Phosphorus pentachloride alkyl alcohols

If the diselenide acid is treated with phosphorus pentachloride the free acid chloride is not isolated, but the compound (HCl.Se.C6H4.COCl)2. The normal acid chloride, however, is formed by using thionvl chloride it crystallises from benzene or toluene in needles, M.pt. 178° to 174° C. If the hydrochloride, M.pt. 65c to 66° C., be boiled with methyl alcohol, a compound is obtained, M.pt. 74° to 75° C., which is the hydrochloride of the methyl ester of the diselenide acid, (Se.C6H4.COMe)2, which may be liberated by dilute sodium hydroxide. The free ester melts at 143° to 144° C. The corresponding ethyl ester melts at 129° to 130° C. and its hydrochloride at 91° to 92c C. Both esters may be formed also from the silver salt of the acid by heating with the alkyl iodide for two to three hours at 1X0° to 1203 C. 

Alkyl chlorides have been obtained by the direct chlorination of hydrocarbons 1 and by the addition of hydrogen chloride to olefines.2 However, they have usually been prepared from the corresponding alcohol by the action of a number of reagents, such as dry hydrogen chloride,3 dry hydrogen chloride in the presence of zinc chloride,4 phosphorus trichloride,3 phosphorus oxychloride,6 phosphorus pentachloride,7 phosphorus trichloride... 

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

Phosphorus pentachloride is one of the most powerful reagents by which the hydroxyl of organic compounds can he replaced by chlorine. Alkyl chlorides, RC1, from alcohols, and acid chlorides, RCOC1, from acids, are often prepared by this method. The pentachloride is thereby converted first into the oxychloride, P0C13, which may itself be used for the substitution of OH by Cl. 

Many of the convenient methods of preparing alkyl halides are based on the reactions of alcohols with reagents such as thionyl chloride and phosphorus pentachloride. These are dealt with in more detail in Section 2.3 on alcohols. The nucleophilic substitution of an alkyl methanesul-fonate or toluene-4-sulfonate with a sodium or potassium halide is a useful method. 

The reaction with thionyl chloride affords a chlorosulfite, the decomposition of which may generate an alkyl chloride by the S i (substitution, nucleophilic, internal) mechanism (Scheme 2.16). This reaction, w hich may proceed by an ion pair, can lead to the retention of configuration of an asymmetric secondary alcohol in the conversion to the alkyl chloride. This is in contrast to the inversion of configuration found with the reaction with phosphorus pentachloride and with the nucleophilic displacement of a leaving group. 

The most familiar of the sulfonic adds derived from camphor is 10-camphorsulfonic add (44, Reychler s acid45). Both enantiomers are commercially available and convenient procedures exist for their preparation by sulfonation of camphor (ref 46 exemplifies the racemate, but the procedure works equally well for optically active camphor). The free acid is often applied to the resolution of basic compounds such as amines. A detailed review on the use of derivatives of this acid as auxiliaries has been given3. Esters of this add are normally obtained by the reaction of the alcohols with the sulfonyl chloride which is also commerdally available (or readily obtained by the reaction of the free acid with phosphorus pentachloride or thionyl chloride46,48). Such esters with unsaturated alcohols have been used for diastereoselective [1,2] sigma tropic rearrangements (Section D.1.6.3.3.). Allyl esters have been used for enantioselective alkylation reactions, in which camphorsulfonic acid reacts as the chiral leaving group (Section D.1.1.2.2.). 

Noncombustible solid. Phosphorus pentachloride fumes in moist air reacts vigorously with water and alcohol. The hydrolysis products are HCl and phosphorus oxychloride, POCI3. The latter reacts further with water to yield HCl and H3PO4. Both these reactions are exothermic. Alcohols decompose it, producing toxic alkyl chlorides. 

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