The Exchange of Hydroxylic Compounds with Water

The mechanism of exchange of oxygen with alcohols can conveniently be discussed as a function of the structure of the carbon skeleton. In the elucidation of the mechanism, studies of the absolute and comparative rates of exchange can be used as well as the rates of loss of optical activity and of various competing rearrangements in alcohols of suitable structure. Hence a large number of studies have centred on secondary and tertiary alcohols, where the stereochemistry can be studied by different means, and less attention has been paid to primary alcohols. 

Two mechanisms of isotopic exchange in alcohols can be considered, corresponding to the S l and Sj 2 mechanisms of nucleophilic substitution. Water is the nucleophile in this case, attacking a saturated carbon atom. In all the studies reported so far, exchange occurs only in acid solution and it has always been assumed that the conjugate acid of the alcohol is the reacting species. 

At high acidities, there is always the possibility of reversible ether and olefiA formation which can provide alternative paths for exchange and which must be taken into account in calculating accurate rate constants. However, the rates of dehydration and hydration of olefins are usually 

The two extreme mechanisms of exchange between a protonated alcohol and water are  

The stereochemical consequences of reaction (a) are that if the carbonium ion becomes planar, and the leaving water molecule equilibrates with the solvent very rapidly, then each ionization of a C—0 bond results in exchange. When the alcohol is optically active due to asymmetry at the a-carbon atom, eachionization will also causeracemization. It follows that 

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