Elimination from Derivatives of Alcohols

Although it is clear that elimination of the hydroxyl (-OH) group and an adjacent proton as water (H2O) can be effected by acid catalysis (through protonation of the hydroxyl and either subsequent E2 or El processes), it is much more common to utilize ester (Chapter 9) intermediates to the same end. In some cases, the esters are not even isolated before they go on to form alkenes. 

Since sulfuric acid is not particularly soluble in higher-molecular-weight alcohols, it is common to attempt to form esters from acids, which both promote elimination (or substitution, vide supra) and which enjoy such solubility. To this end, and as shown in Equation 8.41, even the acetate (ethanoate) ester (formed from the reaction of alcohol—in this case cyclopentanol—with, e.g., acetic acid [ethanoic acid, CH3CO2H] or acetic anhydride [ethanoic anhydride, (CH3CO)20)] or acetyl chloride [ethanoyl chloride, CH3COCI]), by processes that are discussed further in Chapter 9, can be used to good effect to produce cis- or suprafacial elimination of the elements of acetic acid (ethanoic acid). 

When there are no double bonds in the carboxylic adds, they are called saturated tatty adds. Unsaturated tatty adds have at least one double bond present and this unsaturation results from the elimination of water from an alcohol (dehydration). (Z)-9-octadecenoic acid (oleic acid) and (9Z,12Z)-9,12-octadecadinoic acid (linoleic acid) are examples of the latter (Rgure 8.19). 

There are two points to note. Rrst, both of these adds contain an even number (18) of carbon atoms and, second, the double bonds are ds or (Z). 

In some other cases in nature when elimination of a hydroxyl group (-OH) is to be effected, activation of the hydroxyl is through phosphorylation of the hydroxyl to a phosphate (i.e., R-OH to R-0P03 ) using an enzyme (a kinase) to catalyze the process. Conversion of the hydroxyl to the phosphate group does not utilize phosphoric acid but rather, as noted earlier (and see Chapter 14) relies on the cleavage of an oxygen phosphorus bond in a polyphosphate, that is, conversion of ATP to ADR 

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