SYNTHESIS OF UNDEC-10-YNOIC ACID

STEP 1: Purify commercial undecylenic acid by distillation of, say, 250 g under dimin­ished pressure and collect the fraction, b.p. 152-154 °C/6 mmHg; this has a freezing point of 23 °C. Dissolve 108 g (0.58 mol) of the purified undecylenic acid in 285 ml of dry carbon tetrachloride (CAUTION, shown below Note 2) in a 1-litre three-necked flask provided with a sealed stirrer unit, a dropping funnel and a reflux condenser. 

STEP 2: Cool the flask in a freezing mixture of ice and salt, stir the solution and add 96 g (31 ml, 0.6 mol) of dry bromine during a period of 1 hour: allow the mixture to warm up gradually to the tem­perature of the laboratory. 

STEP 3: Remove the carbon tetrachloride with a rotary evaporator and pour the residue into a large evaporating dish (fume cup­board). 

STEP 4: Upon standing 1-2 days (more rapidly when left in a vacuum desic­cator over silica gel), the dibromo acid crystallises completely. 

STEP 5: The yield is quantitative.

STEP 6: Transfer the solid dibromo acid to a 2-litre round-bottomed flask attached to a reflux condenser, add a solution of 263 g of potassium hydroxide in 158 ml of water, and heat in an oil bath at 150-160 °C for 8 hours. 

STEP 7: Consider­able frothing occurs, but this is reduced by the addition of small quantities (about 0.1 g) of a suitable detergent, e.g. sodium dodecyl benzenesulphonate, from time to time. 

STEP 8: Allow the mixture to stand overnight, add 1500 ml of water, shake until all the solid dissolves and acidify with dilute sulphuric acid to Congo red. 

STEP 9: A solid cake of acid separates on the surface of the liquid after standing for several hours. 

STEP 10: Extract with four 250 ml portions of ether, dry with anhydrous sodium or magnesium sulphate and remove the ether by dis­tillation on a water bath. 

STEP 11: Transfer the residue to a 250-ml flask fitted with a Claisen still-head and distil cautiously under diminished pressure using a free flame. 

STEP 12: A little ether and water pass over first and the temperature rises rapidly to 175 °C/15 mmHg. 

STEP 13: Collect separately the fractions (a) b.p. 177-182 °C/ 15 mmHg (52 g) and (b) 182-200 °C/15 mmHg (15 g). 

STEP 14: The flask contains a large residue, which is discarded. Fraction (a) solidifies completely on cooling and has m.p. 37-41 °C; upon recrystallisation from light petroleum, b.p. 60­80 °C, 34 g (32%) of pure undec-10-ynoic acid, m.p. 41-42 °C, are obtained. 

STEP 15: A further quantity of product is obtained from fraction (b), which solidifies to a slightly sticky solid: upon recrystallisation from light petroleum, b.p. 60­80 °C, a sticky solid separates, which, after spreading upon a porous tile, becomes colourless and has m.p. 41-52 °C (3 g).

Cognate preparations. Phenylpropynoic acid: 

STEP 1: Place a solution of 88 g (84 ml, 0.5 mol) of ethyl cinnamate (Expt 6.137) in 50 ml of carbon tetrachloride in a 500-m1 round-bottomed flask. 

STEP 2: Immerse the flask in ice and add 80 g (25.5 ml, 0.5 mol) of bromine from a separatory funnel slowly with frequent shaking. 

STEP 3: The halogen will disappear rapidly at first, but more slowly towards the end of the reaction; no hydrogen bromide is evolved and the time of the addition is about 20-25 minutes. 

STEP 4: Allow the mixture to stand for 1 hour, pour the solu­tion into a large evaporating dish and permit the excess of bromine and the carbon tetrachloride to evaporate spontaneously in the fume cupboard. 

STEP 5: The crude ethyl 2,3-dibromo-3-phenylpropanoate will remain as a solid cake; this can be dried by pressing between large filter papers. 

STEP 6: The yield of crude ester, m.p. 66-71 °C, is 140 g (83%) (1).

STEP 7: Dissolve 85 g of potassium hydroxide in 400 ml of rectified spirit by heat­ing in a 1500-ml round-bottomed flask, provided with a reflux condenser, on a water bath. 

STEP 8: Cool to 40-50 °C, and add 112 g (0.33 mol) of the crude dibromo ester; when the initial exothermic reaction has subsided, heat the mixture on a water bath for 5-6 hours. 

STEP 9: Pour the contents of the flask into a large beaker and, when cold, add concentrated hydrochloric acid with stir­ring until neutral to litmus. 

STEP 10: Cool, filter the precipitated solids at the pump and wash with a little alcohol. 

STEP 11: Set the solids (A) aside. Transfer the filtrate to the original flask and distil the liquid until the temperature of the vapour reaches 95 °C. 

STEP 12: Combine the residue in the flask with the precipitated solids (A), dissolve in 270 ml of water, add about 300 g of crushed ice and cool the flask in an ice bath. 

STEP 13: Stir the mixture mechanically, and add 20 per cent sul­phuric acid slowly until the solution is strongly acid to Congo red. 

STEP 14: Allow to stand for 20 minutes, filter off the dark-coloured crude phenylpropynoic acid at the pump and wash it with three 15 ml portions of 2 per cent sulphuric acid. 

STEP 15: Dissolve the solid in about 300 ml of 5 per cent sodium carbonate solution, add 6 g of decolourising charcoal and heat on a water bath for 30 minutes with occasional shaking. 

STEP 16: Filter through a fluted filter paper, cool the filtrate in ice and then add 70 g of crushed ice. 

STEP 17: Stir the solution mechanically and add 20 per cent sulphuric acid slowly until acid to Congo red. 

STEP 18: After 20 minutes, filter the precipitated acid by suction, wash with 15 ml of 2 per cent sulphuric acid, then with a little water, and dry in the air. 

STEP 19: The yield of pure phenylpro­pynoic acid, m.p. 134-135 °C, is 23 g (47%).

Notes to keep in mind:

1. To obtain the pure dibromo ester, recrystallise from light petroleum, b.p. 60-80 °C; the recovery of the pure ester, m.p. 75 °C, is 85 per cent.

2. Carbon tetrachloride is a suspect carcinogen; avoid breathing vapour and contact with the skin and eyes.