SYNTHESIS OF HEXANEDIAL (Adip aldehyde)

• Dissolve 8.2 g (0. 1 mol) of cyclohexene in 200 ml of pure dry ethyl acetate contained in a 500-ml glass-stoppered wash bottle, cool the solution to —20 to — 30 °C or below (e.g. with solid carbon dioxide-acetone) and attach the wash bottle through a calcium chloride drying tube to another containing acidified potassium iodide solution. 

• Pass ozonised oxygen until the reaction is complete, i.e. until iodine is abundantly liberated. 

• Then add 0.5 g of palladium hydroxide-calcium carbonate catalyst(1) and hydrogenate the cold solution of the ozonide in the usual manner; cool the hydrogenation vessel in ice). 

• Filter off the catalyst and remove the solvent by distillation at normal pressure. 

• Distil the residue under reduced pressure and collect the adipaldehyde at 92-94 °C/12mmHg. The yield is 7g (61%). 

• This aldehyde oxidises readily and should be kept in a sealed tube in an atmosphere of nitrogen or carbon dioxide. It may be converted into the dioxime by warming with aqueous hydroxylamine acetate solution: after recrystallisation from water, the dioxime has m.p. 172 °C.

Cognate preparation: Azelaic hemialdehyde semicarbazone

• Dissolve 7g (0.025 mol) of pure oleic acid in 30 ml of dry ethyl chloride (chloroform may be used but is less satisfactory), and ozonise at about — 30 °C. 

• Remove the bulk of the solvent under reduced pressure, dissolve the residue in 50 ml of dry methanol and hydrogenate as for adipaldehyde in the presence of 0.5 g of palladium-calcium carbonate (2). 

• Warm the resulting solution for 30 minutes with a slight excess of semicarbazide acetate and pour into water. 

• Collect the precipitated semicarbazones and dry: the yield is 8.5 g. 

• Separate the mixture of semicarbazones by either of the following methods: 

1. Treat with dilute sodium hydrogen carbonate solution to extract the semicarbazone of azelaic hemialdehyde; upon acidifying the extract with dilute sulphuric acid, the semicarbazone of azelaic hemialdehyde is precipitated (4.4 g, 77%, m.p. 1 62 °C, after recrystallisation from methanol).The residue from the sodium hydrogen carbonate extraction consists of the semicarbazone of nonanal and melts at 101 °C after recrystallisation from methanol: yield 3.8 g (77%).
2. Extract the dry mixture of semicarbazones with ether: only the semi-carbazone of nonanal dissolves easily.

Notes to keep in mind:

1. The Adams platinum oxide catalyst gives satisfactory results in the reduction of ozonides.

2. An alternative work-up procedure after hydrogenation is to isolate the nonanal by steam distillation and to recover the azelaic hemialdehyde from the residual aqueous layer.