Question

Question about mixed solvents for recrystallization.

I am having a hard time conceptualizing how a mixed solvent would work in a scenario where P and O nitronitroacetanilide are recrystallized with a small amount of hot Ethanol followed by a larger volume of water.

From the structure of the molecules I assumed that the Para form of nitroacetanilide is more polar than the Ortho form. Second, I know that water>ethanol for polarity. Finally, I know that water and ethanol can hydrogen bond with one-another and with itself. Also I know that water and ethanol will hydrogen bond together to form a more densely packed liquid. I imagine that this would look somewhat like this: [ Eth-OH] --[O-H2]---[OH-Eth]

My understanding of dissolving is that the solute gets surrounded by solvent molecules and separates from other solute molecules. This happens to all the solute molecules until no more solvent molecules are left to dissolve the solute.

Having said that, does the following make sense? If not, please correct me

1) the O/P mixture of crude nitroacetanilide dissolves in hot ethanol because ethanol hydrogen bonds with the nitroacetanilide

2) when water is added, ethanol and water hydrogen bond with one another creating a denser solvent because the ethanol hydrogens are attracted to the oxygen in water and vice versa

3) because there's less hydrogens "available" to form new hydrogen bonds after the water is added, the solvent can't make new hydrogen bonds as easily

4) because the P-nitroacetanilide needs more hydrogens to dissolve due to it being more polar, it is now less soluble in the ethanol/water mixture. this causes it to precipitate out while leaving the O-nitroacetanilide that's less polar still in solution

Is this correct? if not please explain how the mixed solvent helps precipitate the p-nitroacetanilide but maintain the o-nitroacetanilide in solution.

Answer 1