SYNTHESIS OF OCTANAL

CAUTION: This reaction should be conducted in an efficient fume cupboard as hydrogen is evolved.

Trioctyloxyboroxine:

• In a dry 250-ml round-bottomed flask provided with a septum inlet, magnetic stirrer bar, and a reflux condenser attached to a connecting tube leading to a mercury bubbler, are placed octanoic acid (9.51 ml, 60mmol) and diethyl ether (75 ml) under nitrogen. 

• The mixture is stirred vigorously and borane-dimethyl sulphide (BMS, 6.12 ml, 60mmol) is added dropwise from a syringe. 

• Following the addition of the initial 2-3 ml of BMS, when gas evolution has ceased, the mixture is heated under gentle reflux to complete evolution of gas (hydrogen). 

• The remainder of the BMS is added at such a rate as to maintain gentle reflux. 

• Following completion of the addition, the mixture is heated under reflux for 1 hour. 

• The solvent and dimethyl sulphide (CAUTION) are removed under vacuum (1) and dichloromethane (20 ml) is introduced to dissolve the product.

 Octanal: 

• To a well-stirred suspension of pyridinium chlorochromate [PCC, 14.3 g, 66mmol] in dichloromethane (100 ml) contained in a 500-ml flask equipped as above, is added dropwise the above solution of trioctyloxyboroxine in dichloromethane. 

• The stirred mixture is heated under reflux for 1 hour and then diluted with ether (150 ml). 

• The supernatant liquid is filtered through Florosil (lOOg) contained in a 300-ml sintered glass funnel; the solid residue is triturated with ether (3 x 50 ml) and passed through the same Florosil column. 

• The colourless filtrate is concentrated and distilled under reduced pressure to give octanal: yield 5.9 g (77%), b.p. 64-65°C/15mmHg.

Notes to keep in mind:

1. Very efficient cooling traps must be used in this distillation, should be consulted for the treatment and disposal of the distillate and trap residues.