Is a strong nucleophile a strong base?

How do I know when a compound is a good base but a bad nucleophile and vice versa?

• Answer:

  Hi! A chemical compound is classified as a good base, as long as it can extract/remove a H+ proton from another compound. This is usually characterised by its high pka value (ie it's a good proton acceptor). Nucleophilicity on the other hand, depends very much on steric effect (i.e how bulky it is). In general, a good nucleophile will also be a good base, but the reverse may not be true. It's very unfavorable for a bulky base (eg (i-Pr)3N) to attack a relatively more hidden C atom, resulting in its poor nucleophilicity. Hope this helps! ^^