Secondary alcohol reaction with hydrobromic acid

A) Sn2 substitution at the allylic alcohol with hydrobromic acid followed by reaction with the requisite secondary amine, or... 

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 reaction reaches equilibrium rapidly if the hydroxyl group is attached to a reactive radical, as in tertiary alcohols. In such cases, excess of the halogen acid is shaken with the alcohol, and the mono-halogen compound is separated. With primary and secondary alcohols, equilibrium is reached more slowly, and a catalyst (such as zinc chloride in the case of hydrochloric acid, and sulfuric acid in the case of hydrobromic acid) is used. On account of its cost, hy-driodic acid is not used, but iodine and phosphorus, which react as follows ... 

It is necessary to use different procedures to prepare secondary alkyl bromides from secondary alcohols because such alcohols are easily dehydrated by concentrated sulfuric acid to give alkenes by way of Equations 14.13 and 14.14. In fact, the acid-catalyzed dehydration of secondary and tertiary alcohols is a common method for synthesizing alkenes (Sec. 10.3). This problem may be circumvented by using concentrated hydrobromic acid however, it is better to prepare secondary alkyl bromides by the reaction of secondary alcohols with phosphorus tribromide, PBrg (Eq. 14.16). 

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