[xlion]

I did an experiment involving the isolation of a pure product from a crude product by recrystallisation, and my report required me to comment on the efficiency of the recrystallisation process, but I have no idea what that means. Can someone explain what is meant by the efficiency of recrystallisation

[Dan]

As far as I know, "efficiency of recrystallisation" is not a clealy defined term. I think you are just being asked to comment on how effective the recrystallisation was. What is the purpose recrystallisation? What were you trying to achieve and to what extent were these goals met?

[fledarmus]

You can determine a theoretical efficiency of recrystallization if you know the solubility of your product and the solubility of your impurity in both hot and cold water. From that information you can calculate an expected % recovery and % purity, given a knowledge of the mass and purity of your starting material and the amount of solvent.

[xlion]

As far as I know, "efficiency of recrystallisation" is not a clealy defined term. I think you are just being asked to comment on how effective the recrystallisation was. What is the purpose recrystallisation? What were you trying to achieve and to what extent were these goals met?

What do you mean by the effectiveness? Do you mean the purity of the compound? In that experiment, the purpose of the recrystallization was to purify a sample of p-toluic acid prepared through a grignard reaction. The melting point of the crystals were then tested and compared to the theoretical values.

You can determine a theoretical efficiency of recrystallization if you know the solubility of your product and the solubility of your impurity in both hot and cold water. From that information you can calculate an expected % recovery and % purity, given a knowledge of the mass and purity of your starting material and the amount of solvent.

But what if I do not know some of that information, like the purity of the starting material and the type of impurity present? The product was synthesized by a reaction between dry ice and a grignard reagent that was self prepared, so i don't know what kind of impurities are there inside. And i still don't get what is meant by efficiency.

[blaisem]

You'll probably have to ask your TA or whomever is in charge to clarify exactly what is meant with recrystallization efficiency.  You can state qualitatively how pure the compound is by its melting point.  To name an exact efficiency if you have no knowledge of the crude product creates some ambiguity to what your reference for efficiency is.

[fledarmus]

If they are asking you to "comment" on the efficiency of the recrystallization process, they are probably looking for comments in general terms rather than in mathematical terms. So how clean was your crude product compared to your crystallized product? How much of your product was left behind in the mother liquor? How much time/solvent/heat/other resources were required in comparison to other methods you may have used? These are all different ways to think about efficiency. You can attach some numbers to a few of these if you were able to measure them, such as purity of crude and final products, and loss of material during recrystallization (if you went back and measured the remaining product in your mother liquor), others you can make some educated guesses about.