[colinsjohn]

why is ethanol a better recrystallizing agent than water?

  I have been trying to work out exactly why for many week but i cant seem to come up with a clear answer. It is aspirin (2-hydroxybenzoic acid) which is going to be recrystallized

This is what i have came up with so far-I know that water is more polar due to being able to make more hydrogen bonds. Ethanol is less polar as it has a polar -OH group and a non-polar CH3.


Maybe a reason could be that because water is more polar it is held together more strongly so does not therfore react with the impurities are readily as ethanol.

Help would be greatly appreciated.

*Ignore me, I am impatient*-helppppppppppppppppppppppppppppppppppppppppppppppppppppp
*Ignore me, I am impatient*

[Yggdrasil]

The best choice for a solvent for recrystalization depends on both the compound you are trying to recrystalize and the impurities you expect to have.  For a good recrystalization solvent, you want your compound to have high solubility in that solvent at high temperatures, but low solubility at low temperatures (this is what allows you to recrystalize your product).  Furthermore, you want your impurities to be somewhat soluble in the solvent at low temperatures so that the impurities don't recrystalize as well.

I don't know the exact solubility properties of aspirin in water and ethanol, but that may be a reason (for example, aspirin may not be very soluble in water even at high temperatures).  Another reason could be because of your impurities.  Maybe you have a large portion of organic material as impurities which would come out of solution at low temperatures in water, but not in ethanol.

Also, 2-hydroxybenzoic acid is salicylic acid.  Aspirin is acetylsalicilic acid (2-(acetyloxy)benzoic acid).

[Borek]

What about dissociation?

[UnintentionalChaos]

The main issue here is that water dissociates into H+ and OH- and solvents like alcohol and acetone do not. Acetyl-salicylic acid is a salicylic acid with an acetyl group tethered to its phenol group. This bond is very weak. Smelling an old  bottle of aspirin is like sniffing a jug of vinegar since tiny quantities of water hydrolyze the bond and make salicylic acid (wart remover) and acetic acid (vinegar) If you simply put aspirin in water and heat, it will eventually dissolve. This is because the H+ from the water replaces the acetyl group and the OH attaches to the acetyl group forming acetic acid. When cooled, needle-like crystals of salicylic acid form. Salicylic acid has a huge difference in water solubility from boiling to freezing. Aspirin is not highly soluble in water at all temperatures and breaks down irreversibly in hot water. In acetone or in alcohol, there are no H+ and OH- ions to break up the structure and the solubility of aspirin in these solvents is high when hot and low when cold.