Base-catalysed mechanisms exchange

Two mechanisms are shown above. The base-catalysed mechanism proceeds through the enolate anion. The acid-catalysed process would be formulated as involving an enol intermediate. Note that the terminal hydrogens in pentan-3-one are not exchanged, since they do not participate in the... 

The rate constant for the base-catalysed hydrogen exchange of [Ir(NH3)6] at 298 K in D2O (reaction 64) has been determined (fc = 1.5 x 10 M s" ). The chloride anation of [Ir(H2)(NH3)5] + is three times faster than the water exchange rate, and has been attributed to an associative interchange mechanism. ... 

The first evidence that an elimination-addition mechanism could be important in nucleophilic substitution reactions of alkanesulfonyl derivatives was provided by the observation (Truce et al., 1964 Truce and Campbell, 1966 King and Durst, 1964, 1965) that when alkanesulfonyl chlorides RCH2S02C1 were treated in the presence of an alcohol R OD with a tertiary amine (usually Et3N) the product was a sulfonate ester RCHDS020R with exactly one atom of deuterium on the carbon alpha to the sulfonyl group. Had the ester been formed by a base-catalysed direct substitution reaction of R OD with the sulfonyl chloride there would have been no deuterium at the er-position. Had the deuterium been incorporated by a separate exchange reaction, either of the sulfonyl chloride before its reaction to form the ester, or of the ester subsequent to its formation, then the amount of deuterium incorporated would not have been uniformly one atom of D per molecule. The observed results are only consistent with the elimination-addition mechanism involving a sulfene intermediate shown in (201). Subsequent kinetic studies... 

The intermediacy of enols or enolate anions may be demonstrated by hydrogen exchange reactions (see Section 4.11.2). Both acid-catalysed and base-catalysed tautomerism mechanisms involve removal... 

The results are consistent with a preliminary base-catalysed fast reversible equilibration of the a-hydrogen leading to exchange. Concurrent attack of the base at the -carbon competes with trans elimination to acetylene, and exchange could be observed for either mechanism, or even in the absence of substitution (Scheme 7). With a weaker base, e.g. PhS-, the exchange is slower than substitution. The trans isomer favours addition-elimination, while the cis isomer reacts, at least partially, via the acetylenic intermediate, showing an element effect. The absence of an isotope effect suggests an ElcB mechanism. 

Hydrolysis is the reaction of the nucleophiles H2O or OH with a chemical where a (leaving) group of the chemical is exchanged with an OH group. Many compounds, especially acid derivatives, are susceptible to hydrolysis. Hydrolysis can both be abiotic and biotic, but in regard to testing only abiotic hydrolysis is considered. Hydrolysis can take place by different mechanisms at different pHs, neutral, adder base-catalysed hydrolysis, and hydrolysis rates may be very dependent on pH. 

D.A. Jencks and W.P. Jencks, On the Characterisation of Transition States by Structure-Reactivity Coefficients, J. Am. Chem. Soc., 1977, 99, 7948. W.P. Jencks, When is an Intermediate Not an Intermediate Enforced Mechanisms of General Acid-Base Catalysed, Carbocation, Carbanion, and Ligand Exchange Reactions, Aces. Chem. Res., 1980, 13, 161. 

W.P. Jencks, When is an Intermediate not an Intermediate Enforced Mechanisms of General Acid Base Catalysed, Carbocation, Carbanion and Ligand Exchange Reactions, Accs. Chem. Res., 1980,13, 161. 

An alternative mechanism in which hydroxide ion displaces a hydroxyl group was rejected as unlikely. A rate equation obs = was derived using the constants shown in the above scheme. It was suggested that the effects of para-substitution on these three constants are in the same direction, hence the overall effect on the observed rate constant is more pronounced in the base-catalysed exchange than in the acid-catalysed one. 

Hydrogen in the stable bonds of aliphatic and aromatic compounds can be exchanged only under particularly energetic conditions. It is favored, for instance, by solid catalysts, by strong bases which ease the removal of protons in such cases, or by strong acids which act as effective deuteron-donors. An acid-base-catalysed ionic mechanism and an electrophilic substitution mechanism have both been discussed as explaining the formation of deuterated compounds.70,71... 

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