# Why do some nucleophiles attack at the carbonyl group while others at an alpha-carbon in ketones?

I was reading about the iodoform reaction and it stated that $\ce{OH^-}$ attacks at the $\alpha$-carbon and makes a substitution reaction possible. But there is also the possibility of attacking the carbonyl group directly and forming a gem-diol. Why doesn't that happen?

Please don’t stop thinking like this (‘what happens when $\ce{X}$ attacks $\ce{Y}$ rather than $\ce{Z}$?’) — it’s a great way to spot side reactions, alternative pathways, problems with functional groups etc.