CONVERSION OF ALCOHOLS TO HALOALKANES

Tertiary phosphine oxides are also produced as significant by-products in several of the reactions of phosphines that have been noted previously, including the Wittig olefination and the conversions of alcohols to haloalkanes with triphenylphosphine as an adjunct reagent. The tertiary phosphine oxides produced in such reactions present a problem in chemical economics, as they themselves possess little chemical utility. The phosphine may be regenerated, but several steps are required, as previously noted with preparations of phosphines by reduction (see Section 3.2). 

On the basis of observations of the relative ease of reaction of alcohols with HX (go > 2° > 1°), it has been proposed that the conversion of tertiary and secondary alcohols to haloalkanes by concentrated FIX occurs by an Sfjl mechanism (Section 7.4) and involves the formation of a carbocation intermediate. Note Recall that secondary carbocations are subject to rearrangement to more stable tertiary carbocations (Section 5.4). 

In addition to the conversion of unactivated alkanes to alcohols, cytochrome P450 hemes transform alkenes to epoxides, arenes to phenols, and sulfides to sulfoxides to sulfones. Furthermore, they are involved in the biosynthesis and biodegradation of endogenous compounds such as steroids, fatty acids, prostaglandins and leukotrienes. Under anaerobic conditions, P450 will reductively dehalogenate haloalkanes to the corresponding alkanes. 

The approach to Chapters 8 and 9 is similar to that used in Chapters 6 and 7. However—and this is very important—alcohols have much more potential for chemical conversion than haloalkanes do. You must be prepared even more than before to focus on the functional groups and their polar bonds as sites of possible retictivity. A compari.son of. first, the bonds and. second, the potential bonding changes available in alcohols and haloalkanes is u.seful. Haloalkane chemistry involves mainly three bonds ... 

In this section, we study the acidity and basicity of alcohols, their dehydration to alkenes, their conversion to haloalkanes, and their oxidation to aldehydes, ketones, or carboxylic acids. 

The native activity of these haloalkane dehalogenases is the conversion of alkyl halides to the corresponding alcohols (Scheme 4.16) [27]. 

Conversion of an alcohol to a haloalkane involves substitution of halogen for —OH at a saturated carbon. The most common reagents for this conversion are the halogen acids (HCl, HBr, and HI) and certain inorganic halides (PBr3, SOCI2, and SOBr2). 

The conversion of carboxylic acids into acyl halides employs the same reagents used in the synthesis of haloalkanes from alcohols (Section 9-4) SOCI2 and PBrs. The problem to be solved in both cases is identical—changing a poor leaving group (OH) into a good one. 

In Summary Several reagents oxidize primary alcohols and aldehydes to carboxylic acids. A haloalkane is transformed into the carboxylic acid containing one additional carbon atom either by conversion into an organometaUic reagent and carbonation or by displacement of halide by cyanide followed by nitrile hydrolysis. 

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