Proton removal from intramolecular hydrogen bonds

4 Proton removal from intramolecular hydrogen bonds 

The reactivity of protons in chemical and biological systems is modified if the proton is held within an intramolecular hydrogen bond. In this section, the 

Over the past 20 years, with the availability of fast reaction techniques (Eigen and de Maeyer, 1963 Hammes, 1974 Bemasconi, 1976), numerous kinetic studies have been made of the reactivity of hydrogen-bonded protons towards an external base (52). The majority of such studies have been made with hydroxide ion as the external base. Some examples of proton transfer to 

All these reactions are thermodynamically favourable in the direction of proton transfer to hydroxide ion but the rate coefficients are somewhat below the diffusion-limited values. In broad terms, the typical effect of an intramolecular hydrogen bond on the rate coefficient for proton removal is to reduce the rate coefficient by a factor of up to ca 10 below the diffusion limit. Correspondingly the value of the dissociation constant of the acid is usually decreased by a somewhat smaller factor from that of a non-hydrogen-bonded acid. There are exceptions, however. 

Proton transfer from intramolecularly hydrogen-bonded nitrogen and oxygen acids to hydroxide ion  

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Results which have an important bearing on the mechanism of proton removal from intramolecular hydrogen bonds have been obtained quite recently for the second ionisation of phenylazoresorcinols (73). Kinetic measurements were made using the temperature-jump technique (Perlmutter-Hayman and Shinar, 1975 Perlmutter-Hayman et al., 1976). 

Proton removal from intramolecular hydrogen bonds. 148... 

In the two examples of buffer catalysis of proton transfer from an intramolecularly hydrogen-bonded acid which have been discussed, it seems reasonably certain that the mechanism in Scheme 7 applies. The reactions are of the first order with respect to the catalyst B and it therefore follows that proton removal from the non-hydrogen-bonded species is rate-limiting k j > 2[B]- If this step consists of diffusion-controlled proton removal from a low concentration intermediate, the value k2 lx 109dm3 moP s-1 will apply for proton transfer to an amine. In the case of proton removal by hydroxide ion from 4-(3-nitrophenylazo)salicylate ion, the reaction was found to be of the first order in hydroxide ion up to the highest concentrations which could be studied (0.003 mol dm-3) with a rate... 

Removal of the proton from an intramolecular hydrogen bond by a base occurs in a two-step mechanism (a rapid equilibrium between H-bonded and non-H-bonded forms followed by base catalyzed proton removal from the non-H-bonded form) rather than by direct attack of the base on the intramolecularly hydrogen bonded species (6). 

The proton in [34] is removed by the hydroxide ion and buffer species on the timescale of minutes. The mechanism of the reaction has been established (see p. 330) as involving opening of the intramolecular hydrogen bond to give a strained open intermediate from which the proton is removed. The... 

In Section 4 data were described that show that removal of the intramolecularly hydrogen-bonded proton by a general base from protonated... 

Figure 3.12. A working hypothesis as to how the binding of phosphate (9) to anthrylpolyamine 8 can afford a CHEF effect. Intracomplex protonation (or strong hydrogen bonding) of 10 by the HP04 2 ion leads to loss of the benzylic nitrogen lone pair (11), thereby removing this source of intramolecular quenching. (Reproduced from Ref. 16. Copyright 1989 American Chemical Society.)...

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