Question 16

[matthew]

Dear Dr. Ferdinand,

Choice B can work too! OH2 leaves, hydride shift and attack of the halide on the carbocation. It was on your recent GS 10 BS I think, or maybe earlier, the one about Sn2 being able to hydride shift.

Partial Credit on this one?

[admin]

Hmmmm, I want to give at least 0.2 for the thought but there is this problem: you have to choose the best answer. And though what you are mentioning is true, the rate of occurrence, in comparison to answer choice C, is low.

What's the big difference? It comes down to the intermediates. B must have a primary carbocation as an intermediate while C must have a secondary carbocation as an intermediate. The mechanism with the most stable intermediate triumphs.

PS: just in case anyone is wondering, of course we are kidding, there is no chance of part marks on the MCAT.

[mvenus9292913]

The molecule can be synthesized through an SN1 reaction with 2-butanol and HBr in which the 1st step involves protonation of the hydroxyl group to produce a good leaving, followed by backside displacement by the bromine ion (ORG 6.2.1 and 6.2.3).

Isn't that Sn2 substitution? Sn1 substitution involves a carbocation, doesn't it?

[admin]

Yes, you are correct.

Both could occur. SN1 as H2O drops off on its own creating a 2-butyl carbocation intermediate. The SN2 mechanism was described in the Explanation.

[admin]

bump