the second thing you said. Remembering that the rate of reaction is determined by the rate of formation of the carbocation, and is not dependent on the nucleophile, the compound that forms the carbocation fastest should best undergo SN1. Since the concentration of the nucleophile doesn't matter, I would argue that this implies that the effect that sterics has on rate is minor relative to the formation of the carbocation.

Also, while sterics always plays at least some role in reactions (large or small), the carbocation is planar, so the substituents would have to be huge in order to cause some significant change in rate due to sterics.