Question

Pre Lab Question Please answer in complete sentences.

Describe the characteristics of a good recrystallization solvent The solubility of a compound is 59 g per 100 mL in boiling methanol and 30 g per 100 mL in cold methanol, whereas its solubility in water is 7.2 g per 100 mL at 95 degree C and 0.22 g per 100 mL at 2 degree C. Which solvent would be better for recrystallization? Explain why. In what circumstances is it necessary to do a hot filtration What is an appropriate heating rate during the determination of a melting point? What happens if the rate is faster? What happens if it is slower?

Answer 1

Concepts and reason

Recrystallization is generally used for the purification of organic solids that are non-volatile in behaviour. It is a purification technique that is based over the effect of temperature over the solubility related to a solvent.

Fundamentals

Melting point: The melting point is generally that range of the temperature at which there occurs a change to liquid state from the solid state.

The steps involved in the recrystallization are as follows.

The first step involves the selection of solvent.

The impure compound is then dissolved in the hot solvent.

If required, then the solution is decolorized using the activated charcoal.

The insoluble material is then filtered from the solution.

The solution is then cooled slowly for the recrystallization to take place.

The crystals are then collected, washed and dried. The last step involved the determination of melting point for checking the purification of the given compound.

(a)

The characteristics corresponding to a good solvent used for recrystallization are as follows.

The compound that has be purified must not dissolve in the solvent at the normal temperature. It must be soluble at the condition of boiling point.

There should be no reaction between the compound that has to be purified and the solvent.

The solvent must possess high volatility so that its removal becomes easy after the recrystallization of the compound.

(b)

Water acts as a better solvent for the recrystallisation for the given compound in comparison to its solubility in methanol.

(c)

The process of hot filtration is done when the impurities do not dissolve in the solvent and are left behind as the solids.

(d)

The heating rate is approximately $1 - 2{\rm{ }}^\circ {\rm{C}}$ for determining the melting point.

If the rate of melting point is high then exact melting point of the given compound cannot be observed.

If the heating rate is slow then the melting point can be accurately determined having a sharp range for the melting point determination.

Ans: Part a

The solvent must dissolve the compound at the high temperature, there should be no reaction between the compound and the solvent and solvent must possess high volatility.

Part b

Water acts as a better solvent for the recrystallisation of the given compound due to large difference in the solubility at high and low temperature.

Part c

The hot filtration is used for filtration of impurities that are insoluble and left behind as solids in the solvent.

Part d

The heating rate for melting point is ${\bf{1 - 2 ^\circ C}}$ ; faster heating rate leads to broad melting point range while slow heating rate gives sharp melting point range.