Question about strong/weak nucleophiles/bases

[jyjuang]

Is it easier just to memorize the lists of typical strong nucleophiles, weak nucleophiles, strong bases, and weak bases when trying to identify E1/E2/Sn1/Sn2 reactions? I know how to compare nucleophilicity and basicity when given a set of reagents but I am not sure how to figure it out when given only one type of reagent in these kind of questions?

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[monsieurwise]

Just do a lot of questions and you will start "absorbing" the trends.
But here is what I see, I hope it helps:

First, you have to know that good nuc are usually good base

Good nuc-Poor base: have lone pairs and/or negative charge and don't enjoy taking H+ (example: Br-, a weak base). If you see neg charge, that's almost sure it's a good nuc.
Poor nuc-Good base: Bulky stuff. They are poor nuc only because they are simply too bulky and can't backside attack for SN2. They have to do E2 instead.
Good nuc-Good base: Everything else that are electron rich and want to share electrons, especially things with an O or O- on it, provide that they are not bulky.
Weak nuc-Weak base: Everything else.

Chad has a really good video on this one, you should watch it =).

 

[jyjuang]

What are some other examples of poor nuc-good base? Also, why doesn't Br- attack H+ to make HBr? I know that basicity increases going up a group so does Cl- behave the same as Br- or different?

 

[monsieurwise]

Any bulky base will do: for example, t-butoxide.
Br- doesn't like H+ because it's big and quite stable with the negative on it (Think: it has a big house so e- can run around -> stable). This guy is best as anion.
Cl- is not as big (e- have less place to run around -> less stable with negative on), this guy will want to take up H+ more than Br-, but will be ok as anion.
F- is super small (e- have NO place to run around -> not stable). This guy is addicted to H+. This is the worse anion.
When something is stable, there's no reason for them to react, like Br-. When something is NOT stable, they will react right away like F-.
Do you remember why HF is a weak acid and HBr is a strong acid? This is why. Think simple: everything want to be stable, if they are not stable yet, they will react all they can to be stable.
Stability is the key.