[NYM]

Why does (dibromomethyl)-benzene and OH- yield benzaldehyde, and not (dihydroxymethyl)-benzene?

The reaction mechanism for benzaldehyde seems pretty complicated.

[sjb]

It's possible that it does produce (dihydroxymethyl)-benzene, but then the base converts one of the OHs into an O-, which then decomposes like a tetrahedral intermediate back down to the carbonyl, kicking out regenerated OH- and forming benzaldehyde. Purely entropy considerations may make this the more favoured or observed final product.