Phosphorus trihalides, reactions with alcohols

The reaction is performed most simply by the addition of the propargylic alcohol to a solution of the phosphorus halide. Rearrangement of the phosphorus ester proceeds at ambient temperature or with mild heating. When phosphorus trihalides are used, the product can be isolated as the phosphonic dichloride.168169 Aqueous workup provides the phosphonic acid.162 In most instances, however, a dialkyl phosphorochloridite with only a single halogen on phosphorus available for reaction with alcohol has been used.165 170 174... 

Reaction with phosphorus trihalides (Section 4 13) Phosphorus trichloride and phosphorus tribromide convert alcohols to alkyl halides... 

Elder, R.C., Florian, L.R., Kennedy, E.R., and Macomber, R.S., Phosphorus containing products from the reaction of propargyl alcohols with phosphorus trihalides. II. The crystal and molecular structure of 2-hydroxy-3,5-di-ferf-butyl-l,2-oxaphosphol-3-ene 2-oxide, /. Org. Chem., 38, 4177, 1973. 

Reaction with phosphorus trihalides Alcohols are converted to alkyl bromides by reaction with phosphorus tribromide (Refer Unit 10, Class XII). 

Mechanism of the Reaction with Phosphorus Trihalides The mechanism of the reaction of alcohols with phosphorus trihalides explains why rearrangements are uncommon and why phosphorus halides work poorly with tertiary alcohols. The mechanism is shown here using PBr3 as the reagent PC13 and PI3 (generated from phosphorus and iodine) react in a similar manner. 

Primary, secondary, and tertiary alcohols undergo nucleophilic substitution reactions with HI, HBr, and HCl to form alkyl halides. These are Sn2 reactions in the case of primary alcohols and SnI reactions in the case of secondary and tertiary alcohols. An alcohol can also be converted into an alkyl halide by phosphorus trihalides or thionyl chloride. These reagents convert the alcohol into an intermediate with a leaving group that is readily displaced by a halide ion. 

This reaction is simple substitution where the halogen, either bromine or iodine will replace the 0H group, and the phosphorus forms the by-product phosphorus acid (HaPOa). With the above facts and the general synthesis at hand, the products of each reaction can be obtained after reacting the alcohol with a phosphorus trihalide. 

Examination indicates that the carbon skeleton remains intact in this synthesis. All that must be done is to alter the carboxylic acid to the alkyl bromide of the same carbon number. Now, if one had n-Ci1H23CH2OH (the corresponding alcohol), 1-bromododecane could be produced by reaction with the phosphorus trihalide PBr3. To obtain the alcohol, use the fact that lithium aluminum hydride, LiAli, is one of the few reagents that reduce a carboxylic acid to an alcohol of the same carbon length. Overall, then, the synthesis becomes ... 

The interaction of phosphorns trihalides with alcohols involves a series of consecntive and concnr-rent reactions. In the case of phosphorus trichloride, these can be represented by Eqnations 6.306 through 6.309 in which triesters are first formed, followed by stepwise dealkylation to phosphorous acid. 

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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