Formation of alkyl bromides

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. 

These are not stable and decompose with formation of alkyl bromide and dialkyl cyanamides, as follows ... 

The system of PhsP-diethyl azodicarboxylate-hydroxy compound-LiBr allows the formation of alkyl bromides under mild conditions (equation An 5N2-type displacement of an intermediate alkoxy-... 

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. ... 

PROBLEM 7.48 Write a mechanism for the formation of alkyl bromides from the reaction of alcohols and PBrg. 

Hydrogen bromide is used as a reagent and catalyst in several types of organic reactions such as the formation of alkyl bromides from alcohols. 

Formation of Alkyl Bromides. Alcohols react with excess TMS-Br (1.5-4 equiv) at 25-50 °C to form the alkyl bromide and hexamethyldisiloxane (eq 12). Benzylic and tertiary alcohols react faster than secondary alcohols. 

Conversion of Alcohols to Alkyl Bromides. The reaction of alcohols with Triphenylphosphine and Carbon Tetrabromide results in the formation of alkyl bromides. The conditions are sufficiently mild to allow for the efficient conversion of alcohols into the corresponding bromides. The uridine derivative (eq 1) is transformed into its bromide with PhsP and CBr4. ... 

As it can be seen, the oxo group in 142 was inactive while the thioxo group at C-3 was very reactive. Formation of pyrazoles in the oxo compound was explained by the reactivity of C-6. When triazepine 141 was treated with 1 or 2 equiv of alkyl bromide, only 152 was obtained but when a large extent of the alkylating agent was used, a small amount of the dialkylated 150 was formed. The different reactivity of 141 and 142 was explained by a softer basic character of the thioxo group in comparison with the oxo group. 

Acetate functional groups are quite prevalent in insect sex pheromones and have been introduced into a wide variety of alkyl bromides by reaction with sodium acetate in DMF372 or acetic acid371. As is generally the case, the use of PTC allows reaction under milder conditions and significantly improves yields and reduces reaction times. In one example the reaction of alkyl halides with formate anion has given excellent yields of a variety of alkyl formates212. 

Replacement of iron(II) ion as a catalyst by radiation and use of peracid esters leads to more efficient production of radicals. Thus with a 1200 W source and t-butyl peracetate, methylation of caffeine is 28 times faster than using r-butyl hydroxide (77JA5096). An alkaline solution of theophylline containing benzyl bromide similarly gave with UV radiation both the 8- and the 7-benzyl derivatives (76AJC891). Formation of alkyl radicals in these various reactions are summarized in Scheme 28. 

The same study showed that Pd gives better results than Pd" catalysis in the yields for formation of alkylated products. Unfortunately, except for a single report, the regioselectivity was not examined. Indeed, when cinnamyl bromide was reacted with phenylmagnesium bromide in the presence of PdlPPhj), the 5, 2 coupled product 255 was the only alkylated compound observed, in 77% yield with 22% of biphenyl [Eq. (85)]. 

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