SYNTHESIS OF MANDELIC ACID

CAUTION: This preparation must be carried out in an efficient fume cupboard.

• Prepare a saturated solution of sodium metabisulphite by stirring 250g of finely powdered sodium metabisulphite with 335 ml of water for half an hour and then filtering to remove excess of the salt. 

• In a 1 -litre three-necked flask equipped with a mechanical stirrer and a dropping funnel, place a solution of 25g (0.5 mol) of sodium cyanide (CAUTION) in 100 ml of water and 53g (51 ml, 0.5 mol) of purified benzaldehyde. 

• Add the sodium metabisulphite solution from the dropping funnel, slowly at first and then more rapidly (the addition occupies 10-15 minutes). 

• During the initial stages of the addition, add 150g of crushed ice to the reaction mixture in several portions through the third neck. 

• Transfer the two-layer liquid mixture to a separatory funnel and remove the crude mandelonitrile(1). 

• Place the crude product at once (2) in a large evaporating dish, add 75 ml of concentrated hydrochloric acid, cover with a clock glass and allow the hydrolysis to proceed at room temperature for 12 hours. 

• Evaporate the solution to dryness on a steam bath, stirring from time to time to break up the deposit of ammonium chloride and mandelic acid which separates. 

• Grind the residue of slightly discoloured mandelic acid and inorganic salts to a fine powder and wash it with two portions of 125 ml of cold benzene (CAUTION); this process will remove most of the colouring matter but a negligible quantity of mandelic acid. 

• To separate the inorganic salts from the mandelic acid, extract the residue in a Soxhlet apparatus with about 200 ml of benzene. 

• Allow the hot benzene extract to crystallise, collect the crystals on a Buchner funnel and dry in air. The yield of pure (±)-mandelic acid, m.p. 118 °C, is 35g (46%).

Notes to keep in mind:

1. A further small quantity of mandelonitrile may be obtained by extracting the aqueous solution with ether, evaporating the ether and adding the residue to the main portion of mandelonitrile. This extraction is hardly worth while except for large-scale preparations.

2. It is important to mix the mandelonitrile with hydrochloric acid immediately it has been separated from the water. Standing results in rapid conversion to the acetal of benzaldehyde and mandelonitrile Ph-CH[OCH(CN)'Ph] 2 ; the yield of mandelic acid will, in consequence, be reduced.

Cognate preparation: Acetone cyanohydrin

• Dissolve 110g of sodium metabisulphite in 200 ml of cold water contained in a 1 -litre round-bottomed flask. 

• Add slowly 58g of acetone while swirling the liquid mixture slowly, followed by a solution of 60g of potassium cyanide in 200 ml of cold water (CAUTION). 

• During this latter slow addition the cyanohydrin separates as the upper layer. 

• When separation is complete the contents of the flask are transferred to a separatory funnel and the lower layer removed. 

• The upper layer is transferred to a flask, sodium sulphate added to effect drying and the flask stoppered and kept in the dark. 

• The dried cyanohydrin (60g, 70%) is slightly discoloured; it may be distilled under reduced pressure when it distils at 80-82°C/15 mmHg.