SYNTHESIS OF 2,2-DIMETHYLPROPANOIC ACID (Trimethylacetic acid; pivalic acid)

•  In a 3-litre three-necked flask, fitted with a thermometer, a mechanical stirrer and dropping funnel, place a solution of 160 g (4 mol) of sodium hydroxide in 1400 ml of water. 

• Cool to 0°C in an ice-salt bath. 

• Add 240 g (77 ml, 1.5 mol) of bromine with vigorous stirring at such a rate as to keep the temperature below 10 °C (15-20 minutes). 

• Cool again to 0°C, introduce 50 g (0.5 mol) of pinacolone keeping the temperature below 10 °C. 

• After the solution is decolourised (c. 1 hour), continue the stirring for 3 hours at room temperature. 

• Replace the thermometer by a knee tube connected to a condenser for distillation and replace the stirrer by a steam inlet to allow the bromoform and carbon tetrabromide (if present) to be separated by steam distillation; heat the flask with a powerful Bunsen burner. 

• Remove the burner, cool the reaction mixture to 50 °C and add 200 ml of concentrated sulphuric acid cautiously through the dropping funnel. 

• Heat the flask again; the trimethylacetic acid passes over with about 200 ml of water. 

• When all the trimethylacetic acid (the upper layer; 35-40 ml) has distilled, a liquid heavier than water (possibly brominated pinacolone) begins to pass over, Stop the distillation at this point, separate the trimethylacetic acid from the aqueous layer, and dry it by distillation with 25 ml of benzene (the latter carries over all the water) or with anhydrous calcium sulphate. 

• Distil under reduced pressure and collect the trimethylacetic acid 75-80 °C/20mmHg. The yield is 33 g (55%), m.p. 34-35 °C.

Cognate preparation: Cyclopropanecarboxylic acid

• Use 42 g (47 mol, 0.5 mol) of cyclopropyl methyl ketone and react with alkaline hypobromite solution exactly as in the above preparation except that the final period of stirring at room temperature need be only 1.5 hours. 

• After removal of bromoform by steam distillation, cool and cautiously acidify the solution to Congo red with 250 ml of concentrated hydrochloric acid. 

• Discharge the pale yellow colour by adding a little sodium metabisulphite solution. 

• Saturate the solution with salt and extract with four 300 ml portions of ether; dry the combined extracts with anhydrous sodium sulphate, and distil off the ether on a water bath through a short fractionating column. 

• Distil the residue under reduced pressure and collect the pure cyclopropanecarboxylic acid (a colourless liquid) at 92°C/22mmHg. The yield is 33 g (76%), ¹³C-n.m.r. spectrum (CDCl₃ , TMS) ẟ 9.2, 13.1 and 181.9.