SYNTHESIS OF ISOPROPYL BROMIDE (2-Bromopropane)

• Mix 40 g (51ml, 0.67 mol) of propan-2-ol (isopropyl alcohol) with 460 g (310 ml) of constant boiling-point hydrobromic acid in a 500-ml flask fitted with a double surface condenser, add a few boiling chips and distil slowly (1-2 drops per second) until about half of the liquid has passed over. 

• Separate the lower alkyl bromide layer (70 g), and redistil the aqueous layer when a further 7g of the crude bromide will be obtained (1). 

• Shake the crude bromide in a separatory funnel successively with an equal volume of concentrated hydrochloric acid (2), water, 5 per cent sodium hydrogen carbonate solution and water, and dry with anhydrous calcium chloride. 

• Distil from a 100-ml flask; the isopropyl bromide passes over at 59 °C. 

• The yield is 66 g (81%). The p.m.r. spectrum (CC1 4 , TMS) shows signals at 3 1.70 (d, 6H, Me 2 CH— ) and 4.21 (sept, 1H, — CH— ).

Notes to keep in mind:

1. The residue in the flask may be mixed with the aqueous layer of the first distillate, 40 g of propan-2-ol added, and the slow distillation repeated. The yield of crude isopropyl bromide in the second distillation is only slightly less than that obtained in the original preparation. Subsequently most of the residual hydrobromic acid may be recovered by distillation as the constant boiling point acid (126 °C).

2. The hydrochloric acid washing removes any unchanged alcohol which may be present.

Cognate preparations: Bromocyclohexane

• Use 50 g (0.5 mol) of cyclohexanol and 260 g (176 ml) of 48 per cent hydrobromic acid and distil all the mixture slowly (6 hours). 

• Add a little water to the distillate, separate the lower layer of crude bromide, and purify as above. 

• Collect the bromocyclohexane at 163—165°C(60g,74%).

                                                                                                                  

Bromocyclopentane:

• Use 43 g (0.5 mol) of cyclopentanol (Refer HEPTAN-2-OL to  see more details) and 260 g (176 ml) of 48 per cent hydrobromic acid. Collect the bromocyclopentane at 135-137 °C (55 g, 74%).