Question

Why is sulfide a better nucleophile than the structurally equivalent alkoxide?

Answer 1

Well, which is the LARGER anion...?

Explanation:

Certainly, third period sulfide anion is LARGER than second period oxide anion....and on this basis its electron cloud is more diffuse, and more polarizable, and thus more likely to participate in a substitution reaction. And CLEARLY, sulfide is a WEAKER base than hydroxide....and clearly the `pK_a` values of water and `H_2S` reflect this.

Taken together, these factors contribute to the superiority of sulfide as a nucleophile...and the superiority of hydroxide as a base.