Diffusion-controlled deprotonation

Whereas base-induced deprotonation at a heteroatom is very fast (practically diffusion-controlled), deprotonation at carbon is generally much slower (Eigen et al. 1964, 1965). Thus, this type of 02 -elimination is observed at higher pH values compared to the reactions discussed before. The elimination of HO2 is subject to steric restrictions, but the OH -induced 02 -elimination is not, and at high pH all hydroxycyclohexadienylperoxyl radicals eliminate 02 bringing the phenolate yield close to 100% [reactions (9) and (14)/(15)] competing reactions (see below) are thereby suppressed. 

M ionic strength solution (with KNOg) at 25°. By way of comparison, the rate constant for the essentially diffusion-controlled deprotonation of the HP04 dianion by OH ion in O.IM ionic strength aqueous solution at 25° is —2 X 10 sec . Eigen s explanation for the comparative slowness... 

This section will only cover reactions in aqueous solutions. Water molecules acting as either a proton acceptor or a proton donor will thus be in close contact with an acid or a base undergoing excited-state deprotonation or protonation, respectively. Therefore, these processes will not be diffusion-controlled (Case A in Section 4.2.1). 

The pK of tyrosine explains the absence of measurable excited-state proton transfer in water. The pK is the negative logarithm of the ratio of the deprotonation and the bimolecular reprotonation rates. Since reprotonation is diffusion-controlled, this rate will be the same for tyrosine and 2-naphthol. The difference of nearly two in their respective pK values means that the excited-state deprotonation rate of tyrosine is nearly two orders of magnitude slower than that of 2-naphthol.(26) This means that the rate of excited-state proton transfer by tyrosine to water is on the order of 105s 1. With a fluorescence lifetime near 3 ns for tyrosine, the combined rates for radiative and nonradiative processes approach 109s-1. Thus, the proton transfer reaction is too slow to compete effectively with the other deactivation pathways. 

At pH <4 both 2"+ and 3,+ undergo C-H deprotonation as the exclusive reaction while in basic solution they behave as oxygen acids undergoing OH-induced OH deprotonation in a diffusion controlled process. Results indicate that in alkylaromatic radical cations, overlap between the scissile bond and the jt-system containing the unpaired electron is a fundamental requirement for the bond cleavage. The observation that for both 2 and 3, the 1,2-H atom shift occurs more rapidly than C-C ft-scission whereas the radical derived from OH de-... 

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

Most of the pKd values of free radicals have been determined by pulse radiolysis, and it is therefore useful to recall, how fast pK equilibria become established. In general, the reaction of H+ with an acid anion is practically diffusion-controlled [reaction (2) k ranging between 5 x 109 dm3 mol1 s1 and 5 X 1010 dm3 mol"1 s 1 (Eigen et al. 1964 Perrin et al. 1981)]. The same holds for the deprotonation of an acid by OH [reaction (3)]. The rates of reaction (4) can be calculated from the pKa value taking into account that Kw = [H+] x [OH ] = 1014 mol2 dm 6. 

The rate of deprotonation of an acid by a base depends on their structures [41], on the solvent and temperature, and on the difference (ApKa) between the pKa of the acid and that of the base. When acid and base have the same pfCa (ApKa=0) the change of free energy for proton transfer becomes zero and the reaction becomes thermoneutral. Under these conditions the rate of proton transfer is limited only by the so-called intrinsic barrier [34], which is particularly sensitive to structural changes in the reaction partners [39]. When ApKa increases, the rate of proton transfer also increases and approaches a limiting value, which depends on the structures of the acid and base and on the experimental conditions. For normal acids (O-H, N-H) in water the rate of proton transfer becomes diffusion-controlled (ka=10loL mol-1 s"1) when ApKa>2, but in aprotic solvents the limiting proton transfer rate can be substantially lower [42]. 

The reverse situation holds for radicals with low p/if-values when (39f) becomes diffusion-controlled. Buffers present in solution also enter these proton transfer reactions and affect the rate of protonation and deprotonation according to their pAT-valucs. Buffers are therefore used to increase the rate of exchange between acid-base forms at the pH range where these forms may exchange slowly, and as a result e.s.r. lines may be narrowed by the effect of buffers (see Section 6). 

The acidity at the C2a position had been estimated before based on the loss of deuterium or tritium to solvent as a measure of deprotonation rates, and assuming diffusion-controlled rate constants for the reprotonation by Sable and coworkers24,25... 

The reaction of hydrated electrons formed by radiolysis with peroxydisulfate yields the sulfate radical anion SO4 which is a strong chemical oxidant (Eqx = 2.4 V/NHE) [50, 58]. The oxidation of both purine and pyrimidine nucleotides by S04 occurs with rate constants near the diffusion-controlled limit (2.1-4.1 x 10 M s ). Candeias and Steenken [58a] employed absorption spectroscopy to investigate acid-base properties of the guanosine cation radical formed by this technique. The cation radical has a pKa of 3.9, and is rapidly deprotonated at neutral pH to yield the neutral G(-H) . Both G+ and G(-H) have broad featureless absorption spectra with extinction coefffcients <2000 at wavelengths longer than 350 nm. This has hampered the use of transient absorption spectra to study their formation and decay. Candeias and Steenken [58b] have also studied the oxidation of di(deoxy)nucleoside phosphates which contain guanine and one of the other three nucleobases by SO4 , and observe only the formation of G+ under acidic conditions and G(-H) under neutral conditions. 

The effects of substituents on the carbon acidity of 5H-dibenzo[a,d]cyclohepta-triene (suberene) have been studied, and deprotonation has been shown to approach the diffusion controlled limit in acetonitrile solution in the presence of primary amines. However, secondary and tertiary amines facilitate photoreduction of the substrate. 

The speed of proton transfer is usually very fast, often at the diffusion-controlled limit. Exceptions occur when the acidic H is very hindered, and when a carbanion deprotonates a C-H bond, which can be so slow that other reactions can easily compete. 

Kinetics Proton transfer catalyzes many reactions. Proton transfer between heteroatom lone pairs is very fast, often at the diffusion-controlled limit. Under reversible (equilibrium) conditions, the most acidic proton is removed preferentially. However, if the deprotonation is done under irreversible conditions, the proton removed is determined by kinetics, not thermodynamics (Section 9.3). Anion basicity always competes with nucleophilicity. Proton transfer is slow enough between organometallics and protons adjacent to carbonyls (carbon bases with carbon acids) that addition of the organometallic to the carbonyl is the dominant process, path AdN. 

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