Poor Nucleophile but Strng Base?

[SuperSneaky24]

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So when a molecule is a poor nucleophile and a strong base, it is usually sterically hindered. Is the reason why it's a strong base is that the water molecules it deprotonates are too small to be affected by steric hinderance?

And is there ever a case where a molecule is a strong nucleophile but a poor base?

 

[Teleologist]

Yes; water molecules tend to be less bulky than many other molecules.

 

[Czarcasm]

So when a molecule is a poor nucleophile and a strong base, it is usually sterically hindered. Is the reason why it's a strong base is that the water molecules it deprotonates are too small to be affected by steric hinderance?

And is there ever a case where a molecule is a strong nucleophile but a poor base?

Potassium Tert-butoxide vs. Tert-Butyl Alcohol. Tert-butoxide, having a negative charge on oxygen is a strong nucleophile. Alcohols have a pKa very near water, and water as you may have realized, is not very basic. Both however, are very bulky and wouldn't be ideal for back-side attack (SN2).