Internal conjugate base

The Internal Conjugate Base Mechanism.—Also in 1966, Rorabacher reported some temperature-jump data involving the monoammine complexes of Co", Ni", and Zn". The rate constants were discussed in terms of the normal mechanism but they were particularly interesting inasmuch as they suggested that the rates which had been reported previously for the reaction of nickel with polyamines e.g. en) had been unusually high. In the 1966 paper, Rorabacher proposed an internal conjugate base mechanism to explain this acceleration, and this is shown in Figure 5. 

The fact that the rate constant for the reaction of FeOH + with Hanta" is 20 times that for reaction with SCN is ascribed to the operation of the internal conjugate-base mechanism of Rorabacher. 

Are internal proton transfers within the conjugate base possible ... 

A very plausible mechanism for this would involve loss of hydrogen chloride from XXII to form the bicyclohexanone (XXXII) this is a cyclopropanone derivation and would certainly react at once with alkali to give XXXI. The problem is to explain how this comes about. One possibility might be a-elimination of HC1 from XXX to form a carbene but this seems unlikely such a carbene would in any case be expected to rearrange to cyclohexenone rather than to XXXII. Another possibility would be an internal displacement of chloride ion from the conjugate base of XXX, as indicated in XXXIII this, however, is sterically improbable since in the grouping —CH2—CH=-CO—CHC1— the... 

The oxidation potentials for the conjugate bases, the neutral monohydrides, were obtained by cyclic voltammetry in THF or dichloromethane. Irreversible waves were often reported, also for CpRu(PPh3)2H, CpRu(dppe)H, and CpRu(dppp)H for which other workers had reported reversible voltammetry [27, 30, 31]. It is not clear to what extent, if any, these differences will affect the calculated BDEs, and for the sake of internal consistency of data we will mostly use Morris s data. The data... 

Figure 22. The correlation between the rate of proton recombination with its conjugate base and the internal diameter of the liposome. Rates of proton recombination were determined either by steady-state fluorescence ( ) or time-resolved (O) measurements. The line is drawn according to Equation (21).

A third member of the bimolecular then unimolecular reaction class is a variant of the previous mechanism. In this case, the conjugate base of biotin reacts with bicarbonate to produce an addition intermediate that then reacts with ATP (Scheme 23). It is likely that the phosphorus of the terminal group of ATP would preassociate with an oxygen of bicarbonate. In particular, if the anionic center of bicarbonate associates with a cation, the 7r-electron density of bicarbonate would align with the phosphorus of the terminal phosphate of ATP. The addition of the conjugate base of a urea to a carboxylate is an appropriate model for this mechanism. The intermediate should be very reactive toward ATP based on the observation that the conjugate base of a carbonyl hydrate reacts rapidly with an internal phosphate ester (59). 

Another carbon compound has an acidic proton, and the acid-base reaction is quite common. Alkynes (Chapter 5, Section 5.2) are categorized as internal alkynes (R-C=C-R) or terminal alkynes (R-C=C-H). The hydrogen atom of a terminal alkyne is a weak acid (pK of about 25), and strong bases such as sodium amide (NaNH2) remove that proton to give an alkyne anion (R-C=C ). In this reaction, the alkyne anion is the conjugate base of the alkyne, which is a weak acid. Alkyne anions are useful nucleophiles in many reactions (see Section 6.7). 

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