Question about Sn2!!!

[pup329]

The Kaplan book for Orgo mentions that the Gabriel synthesis uses a very bulky nucleophile (phthalimide) for the Sn2 reaction with diethylbromomalonate. The book mentions that "the bulkiness of the nucleophile creates steric hindrance, which prevents the substrate carbon from undergoing multiple substitutions" (page 241).

I don't really get this??? I thought that Sn2 reactions were inhibited by steric hindrance???

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[PositivePhotoresist]

Kinetically, Sn2 is a bimolecular reaction. You can define the rate as r = k [nucleophile] [leaving group]. Remember that nucleophilicty is all about kinetics. The nucleophile wants to attack the substrate as quickly as it can. Since this reaction depends a lot on the nucleophile, steric hindrance from the substrate can drastically lower the rate of reaction because you’re making it harder for the nucleophile to attack the substrate (or more specifically, the electrophile).

Now, let’s run through what the excerpt is trying to say. You have a bulky nucleophile that first attaches to the substrate. Note that the substrate is not initially sterically hindered. After you attach the huge nucleophile, another nucleophile can’t really come in because you now made your new substrate bulky. This is why there is a limitation on multiple Sn2 substitutions.

Hope that helps!