Homework Help: How reaction proceeds

I'm not quite sure what PhCHO is, it seems as though it is benzaldehyde. If that's the case lets look at your electrons that can participate in bonding. You have two lone pairs on an oxygen, two electrons in a pi bond, and 6 pi electrons in resonance in your ring. These are the only electrons that will be able to form a bond and given the reactivity of benzene pi bonds, it's unlikely that they will be involved. Also think about you the stability. The positive carbon is fairly stable due to the benzene ring on one side and oxygen on the other.

 

Your last compound is wrong. The sulphur shall have a -ve charge due to accumulation of another pair of electrons and the carbonyl carbon shall have +ve charge. That should help you get the mechanism. Post that, think about the stability of the sulphur and how it can become more stable.