Simple proton transfer

Let us consider the proton transfer from acetone to a series of bases [3] and compare the logarithm of the rate constant with the pK of the conjugate acid of the base. A linear relationship is observed (Fig. 1) which is an example of a Bronsted correlation. If we had plotted log kf (a measure of the free-energy difference between ground and transition state) versus log k /kf (a measure of the free energy of the reaction) it is clear intuitively that the transition state would be product-like for a slope of unity and reactant-like for zero slope. It is difficult to measure equilibrium constants such as in Eqn. 1 but ionisation constants are easily estimated (using pH-titration equipment for example) so that the majority of comparisons are with these. Inspection of Eqns. 1 and 2 shows that the only identities are the base and the acid comparison of oxonium ion with acetone and water with the conjugate base of acetone is doubtful. 

In the event, the correlation of Fig. 1 is tantamount to a correlation between rate and equilibrium constant for Eqn. 1 because the two equilibria are related by Eqn. 3 and the value iLgu constant throughout the series. 

The Bronsted relationship for proton abstraction is usually expressed in the hnear form (Eqn. 4) 

The sign of a is defined as positive when it is derived from a negative slope we may then calculate from Eqns. 3, 4 and 5 the following relationship  

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Furan-2-boronic acid (89) and furan-3-boronic acid are readily prepared by interaction of the furyllithium with methyl borate (MeO)3B followed by acid hydrolysis.233 234 Like most boronic acids they owe their acidity more to coordination with a water molecule than to simple proton transfer they... 

Simple proton transfers of oxygen and nitrogen acids 115... 

Acid and base catalysis of a chemical reaction involves the assistance by acid or base of a particular proton-transfer step in the reaction. Many enzyme catalysed reactions involve proton transfer from an oxygen or nitrogen centre at some stage in the mechanism, and often the role of the enzyme is to facilitate a proton transfer by acid or base catalysis. Proton transfer at one site in the substrate assists formation and/or rupture of chemical bonds at another site in the substrate. To understand these complex processes, it is necessary to understand the individual proton-transfer steps. The fundamental theory of simple proton transfers between oxygen and nitrogen acids and... 

To conclude our description of techniques, the use of nanosecond and picosecond spectroscopy which has been applied to excited state intramolecular proton transfer (ESIPT) will be mentioned briefly (Beens et al., 1965 Huppert et al., 1981 Hilinski and Rentzepis, 1983). A large number of inter-and intramolecular proton transfers have been studied using these methods (Ireland and Wyatt, 1976) but in the case of processes which are thought to involve simple proton transfer along an intramolecular hydrogen bond it is usually only possible to estimate a lower limit for the rate coefficient. 

For simplicity (and to avoid strong net change in dipolar character), we examine the simple proton-transfer reaction between two strong anionic Lewis bases, H ... 

Of these four reactions (ii) and (iv) involve simple proton transfers to and from oxygen atoms, and experience shows that such equilibria will be set up very rapidly. The rate-limiting steps then become (i) and (iii), which involve greater structural changes and are likely to be slow. They are both formally termolecular reactions, and it is of interest to enquire whether either of them can be split up into consecutive bimolecular processes, one of which is rate-limiting. The only possibilities are as follows ... 

It appears that all these possibilities can be excluded. If reactions (a) or (gf) were rate-limiting the reaction velocity would be independent of the concentration of the substrate, while reaction (e) (identical with (Z)) would predict no catalysis by acids or bases. If reactions (b), (d) or (h) determined the rate the reaction would show specific catalysis by hydrogen or hydroxide ions, in place of the general acid-base catalysis actually observed. Reactions (c), (f) and (m) are unacceptable as rate-limiting processes, since they involve simple proton transfers to and from oxygen. Reactions (j) and (k) might well be slow, but their rates would depend upon the nucleophilic reactivity of the catalyst towards carbon rather than on its basic strength towards a proton as shown in Section IV,D it is the latter quantity which correlates closely with the observed rates. 

An even more extreme case is the reaction of hydroxide with acetonitrile. In the ICR spectrometer, the bare hydroxide ion yields a simple proton transfer product, by way of reaction (9). In contrast, in aqueous solution the bulk-solvated hydroxide ion reacts to hydrolyze the nitrile group to give the carboxylate ion plus... 

Figure 8.1 Logarithm of the rate of a simple proton transfer of the type...

