Water-Catalysed Reactions

For the spontaneous decomposition of nitramide (NH2N02) the transition state is assumed to have formula 8, on the supposition that the rate-limiting step is base-catalysed removal of a proton from the (less stable) aci-tautomer (NH N02H). This suggestion is made in the 

For the spontaneous hydrolysis of acetic anhydride we consider a transition state structure 9, which can be regarded as a generalized 

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For water, the second-order rate coefficient was determined as 9.5 x 10 12 by extrapolation from data at higher temperatures and using the presence of hydroxide ion to suppress any reaction with hydronium ion. For reaction with solutions of biphosphate and ammonium ions, since reaction via hydronium ions in these media is negligible (ca. 1 % of the total rate), the second-order rate coefficients were evaluated from exchange data at a single acid concentration as k2 (H2PC>4 ) = 3.89 xlO-7 and (NH ) = 5.0 x 10-9, the latter value being corrected for the water-catalysed reaction. 

In the discussions of micellar effects thus far there has been essentially no discussion of the possible effect of micellar charge upon reactivity in the micellar pseudophase. This is an interesting point because in most of the original discussions of micellar rate effects it was assumed that rate constants in micelles were affected by the presence of polar or ionic head groups. It is impracticable to seek an answer to this question for spontaneous reactions of anionic substrates because they bind weakly if at all to anionic micelles (p. 245). The problem can be examined for spontaneous unimolecular and water-catalysed reactions of non-ionic substrates in cationic and anionic micelles, and there appears to be a significant relation between reaction mechanism and the effect of micellar charge upon the rate of the spontaneous hydrolysis of micellar-bound substrates. 

The acid-catalysed reaction 66 The hydroxide-ion catalysed reaction 74 Spontaneous or water-catalysed reaction 80 Formation of oxocarbocations equilibrium constants for formation of hemiorthoesters 84... 

The breakdown of hemiorthoesters is catalysed by hydronium ions and hydroxide ions and a spontaneous or water-catalysed reaction can also often be detected. Rate constants for these processes are summarised in Tables 1 to... 

General-base catalysis was also detected in this reaction and attributed to general-acid catalysis of breakdown of the monoanion (18). The Bransted P-value was 0.57 and so the a-value for expulsion of the alcohol from the ionized form is 0.43. The point for the water-catalysed reaction lies on the Bransted plot for catalysis by other bases which supports the view that these reactions proceed by similar mechanisms as formulated in (17) and (18). 6... 

Figure 1.17 Cartoon of the observation of buffer catalysis and separation of the catalytic constants into acid and base components. The intercepts on a series of primary (LHS) plots permit a pH-rate profile at zero buffer concentration to be constructed, and hence the values of ko, the water-catalysed reaction, the hydroxide-ion catalysed reaction, and kH+, the proton-catalysed reaction, to be extracted.

The enolisation is catalysed by (relatively unhindered) substituted quinucli-dine bases with a Bronsted (3 value of 0.45 and 0.48 for glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. The elimination reaction of the enediolate (Figure 6.3) gives a clock whereby the relative efficiency of reprotonation can be measured from slopes of the plot of the ratio of reprotonation to elimination product against buffer concentration. This leads to a Bronsted a value of 0.47 for reprotonation to dihydroxyacetone phosphate (reprotonation to glyceraldehyde phosphate could not be observed in the quinuclidinium-catalysed reaction, but in the water-catalysed reaction. 

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