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Here's one way to prepare cholesteryl benzoate crystals:

Step 1 Dissolve 1.0 g of cholesterol in 3 cm3 of pyridine in a conical flask.

Step 2 Add 0.40 cm3 of benzoyl chloride.

Step 3 Heat the mixture on a steam bath for about 10 minutes.

Step 4 Cool the mixture, and add 15 cm3 of methanol.

Step 5 Collect the solid cholesteryl benzoate by suction filtration. Rinse the flask and the crude crystals with a little cold methanol.

Step 6 Recrystallize the cholesteryl benzoate using ethyl ethanoate as the solvent.

The methanol is said to be added to react with the benzoyl chloride. Why is this required?

Usually, in a preparation such as that of 1-bromobutane, some substance such as sodium carbonate is added in order to allow for separation of the aqueous substances from the organic layer. That makes sense. However, in the case of the above preparation, by adding methanol the substances within the conical flask go from benzoyl chloride, pyridine and cholesteryl benzoate to HCl, pyridine, methyl benzoate and cholesteryl benzoate. What's the point? Either way, both mixtures are going to eventually be separated from the cholesteryl benzoate by suction filtration.

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Pyridinium hydrochloride and methyl benzoate are both soluble in methanol and thus will be washed through the filter leaving behind clean cholesteryl benzoate. If you don't use the methanol the solid you produce will be impure and sticky; the methyl benzoate and pyridinium hydrochloride will not pass through the filter but will bind to the product crystals.. Methanol also removes any excess of benzoyl chloride by reacting with it.

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  • $\begingroup$ There is a pattern in all organic preparations that there is some kind of intermediate substance that is usually added in order to allow for 'better separation'. In the british education system the reason for adding this 'intermediate substance' (not an official name- just a term that I am using) is always just said to be added to react with some substance within the solution. Should I assume that the necessity of this is ALWYAS to allow you to separate away your desired product from most of the stuff within the mixture? $\endgroup$ – Mathematician May 15 '17 at 12:54
  • $\begingroup$ The usual phrase is "work up" by addition of....whatever. This can be as simple as ammonium chloride soln to neutralise anions, or can be something to react with excess reagent and stop the reaction. However as you move forward into more complex procedures the problem of removing excess reagents and unreacted starting materials becomes far more significant. $\endgroup$ – Waylander May 15 '17 at 12:57
  • $\begingroup$ It seems like just saying that we want to remove a certain substance within the mixture is a cruel oversimplification. As illustrated by your answer, this is only a proxy to the true reason, which is not to leave behinde a sticky substance that contains a complicated mixture that is not the cholesteryl benzoate that we want. $\endgroup$ – Mathematician May 15 '17 at 13:14
  • $\begingroup$ It is certainly a simplification. Having a well-worked out procedure that delivers pure product is a rarity in synthetic chemistry. All too often getting hold of the clean desired product is far more challenging than running the reaction. $\endgroup$ – Waylander May 15 '17 at 14:21

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