Alkyl bromides, preparation

Substitution of Br for alcoholic OH by means of HBr In principle, alcohols are allowed to react with 48% aqueous HBr, with dry HBr, or with aqueous HBr containing concentrated H2S04. The last of these procedures is the most convenient, but it has the disadvantage that alkyl bromides prepared in that way tend to lose HBr on storage with HBr the resulting olefins form polymers that may discolor the product.881... 

Alkyl fluorides may be prepared in moderate yield by interaction of an alkyl bromide with anhydrous potassium fluoride in the presence of dry ethylene glycol as a solvent for the inorganic fluoride, for example ... 

Owing to the comparatively negligible difference in the cost of bromine and the equivalent quantity of constat boiling point hydrobromio acid, there is little to be gained—apart from the instructional value—in preparing the hydrobromio acid from bromine in the preparation of alkyl bromides. 

Pure dialkylanilines may be prepared by refluxing the monoalkylaniline (1 mol) with an alkyl bromide (2 mols) for 20-30 hours the solid product is treated with excess of sodium hydroxide solution, the organic layer separated, dried and distilled. The excess of alkyl bromide paases over first, followed by the dialkylaniline. Di-n-propylaniline, b.p. 242-243°, and di-n-butylaniline b.p. 269-270°, are thus readily prepared. 

Other alkylbenzenes may be prepared similarly by using the appropriate primary or secondary alkyl bromide. 

The lithiation of allene can also be carried out with ethyllithium or butyl-lithium in diethyl ether (prepared from the alkyl bromides), using THF as a cosolvent. The salt suspension which is initially present when the solution of alkyllithium is cooled to -50°C or lower has disappeared almost completely when the reaction between allene and alkyllithium is finished. 

The alkylations proceeded much more slowly, when ethyl- or butyllithium in diethyl ether, prepared from the alkyl bromides, had been used for the metallation of allene, in spite of the presence of THF and HMPT as co-solvents. 

Note 1. Butyl- or ethyllithium in diethyl ether, prepared from the alkyl bromide, contains LiBr, which may react with chlorine to form bromine, so that RCeC-Br will also be formed. 

Nickel-allyl complexes prepared from Ni(CO)4 and allyl bromides are useful for the ole-fination of alkyl bromides and iodides (E.J. Corey, 1967 B A.P. Kozikowski, 1976). The reaction has also been extended to the synthesis of macrocycles (E.J. Corey, 1967 C, 1972A). 

The synthesis of these disubstituied thioureas takes place in three steps. First the alkyl bromide is prepared by the action of hydrobromic acid on the corresponding alcohol (518). Then the dialkylcyanamide is obtained by treatment at 25°C with calcium cyanamide. The yields are of the order of 30 to 60%. Thioureas are obtained in a third step from the cyanamide by reaction at 40 C with HjS in the presence of pyridine. Yields ranged from 57 to 90% (518),... 

Bromination is normally used only to prepare tertiary alkyl bromides from alkanes... 

Triphenyl phosphine dibromide [1034-39-5] M 422.1, m 23.5 , 245-255 (dec). Recrystd from MeCN-Et20. Although it has been recrystd from EtOH, this is not recommended as it converts alcohols to alkyl bromides. It deteriorates on keeping and it is best to prepare it afresh. [Anderson and Freenor J Am Chem Soc 86 5037 1964 Homer et al. Justus Liebigs Ann Chem 626 26 1959.]... 

Another improvement was reported by Leonard et al. in their preparation of a promising antimalarial, Endochin. The improvement was the alkylation of intermediate enamino-ester 28 by reaction with NaOEt followed by alkylation with an alkyl bromide, rather than forming 29 by reaction of 27 and a suitable P-keto-ester. This provided the important intermediate 29 required for cyclization to Endochin (30). Endochin was first reported by German scientists but was not publicly disclosed until the Department of Commerce made this information available after World War II.Leonard was able to improve upon the chemistry reported by Andersag and Salzer in 1940 and isolated Endochin in 40% overall yield from m-anisidine (27). 

Cradally, dais allows orgatiozitic reagents to be prepared from less reactive aryl bromides and secondary or tertiary alkyl bromides.. Alternatively, orgatiozitic iodides can be prepared by means of a paliadiurniOj-catalyzed reaction between alkyl iodides and Et2Zn iSdaeme 2.25) [53-56],... 

Differences in solubility of the reactants may for example be utilized as follows. Sodium iodide is much more soluble in acetone than are sodium chloride or sodium bromide. Upon treatment of an alkyl chloride or bromide with sodium iodide in acetone, the newly formed sodium chloride or bromide precipitates from the solution and is thus removed from equilibrium. Alkyl iodides can be conveniently prepared in good yields by this route. Alkyl bromides are more reactive as the corresponding chlorides. Of high reactivity are a-halogen ketones, a-halogen carboxylic acids and their derivatives, as well as allyl and benzyl halides. 

When carbon tetrabromide is used, the alkyl bromide is formed. Providing moisture is excluded from the reaction mixture (HX is formed otherwise), the reaction conditions are completely neutral, affording a convenient preparation of the halides of acid-sensitive substrates (for example, sugars). 

The procedure for the preparation of other alkyl bromides is the same. The following alkyl bromides have been prepared by this reaction in the yields shown in Table 6.2. 

The following alkyl bromide can be prepared by reaction of the alcoho (S)-2-pentanol with PB13- Name the compound, assign (R) or (S) stereochemistry and tell whether the reaction of the alcohol occurs with retention of the same stereochemistry or with a change in stereochemistry (reddish brown = Br). 

The trimethylene chlorobromide used boiled at 142-1470. It may be prepared in 75-85 per cent yields from trimethylene chlorohydrin (p. 112) by the general method for the preparation of alkyl bromides described in Org. Syn. 1,1. 

The addition of ethylene oxide to the alcohol causes a decrease in the melting point of the corresponding salt of the alcohol ether sulfate in comparison with the same alcohol sulfate. Weil et al. [65] prepared pure hexadecyl and octadecyl ether alcohols from the corresponding alkyl bromide and glycols with... 

Monoalkyltin(IV) compounds can be prepared under suitable conditions from tindl) compounds (44-46). Tin(II) bis(acetylacetonate) (acac) and related compounds react readily with a variety of alkyl bromides or iodides, to give the product RSn(acac)2X, e.g.,... 

Boiling-point elevation and freezing-point depression measurements have demonstrated that at all concentrations in THF and at low concentrations in ether (up to 0.1M) Grignard reagents prepared from alkyl bromides and iodides are monomeric, that is, there are few or no molecules with two magnesium atoms. Thus, part of the Schlenk equilibrium is operating... 

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