Is there a Reaction mechanism for the Recrystallization of benzoic acid from water

Question

After watching a lab practical experiment on the Recrystallization of benzoic acid from water, I'd please like to know is there a reaction mechanism for this?

Also, am I correct in saying that the benzoic acid during dissolution would dissociate into hydronium ions and benzoate ions?

Answer 1

Yes, since water is amphoteric it can act as a base for the more acidic benzoic acid. Like many acid-base reactions, this will be an equilibrium and since the resultant hydronium ion is considerably more acidic than benzoic acid (pKa of ~ -2 vs ~ 4 respectively), the equilibrium will lie mostly towards the protonated benzoic acid rather than the benzoate. As far as recrystallization goes, there's not an arrow pushing mechanism in the traditional sense, but a simple way to think about it would be that the molecules are becoming more ordered and packed together. Benzoic acid has some strong intermolecular forces at play such as hydrogen bonding and pi-pi stacking, so it can crystallize readily.