Favorable nucleophilic substitution reactions are characterized by having...

[csx]

the following except....

A weak Lewis base acting as a Nuc:
or
a strong lewis base acting as a Nuc:

The answer is the first option. I said the second. I thought that if the lewis base is too strong, it will undergo an elimination reaction instead of a subst.

can someone explain why the first option is the correct answer??

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[Teleologist]

Where did you get this question? There is no widely accepted way of characterizing a Lewis base's strength. Just one possible way of millions is Ka .... but again ... that's for a very specific Lewis acid/base pair: the LB and the hydrogen proton.

 

[csx]

Where did you get this question? There is no widely accepted way of characterizing a Lewis base's strength. Just one possible way of millions is Ka .... but again ... that's for a very specific Lewis acid/base pair: the LB and the hydrogen proton.

It was a Q from TBR iirc

 

[justadream]

@csx

I might be wrong but generally, aren't strong bases often good nucleophiles (at least the molecules that don't have huge steric hindrance)?

If so, maybe the answer is "a weak Lewis base" because in this situation, the nucleophilicity would be so low that you wouldn't get a reaction at all.

 

[Teleologist]

@csx

I might be wrong but generally, aren't strong bases often good nucleophiles (at least the molecules that don't have huge steric hindrance)?

If so, maybe the answer is "a weak Lewis base" because in this situation, the nucleophilicity would be so low that you wouldn't get a reaction at all.

This really depends on the solvent. For example F- is a stronger base than Cl- but F- is a crappy nucleophile in water since it is so heavily solvated as opposed to Cl-.

In aprotic solvents, however, F- can be a great nucleophile.

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Nucleophilicity aside, TBR dropped the ball on this question. Bad question. Would not appear on actual MCAT. Again the issue I have with this question is as follows. @BerkReviewTeach

What's a "weak" or "strong" Lewis base? Lewis basicity depends VERY much on the corresponding Lewis acid. Consider 2,2,6,6-Tetramethylpiperidine. This is a Lewis base that functions only has a Bronsted base because of its highly sterically hindered lone pair. If it weren't for the methyl groups we'd expect 2,2,6,6-Tetramethylpiperidine to actually be rather basic - perhaps comparable to ammonia - given that the nitrogen and its lone pair resemble ammonia and there are no resonance or inductive effects here screwing around with the lone pair. We can't generalize and say that 2,2,6,6-Tetramethylpiperidine is a "weak" Lewis base because, hell, it doesn't even function as a Lewis base most of the time. Terrible question.

 

[justadream]

@Teleologist

Where in TBR is this?

 

[Teleologist]

@justadream - I don't have TBR (or any review book for that matter) ... someone above me said the question came from TBR.