SYNTHESIS OF 2-BROMOHEXANOIC ACID

• Place 100g (107 ml, 0.86 mol) of freshly distilled, dry hexanoic acid (b.p. 202-205 °C) (1) and 150g (48 ml, 0.94 mol) of dry bromine in a 500-ml flask equipped with a reflux condenser, the top of which is connected to a gas absorption device. 

• Momentarily remove the condenser and add cautiously 1.5 ml of phosphorus trichloride. 

• Heat the mixture on a water bath to 65-70 °C, when reaction will commence and hydrogen bromide is smoothly evolved. 

• Towards the end of the reaction allow the temperature of the bath to rise to 100 °C. 

• The reaction is complete when all the bromine has reacted (about 4 hours).

• Distil the reaction mixture under reduced pressure using a water pump; much hydrogen bromide is evolved and a fraction of low boiling point passes over. 

• When all the low boiling point fraction has distilled, connect the flask to an oil pump via a trap containing sodium hydroxide pellets and collect the 2-bromo-hexanoic acid at 116-125 °C/8mmHg (or at 132-140°C/15mmHg). The yield is 145 g (86%). Upon redistillation the 2-bromohexanoic acid passes over almost entirely at 128-131 °C/10mmHg.

Notes to keep in mind:

1. The acid may be dried by adding toluene and distilling out the toluene-water azeotrope, before distilling the dried acid.

Cognate preparations: 2-Bromopropanoic acid

• Proceed as detailed for 2-bromohexanoic acid using 64g (64.5 ml, 0.86 mol) of freshly distilled, dry propanoic acid (b.p. 139-1 42 °C), 150g (48 ml, 0.94 mol) of dry bromine and 1.5 mol of phosphorus trichloride. 

• The reaction commences on warming to about 50 °C. 

• Collect the 2-bromopropanoic acid at 95-97 °C/10mmHg or at 100-102 °C/1 5 mmHg. The yield is 110g (83%).

𝛂-Bromovaleric acid (2-bromopentanoic acid):

• Use 88g (0.86 mol) of valeric acid; allow reaction to proceed at 80 °C and finally heat for 2 hours at 100 °C. 

• Collect the a-bromovaleric acid at 145 °C/15 mmHg; the yield is 125g (80%).

𝛂-Bromoisovaleric acid (2-bromo-3-methylbutanoic acid):

• Use 88g of isovaleric acid; proceed as for 2-bromohexanoic acid, allowing the reaction to proceed at 70-80 °C for about 10-20 hours (until bromine vapour is no longer evident), then add a further 2.5 ml of bromine and heat at 100 °C for about 2 hours. 

• Distil and collect a- bromoiso valeric acid at 100-125 °C/15mmHg. The yield is in the region of 80 per cent.