Carbonyl hydrate dissociation

Stoichiometry (28) is followed under neutral or in alkaline aqueous conditions and (29) in concentrated mineral acids. In acid solution reaction (28) is powerfully inhibited and in the absence of general acids or bases the rate of hydrolysis is a function of pH. At pH >5.0 the reaction is first-order in OH but below this value there is a region where the rate of hydrolysis is largely independent of pH followed by a region where the rate falls as [H30+] increases. The kinetic data at various temperatures both with pure water and buffer solutions, the solvent isotope effect and the rate increase of the 4-chloro derivative ( 2-fold) are compatible with the interpretation of the hydrolysis in terms of two mechanisms. These are a dominant bimolecular reaction between hydroxide ion and acyl cyanide at pH >5.0 and a dominant water reaction at lower pH, the latter susceptible to general base catalysis and inhibition by acids. The data at pH <5.0 can be rationalised by a carbonyl addition intermediate and are compatible with a two-step, but not one-step, cyclic mechanism for hydration. Benzoyl cyanide is more reactive towards water than benzoyl fluoride, but less reactive than benzoyl chloride and anhydride, an unexpected result since HCN has a smaller dissociation constant than HF or RC02H. There are no grounds, however, to suspect that an ionisation mechanism is involved. 

The u.v. spectrum of chloral hydrate has been recorded in a number of solvents.> 5. In solution in water or in dioxan containing a few percent of water, the hydrate is so stable that no carbonyl absorption is detectable. However, if the crystalline hydrate is dissolved in cyclohexane, it dissociates sufficiently to give a considerable absorption at 290my.l (giving an 0 value of 38.3). 

Only rarely are the hydrates sufficiently stable to be isolated, but the hydration reaction allows an examination of the factors that influence addition to the carbonyl group. It is common to represent the equilibrium between an aldehyde or ketone and the corresponding hydrate in terms of a dissociation constant, K, as shown in equation 7.60. 

TABLE 7.6 Equilibrium Constants for Dissociation of Hydrates of Carbonyl Compounds ... 

Over the years, spectrophotometry (UV-VIS) has been used to study such physicochemical phenomena as heats of formation of molecular addition compounds and complexes in solution, determination of empirical formulas, formation constants of complexes in solution, hydration equilibria of carbonyl compounds, association constants of weak acids and bases in organic solvents, tautomeric equilibria involving acid base systems, protein-dye interactions, chlorophyll-protein complexes, vitamin A aldehyde-protein complex, association of cyanine-dyes, determination of reaction rates, determination of labile intermediates, and dissociation constants of acids and bases. 

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