Alcohols reaction with PBrj

Reaction with PBrg (Section 10.5B) Alcohols react with PBrj to initially displace Br to give a protonated dibromophosphite intermediate, which is displaced from the backside by Br to give the bromoalkane. Although some rearrangement may occur with this reagent, it is less likely than in the reaction of an alcohol with HBr. 

Conversion of a pr maiy alcohol into alkyl halides by (,2 reactions with OCl2 and PBrj. 

Conversion of a primary alcohol into alkyl halides by S 2 reactions with SOCI2 and PBrj. 

ROM—>RBr. In the cyclohexane series reaction of diols with PBr is attended with rearrangements. Thus treatment of the 1,3- and 1,4-cyclohexanediols with PBrj affords mixtures of cis- and trans-1,3- and l,4-dibromides. Eliel and Haber found that cu-d-i-butylcyclohexanol reacts with PBr to give trani-4-/-butylcyclo-hexyl bromide, together with a small amount of olefin and a mixture of dibromides. Thus PBr, is evidently superior to PBr, for conversion of secondary alcohols into bromides. However, the Hunsdiecker reaction seems to be the method of choice for preparation of cyclohexyl bromides. ... 

If we know which mechanism a compound reacts by, we know what sort of conditions to use to get good yields in substitutions. For example, if you look at a commonly used nucleophilic substitution, the replacement of OH by Br, you ll find that two quite different reaction conditions are used depending on the structure of the alcohol. Tertiary alcohols react rapidly with HBr to give tertiary alkyl bromides. Er/ma/y alcohols, on the other hand, react only very slowly with HBr and are usually converted to primary alkyl bromides with PBrj. The reason is that the first example is an S 1 reaction while the second is an 5 2 reaction by the end of this chapter you will have a clear picture of how to predict which mechanism will apply and how to choose appropriate reaction conditions. 

Reaction of a Primary Alcohol with PBrj (Section io.5B)... 

Now let s draw the forward scheme. Hydrolysis of acetonitrile gives acetic acid which is subsequently reduced to ethanol upon treatment with excess LAH, followed by water work-up. Upon treatment with PBrj, ethanol is converted to ethyl bromide which is then converted to ethyl magnesium bromide (a Grignard reagent). A Grignard reaction with acetaldehyde (produced by PCC oxidation of ethanol, as shown), followed by water work-up, produces the desired alcohol, 2-butanol. 

PBrj, SOCI2, or SOBr2 react well with secondary and primary alcohols. These reactions involve an Sj,j2 displacement, so inversion of stereochemistry is seen when the OH group is bonded to a chiral center. 

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