SYNTHESIS OF HEXANAL

• Assemble the apparatus with the catalyst of copper-chromium oxide deposited on pumice packed into the Pyrex combustion tube in sections of about 25 cm long, each being separated by a small plug of glass wool. 

• Place lOOg (122ml, 0.98mol) of hexan-1-ol in the dropping funnel. 

• The gas outlet from the Drechsel bottle, E, should be led into a fume cupboard or to an outside window, since hydrogen is evolved in the reaction. 

• Place 0.1 g of hydroquinone in the receiver to act as a 'stabiliser' for the aldehyde. 

• Pass a gentle flow of nitrogen gas through the combusion tube and adjust the temperature of the furnace to 330 °C. After 2 hours, allow the alcohol to pass into the combusion tube at the rate of 1 drop every 3-4 seconds. 

• The commencement of the dehydrogenation will be indicated by the production of white fumes at the point where the combustion tube enters the condenser; it will also be indicated by a gas flow (hydrogen) through the Drechsel bottles which continues after the nitrogen flow has been temporarily stopped. 

• When all the hexanol has passed through the catalyst tube, remove the aqueous layer from the distillate, dry the organic layer with a little magnesium sulphate and distil from a flask carrying a fractionating side arm. 

• Collect the fraction (30 g) (1) having b.p. 125-135 °C, and redistil to obtain 21 g (21%) of hexanal (2) having b.p. 127-129 °C.

Notes to keep in mind:

1. If the high boiling residue is transferred to a smaller flask and fractionally distilled, some hexanol passes over first, followed by hexyl hexanoate (Me⋅(CH₂)₄⋅C0₂(CH₂)₅ ⋅Me) (2g) at 240-250 °C (mainly at 245 °C).

2. About 0.1 per cent of hydroquinone should be added as a stabiliser since hexanal exhibits a marked tendency to polymerise. To obtain pure hexanal, treat the 21 g of the product with a solution of 42 g of sodium metabisulphite in 125 ml of water and shake; much bisulphite derivative will separate. Steam distil the suspension of the bisulphite complex until about 50 ml of distillate have been collected; this will remove any non-aldehyde impurities together with a little aldehyde. Cool the residual aldehyde-bisulphite solution to 40-50 °C, and add slowly a solution of 32 g of sodium hydrogen carbonate in 80 ml of water, and remove the free aldehyde by steam distillation. Separate the upper layer of hexanal, wash it with a little water, dry with magnesium sulphate and distil: the pure aldehyde passes over at 128-128.5 °C.

Cognate preparations: Valeraldehyde (pentanal)

• Use 100g(123ml, 1.14mol) of pentan-1-ol, and fractionate the dried distillate. 

• Collect the fraction of b.p. 98-1 10 °C (23 g); upon redistillation 20 g (20%) of valeraldehyde, b.p. 101-105 °C, are obtained. 

• From the high boiling fractions 25 g of pentan-1-ol (b.p. 135-139 °C) may be recovered, together with 1.5 g of pentyl pentanoate (b.p. 205-210 °C).

Butyraldehyde (butanal)

• Use lOOg (123.5ml, 1.35 mol) of butan-1-ol. The yield of butyraldehyde, b.p. 70-75 °C, is 38 g (39%), and of butyl butanoate (b.p. 165-170 °C) is 2g; 40g of butan-1-ol are recovered.

Propionaldehyde (propanal)

• Use 100 g (125 ml, 1.67 mol) of propan-1-ol and surround the receiver by a freezing mixture. The yield of propionaldehyde, b.p. 48-49.5 °C (mainly 49 °C), is 35 g (36%), and of propyl propanoate, b.p. 120-125 °C, is 1 g; 30 g of propan-1-ol are recovered.