It has been reported that rates of proton transfer from carbon acids to water or hydroxide ion can be predicted by application of multi-dimensional Marcus theory to a model whereby diffusion of the base to the carbon acid is followed by simple proton transfer to give a pyramidal anion, planarization of the carbon, and adjustment of the bond lengths to those found in the final anion.124 The intrinsic barriers can be estimated without input of kinetic information. The method has been illustrated by application to a range of carbon acids having considerable variation in apparent intrinsic barrier. 

The reactions including C02 obey first- and second-order kinetics, whereas the other reversible reactions are based on simple proton transfers and are therefore regarded as instantaneous by the corresponding mass action law equations. The formation of bicarbonate ions (HC03 ) takes place via two different... 

As pointed out in the previous section, chemical modification of a nucleobase can lead to the situation that a cationic (protonated) nucleobase is involved in base pairing. Two cases are to be differentiated First, the modified base is cationic. This situation is realized in the base pair between 7,9-dimethylguani-nium (58a) or 7-methylguanosinium (58b) and the corresponding neutral betain (cf. Fig. 2). Second, the modified base forces the complementary base to become protonated in order to form hydrogen bond for increased base pair stability. This situation is realized in the pair formed between 06-alkylguanine and protonated cytosine (59b) or adenine (69), and also in the pair between 3-methylguanine and protonated cytosine (60) (cf. above). Of course, the above differentiation is in a way superficial in that simple proton transfer will interconvert the two cases. 

The last nucleophile of this chapter, sodium bisulfite, NaHSC, adds to aldehydes and some ketones to give what is usually known as a bisulfite addition compound. The reaction occurs by nucleophilic attack of a lone pair on the carbonyl group, just like the attack of cyanide. This leaves a positively charged sulfur atom but a simple proton transfer leads to the product. 

Don t be put off by the number of steps in this mechanism—look carefully, and you will see that most of them are simple proton transfers. The only step that isn t a proton transfer is the addition of water. 

For mechanism (6) the observed rate coefficient (feBH + ) is the rate coefficient for a proton transfer step, whereas in (7) the observed rate coefficient does not refer to a single proton transfer step. The following examples illustrate the ways in which rate coefficients for a simple proton transfer to or from carbon have been obtained by measuring the rate of an overall reaction composed of a number of steps. 

The reaction does not involve internal return and the rate of exchange, as discussed above, refers to a proton transfer step. This mechanism applies to most carbon acids in aqueous solution and the expected general base catalysis is observed, (ii) k2 < fe t, feobs = kxk2/k- - The observed rate coefficient is composite and the rate of exchange does not refer to a simple proton transfer step. It has been argued that the reaction will then show catalysis by hydroxide ion only and not by general bases when carried out in aqueous solution [26]. This arises because the rate of reaction depends upon the equilibrium concentration of intermediate in eqn. (11) which will depend upon the concentration and basicity of B. It... 

The mechanism of hydrolysis of vinyl ethers (28) resembles the mechanism of hydration of olefins and has been studied extensively to obtain kinetic results for simple proton transfer to carbon [49]. As in many of the examples previously discussed in this section, the overall reaction is conveniently followed spectrophotometrically and the measured second-order rate coefficient refers to the rate coefficient for proton transfer to olefinic carbon (feH A ). 

Equation 4 is not a simple proton-transfer reaction but rather involves displacement of an H2O molecule contained in an (HiOln cluster ion by a molecule C. Usually, C must have a substantially larger proton affinity than H2O (170 kcal mole ) in order to make Eq. 4 exothermic [13, 14]. This is true because Il20-molecules mostly bond more strongly to HaO than to H C. 

A test of the validity of the hypothesis that the active site is a carbenium ion residue was carried out in the following manner (55) If the transformation of n-butenes into isobutylene via a monomolecular reaction is initiated by a simple proton transfer to the reactant, this transformation will involve an unstable primary carbenium ion, as shown previously. In contrast, for a larger olefin like pentene, the monomolecular reaction will occur via a secondary carbenium ion rather than a primary carbenium ion and take place more rapidly as follows ... 

The dielectric constant is a property of major concern in understanding acid-base behavior in various solvents. When the dielectric constant of a solvent is low, ion association and homoconjugation can take place, resulting in modification of otherwise simple proton transfer reactions. 

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