Sn1, Sn2, E1, E2 reactions

[nemzy]

okay, i understand how these reactions works and in which circumstances, but there are some general concepts that i am confused about.

 

first of all, i know that in Sn2 reactions, generally weak bases are best as nucleophiles..but how can you know what a weak base is? for example, why is CH3CO2- a weak base (sn2 will predominate) and CH3CH2O- a strong base (E2 will predominate)?

 

And..how can u tell what is an aprotic or protic solvent?

 

for example, CH3OH is protic, but CH3CN is aprotic?