Question

Predict the effect on each of the following if the product was not completely dried after the recrystallization:
(a) percent yield; (b) melting point; (c) IR spectrum.

Answer 1

Solution

a) Percent yield.

If the product is not well dried and if its mass is recorded as the experimental yield then the moisture present in the moistened solid will lead to increase in the mass of the product which would be higher than its actual mass and it will given higher mass of the product obtained than its actual mass. Therefore the percent yield will be higher.

b) Melting point

If the product is not dried and contains the moisture then the melting point determinations would not be correct because the moistened substance will not filled well in the capillary tube to record the melting point which will not give the correct readings for the temperature of the melting point. Also the solvent present in the product affect the melting point because the melting point and boiling points of the impure compounds are different than the pure compounds. So we may get higher or lower melting point depends on the solvent present in the product.

c) IR spectrum

If the product is not well dried and contains the solvent portion in it then this solvent portion is nothing but the impurity present in the product. Therefore this solvent portion will give the unnecessary IR spectrum peaks in the IR spectrum of the product which would give the incorrect IR spectrum peaks for the product and this will make difficult to exactly figure out the functional groups present in the product. The IR peak intensity and the number of peak will be depend on the nature (type) of the solvent used. Therefore to avoid these errors the product must be well dried.