SYNTHESIS OF BUTYRALDEHYDE (Butanal)

• Equip a two-necked 500 ml flask with a dropping funnel so that the outlet extends to the bottom of the flask, and a Hempel column (15 cm x 1.5 cm) filled with 6-mm glass or porcelain rings, to which is attached a simple distillation head. 

• Fit to the distillation head a thermometer and a condenser set for downward distillation. 

• Attach to the condenser outlet a receiver adapter and a flask immersed in a bath of crushed ice/water. 

• Dissolve 56 g (0.188 mol) of sodium dichromate dihydrate in 300 ml of water and add cautiously, with stirring, 40 ml of concentrated sulphuric acid. 

• Place 41 g (51 ml, 0.55 mol) of butan-1-ol together with a few small chips of porous porcelain in the flask, heat the butan-1-ol to boiling and run in the dichromate solution via the dropping funnel during about 20 minutes. 

• The oxidation to butyraldehyde proceeds with the evolution of heat, but it is necessary to con-tinue to heat the flask so that the mixture boils vigorously to maintain steady distillation. 

• The temperature at the top of the column, however, should not exceed 80-85 °C. 

• When all the oxidising agent has been added, continue heating the mixture for 15 minutes and collect all that passes over below 90 °C. 

• Separate the water from the distillate and dry the residue (29 g) for 30-60 minutes with 3-4 g of anhydrous sodium sulphate. 

• Meanwhile detach the fractionating column from the apparatus and dry the glass or porcelain rings by washing with acetone and blowing hot air through them. 

• Fit the column into a 100-ml flask and arrange for distillation as before. 

• Distil the dried distillate slowly (1-2 drops per second) through the column and collect as fairly pure butyraldehyde all that distils below 76 °C. The yield is 13 g (32%). 

• Pure butyraldehyde boils at 74.5 °C. The i.r. spectrum reveals absorptions at 2700, 2800 (0=C— H) and 1705 cm" 1 (C=0). Assign the p.m.r. absorptions which occur at (50.95 (t, 3H), 1.61 (m, 2H), 2.36 (d of t, 2H) and 9.68 (t, 1H). The m.s. shows principal fragment ions at m/z 72, 56, 44, 43, 41, 29.

Cognate preparation: Propionaldehyde (propanal) 

• Use 34 g (42.5 ml, 0.567 mol) of propan-1-ol and a solution containing 56 g (0.188 mol) of sodium dichromate dihydrate, 300 ml of water and 40 ml of concentrated sulphuric acid. 

• The experimental details are identical with those for butyraldehyde, except that the temperature at the top of the column is not allowed to rise above 70-75 °C, and during the subsequent heating for 15 minutes the liquid passing over below 80 °C is collected; the receiver must be cooled in ice.The yield of propionaldehyde, b.p. 47-50 °C, is 12 g (36%).