Preparation of alkyl bromides from alcohols

The formation of alkyl bromides is more ready than that of the alkyl chlorides. Hence secondary as well as tertiary bromides can be obtained directly from the corresponding alcohols by heating with constant boiling hydrobromic acid (e.g. Expt 5.53), although in the case of primary alcohols the presence of sulphuric acid results, as a rule, in a more rapid reaction and in improved yields. 

The method is readily adapted for the preparation of dibromides from diols. Typical examples are provided in Expt 5.54. The cyclic ethers tetrahydrofuran and tetrahydropyran are readily cleaved by the hydrobromic acid-sulphuric acid medium, and this provides an alternative and convenient preparation of the corresponding a, co-dihalides. 

Promotion of an SN2 displacement mechanism, and hence greater regioselectivity, may be effected by the addition of liquid bromine to a warm suspension of purified red phosphorus in the appropriate alcohol. The reaction is of general application with primary alcohols (isobutyl alcohol to hexadecan-l-ol) the yields are over 90 per cent of the theoretical, but with secondary alcohols the yields are in the range 50-80 per cent (Expt 5.55). This method is to be preferred to the direct use (rather than the in situ generation) of phosphorus tribromide which is the more hazardous reagent. The outline mechanism may be represented thus  

Other highly regioselective brominating reagents are chlorotrimethylsilane/lith-ium bromide and hexamethyldisilane/pyridinium perbromide.88 An outline mechanism of the former reaction is formulated below and a preparative example is given in Expt 5.56. 

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 7 g 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 (CC14, TMS) shows signals at 51.70 (d, 6H, Me2CH—) and 4.21 (sept, 1H, —CH—). 

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