Phosphorus tribromide alkyl alcohols

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

PBrs (phosphorus tribromide) converts alcohols into alkyl bromides. 

Phosphorus tribromide converts alcohols to alkyl bromides. 

Alcohol Phosphorus tribromide Alkyl bromide Phosphorous acid... 

Thionyl chloride and phosphorus tribromide are specialized reagents used to bring about particular functional group transformations For this reason we won t present the mechanisms by which they convert alcohols to alkyl halides but instead will limit our selves to those mechanisms that have broad applicability and enhance our knowledge of fundamental principles In those instances you will find that a mechanistic understand mg IS of great help m organizing the reaction types of organic chemistry... 

Reaction of alcohols with phosphorus tribromide (Section 4 13) As an alternative to converting alco hols to alkyl bromides with hydrogen bromide the inorganic reagent phosphorus tribromide is some times used... 

Alkyl halides are such useful starting materials for preparing other functional group types that chemists have developed several different methods for converting alcohols to alkyl halides. Two methods, based on the inorganic reagents thionyl chloride and phosphorus tribromide, bear special mention. 

Phosphorus tribromide reacts with alcohols to give alkyl bromides and phosphorous acid. 

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

The mechanism for the reactions with phosphorus halides can be illustrated using phosphorus tribromide. Initial reaction between the alcohol and phosphorus tribromide leads to a trialkyl phosphite ester by successive displacements of bromide. The reaction stops at this stage if it is run in the presence of an amine which neutralizes the hydrogen bromide that is formed.9 If the hydrogen bromide is not neutralized the phosphite ester is protonated and each alkyl group is successively converted to the halide by nucleophilic substitution by bromide ion. The driving force for cleavage of the C—O bond is the... 

Both reactants in the Williamson ether synthesis usually originate in alcohol precursors. Sodium and potassium alkoxides are prepared by reaction of an alcohol with the appropriate metal, and alkyl halides are most commonly made from alcohols by reaction with a hydrogen halide (Section 4.7), thionyl chloride (Section 4.13), or phosphorus tribromide (Section 4.13). Alternatively, alkyl p-toluenesulfonates may be used in place of alkyl halides alkyl p-toluenesulfonates are also prepared from alcohols as their immediate precursors (Section 8.14). 

Exercise 15-21 Write the steps that could plausibly take place in the reaction of a primary alcohol with phosphorus tribromide in the presence of the weak base pyridine to give an alkyl bromide. 

A concise synthesis of the methobromide of ungeremine (45) featured the oxidative photocyclization of the Schiff base 43 to provide 44 (Scheme 2). When 44 was reduced with LiAlH4 followed by treatment of the resulting alcohol with phosphorus tribromide, an alkyl bromide was generated that suffered spontaneous cyclization to furnish 45 (106). 

Fig. Conversion of an alcohol to an alkyl halide using (a) thionyl chloride (b) phosphorus tribromide.

Phosphorus tribromide has three bromine atoms present and each PBr, molecule can react with three alcohol molecules to form three molecules of alkyl bromide. 

R—OH + SOCl2 — or PBr3 — R —Cl or R—Br SNlor Sm2 thionyl chloride, phosphorus tribromide, and phosphorus triiodide provide a good one-step procedure for conversion of an alcohol to an alkyl chloride, bromide, or iodide... 

Several phosphorus halides are useful for converting alcohols to alkyl halides. Phosphorus tribromide, phosphorus trichloride, and phosphorus pentachloride work... 

The synthesis of bufotenine itself followed closely upon the proof of its structure. Hoshino and Shimodaira reduced the ethyl ester of 5-ethoxy-indole-3-acetic acid by the Bouveault-Blanc procedure to the corresponding primary alcohol, which was treated with phosphorus tribromide and then dimethylamine, to give the ethyl ether of bufotenine, which was demethylated with aluminum chloride (130). In a later synthesis, 2,5-dimethoxybenzyl cyanide (XXIII) was alkylated by Eisleb s method with dimethylaminoethyl chloride in the presence of sodamide to give l-(2,5-dimethoxyphenyl)-3-dimethylaminopropyl cyanide (XXIV), which was then hydrogenated over Haney nickel to yield 2-(2,5-di-methoxyphenyl)-4-dimethylaminobutylamine (XXV R = Me). De-methylation of this with hydrobromic acid, followed by oxidation of the product (XXV R = H) with potassium ferricyanide yielded bufotenine (XIX) via the related quinone (109). 

Phosphorus tribromide, PB Reacts with alcohols to yield alkyl bromides (Section 10.6). 

Primary and secondary alcohols are best converted into alkyl halidesby treatment with either thionyl cliloride (SOCb) or phosphorus tribromide (PBr3). These reactions, which normally take place readily under mild conditions, are less acidic and less likely to cause add-catalyzed rearrangements than tlie HX method. 

In a similar fashion, the treatment of a 1° or 2° alcohol with phosphorus tribromide, PBr3, forms an alkyl bromide. 

Reactions with —OH Groups and Epoxides.—The formation of A -l,2-oxaphos-pholen derivatives from propargylic alcohols and phosphorus trichloride has been studied in detail. Intermediate phosphites (24) and allenic phosphonates (25) are described, and the A -l,2-oxaphosphoIen is produced in the final stage, as shown. Improved conditions have been outlined for the preparation of allylic bromides (26) from allylic alcohols and phosphorus tribromide. Related reactions of primary alcohols with the complex of phosphorus trichloride and DMF lead to the chloride (27) 22 addition of zinc bromide to the reaction results in the formation of alkyl bromides, but an attempt to extend this exchange to the preparation of cyanides was not successful. ... 

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