Secondary phosphorus tribromide

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 ... 

Prim ry and secondary alcohol are best converted into olWyl halides by treatment with such reage nts as thicmyL chloride CSOClj> or phosphorus tribromide (PBra). These reactions, which normally take place readily under mild conditions, are loss acidic and less tikoly to cause acid-catalyzed rearrangemetitB than the HX method. 

The method has been extended to the preparation of difunctional compounds. Dihalides Including the mixed variety are formed in 90 to 98% yields. Primary unsaturated bromides of the type RCH=CHCH,Br have been formed from the corresponding alcohols by the action of phosphorus tribromide and pyridine at a low temperature without any apparent rearrangement. However, the corresponding secondary-carbinol system, RCHOHCH=CH is very susceptible to allylic isomerization. The formation of a, -acetylenic bromides from acetylenic alcohols and phosphorus tribromide is common (40-70%). An acetylenic-allenic isomerization has been observed, viz.,... 

The allylic secondary alcohol l,5-hexadiene-3-ol reacts with phosphorus tribromide plus 2 drops of 48% hydrobromic acid to give in high yield a mixture of... 

Preparation Alkenes when treated with hydrogen halides (HCl, HBr, and HI) or halogens (Cl and Br ) yield alkyl halides and dihaloalkanes respectively. Another useful method of preparing alkyl halides is by treating an alcohol with a hydrogen halide (HX = HCl, HBr, or HI). The reaction works best for tertiary alcohols. Primary and secondary alcohols can be converted to alkyl halides by treating them with thionyl chloride (SOCy or phosphorus tribromide (PBr ). 

Since rearrangements can occur when some alcohols are treated with hydrogen halides, how can we successfully convert a secondary alcohol to an alkyl halide without rearrangement The answer to this question comes in the next section, where we discuss the use of reagents such as thionyl chloride (SOCI2) and phosphorus tribromide (PBrs). 

Primary and secondary alcohols react with phosphorus tribromide to yield alkyl bromides. 

The l,2-dimethyl-3-ethylcyclopropane 12 was similarly synthesized from 3-methyl-2,4-dibromohexane 11. A yield of 90% was obtained in the ring closure step. The secondary-tertiary and secondary-secondary 1,3-dibromides, 8 and 11, were prepared in 90% yields by the reaction of phosphorus tribromide with the diols, 7 and 10, at low temperature (-24 °C). The low temperature prevents the loss of hydrogen bromide from the reaction mixture. 

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). 

Primary and secondary alcohols, which react only slowly with HBr and HCl, react readily with thionyl chloride and phosphorus trihalides, such as phosphorus tribromide, to give the corresponding alkyl hahdes. The products of these reactions are easily separated ftom the inorganic by-products. Thionyl chloride produces hydrogen chloride and sulfur dioxide, which are released from the reaction as gases. The chloroaUtane remains in solution. 

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