SYNTHESIS OF 3,3-DIMETHYLACRYLIC ACID (3-Methylbut-2-enoic acid)

• Fit a 1 -litre three-necked flask with two double surface condensers and a sealed stirrer unit. 

• Place 25 g (29 ml, 0.25 mol) of mesityl oxide, 50 ml of dioxane and a cold (10 °C) solution of sodium hypochlorite in 750 ml of water (1) in the flask, and stir the mixture. 

• Heat is evolved in the reaction and after about 5 minutes chloroform commences to reflux. 

• As soon as the reaction becomes very vigorous, stop the stirrer and cool the flask with water so that the chloroform refluxes gently; after 20-30 minutes, when the reaction has subsided, resume the stirring and continue it until the temperature of the mixture has fallen to that of the laboratory (2-3 hours). 

• Decompose the slight excess of hypochlorite by the addition of sodium metabisulphite (about 1 g), i.e. until a test-portion no longer liberates iodine from potassium iodide solution. 

• Replace one of the reflux condensers by a dropping funnel and add 50 per cent sulphuric acid (about 50 ml) with stirring and cooling until the solution is acid to Congo red paper. 

• Extract the cold solution with eight 50 ml portions of ether (2) and shake the mixture well during each extraction. 

• Dry the combined ethereal extracts with anhydrous calcium sulphate, and remove the ether and chloroform slowly on a water bath. 

• Distil the residue from a flask fitted with a Claisen still-head and a short fractionating column under diminished pressure and collect the acid at 100-106 °C/20mmHg; this fraction solidifies on cooling and melts at 60-65 °C. The yield is 13 g (51%), Recrystallise from hot water (1 g of acid in 10 ml of water) (3), cool the solution in ice for 2-3 hours, filter and dry overnight in a vacuum desiccator. 

• Alternatively, recrystallise from light petroleum, b.p. 60-80 °C. 

• Pure 3,3-dimethylacrylic acid has m.p. 68 °C; p.m.r. spectrum (CCl₄ , TMS) ẟ 1.95 (s, 3H), 2.19 (s, 3H), 5.69 (m, 1H) and 12.22 (s, 1H).

Notes to keep in mind:

1.  This solution is prepared by diluting 300 ml of commercial sodium hypochlorite (containing 10—14% available chlorine) to 750 ml with water.

2.  A continuous ether extractor (Fig. 2.92) gives more satisfactory results.

3.  Do not boil the aqueous solution for a long time as the acid is markedly steam-volatile.