Question

What are the desirable characteristics of a solvent to be used in the recrystallisation of a solid product? (Hint: what is the underlying purpose of doing recrystallisation, aside from making the product look attractive?)
EXPERIMENT5 PREPARATION OF BENZOIC ACID Oxidation of a Primary Alcohol In this experiment, a primary alcohol (benzyl alcohol) is oxidised to a carboxylic acid (benzoic acid) by a strong oxidising agent (potassium permanganate) Because the synthesis reaction is carried out in basic conditions, the carboxylic acid is dissolved as the carboxylate ion at the end of the preparation step CH2CH NaCH +4 KMno Benzyl alcohol MW 108 Density 1.04 gfmL Sodium benzoate (water-soluble) To isolate the product, the benzoate ion is protonated by adding dilute solution of a strong acid (HCI) to form insoluble benzoic acid CoO COOH Benzcate ion (water soluble) Benzoic acid water insoluble) MW 122 The oxidation of benzyl alcohol is typical of the ease with which alcohols are oxidised to aldehydes, ketones and carboxylic acids The product is purified by recrystallisation from water -40

Answer 1

Characteristics of appropriate solvents for recrystallization or fractional crystallization of solid product-

a) Solvent should have lower boiling point than melting point of desired product.

b) Solvent should solubilize product easily at high temperature but reproduces it when cooled.

c) Solvent should have some difference in polarity compared to product to facilitate crystallization.

d) Solvent shouldn't react with product.

e) Solvent should capable of dissolving minor impurities present along with product at all temperatures.

f) Solvent should be separable from product in mild conditions.

Purpose of fractional crystallization, apart from making it look attractive, is to remove impurities, when pure products are to be procured from mixture. Solvent extraction technics are there to help but desired purity is easily achievable using recrystallization. This technique is time-consuming but yields pure & easily separable product(NMR level purity). Decrystallization followed by recrystallization eliminates minor impurities.

Recrystallization is also popular because it's easier to perform, minimal arrangements is required along with cost-effectiveness(cheap) than other methods of purification.