Deprotonation bimolecular reaction

At pH < 7 the nitroxyl radicals do not undergo an observable heterolysis (khs 10 s ), but decay by bimolecular reactions. However, in basic solution an OH -catalyzed heterolysis takes place to yield the radical anion of the nitrobenzene and an oxidized pyrimidine. In the case of the nitroxyls substituted at N(l) by H (i.e. those derived from the free bases), the OH catalysis involves deprotonation at N(l) which is adjacent to the reaction site [= C(6)] (cf. Eq. 15) [26] ... 

Benzyloxadiazolium salts (67a) deprotonate in alkali to form coloured anhydro bases (68). Treatment of salts (67a) with acetyl chloride, or salts (67b) with triethylamine, results in a bimolecular reaction, with rearrangement, to give 3-(2,2-diacylhydrazino)pyrazoles (71JCS(C)2314). 2-Methyl-3-aryl-l,3,4-oxadiazolium salts condense with aldehydes to yield 2-styryl-l,3,4-oxadiazolium salts but no reaction occurs between aldehydes and the isomeric 5-methyl-3-aryl-l,3,4-oxadiazolium salts. 

There are many kinetic evidences for the fact that the nitronium cation and the aromatic substrate are involved in a reversible bimolecular reaction to form a o-complex, which, being a strong acid, undergoes fast deprotonation (Scheme 2). 

The mechanism discussed above for the deprotonation of alkylaromatic radical cations, involving a bimolecular reaction between the radical cation and the base (B), leading to a carbon centered neutral radical and the conjugated acid of the base (BH" ") as described in Scheme 28, has been recently questioned by Parker who provided evidence for an alternative mechanism in proton-transfer reactions between methylanthracene radical cations and pyridine bases [154] this involved reversible covalent adduct formation between the radical cation and the base followed by elimination of BH+ (Scheme 36). 

A bimolecular reaction of C-ethanemethonium or C-proponium ions with propane molecule may result in formation of ethane and buthonium ion. The latter may further evolve to n-butane or i-butane via deprotonation and recovering the Brpnsted acid site. This reaction step may occur either via consecutive mechanism [15, 16] involving formation of CH3 carbenium ion bound to the zeolite framework, followed by its addition to propane molecule, or via concerted mechanism involving CHs transfer from carbonium ion to the propane molecule. This mechanistic pathway explains butanes primary formation. Ethane was also found to be a primary product, however selectivity to ethane formation was much lower comparetively to butanes. It may be due to the fact that ethane... 

The reaction of [OH] with [Co(NH3)jNC5H5] yields an OH adduct to the aromatic ring (A = 325 nm = 1.7 X 10 M cm ) , which can undergo disproportionation in competition with first-order intramolecular electron transfer to produce Co (k = 2.3 X 10 s ) and second-order bimolecular reaction to yield Co(III) products with modified ligands a similar competition affects the deprotonated ligand radical (k = 11 s )-, ... 

With the exception of a few examples, bimolecular reactions in micelles are largely controlled by the local concentration (and pH) realized at the micellar pseudophase. The data reported in Table 2 [28, 31] give a comparison of the second-order rate constants measured for a series of functional derivatives in aqueous and micellar pseudophases. The ratios of the two rate constants (taking into account concentration and deprotonation effects in micelles) are all close to unity, confirming the above assertion. Finally, although the quantification of rate accelerations has been done mainly with micellar aggregates, the reactivity in vesicles appears to follow basically the same rules with minor differences due to the different lipophilicity and/or order of the membrane [37]. 

The observed first-order rate constant for carbanion formation may be controlled through the choice of the basic proton acceptor. Relatively strong carbon acids undergo detectable deprotonation by the weak base water in a pseudo-first-order reaction (Scheme I.IA), but stronger general bases (Scheme I.IB) or hydroxide ion (Scheme I.IC) are required to give detectable deprotonation of weaker carbon acids in bimolecular reactions. 

It is evident that for bimolecular reactions in non-functional micelles in water the key factor in rate enhancement is the increased concentration of the two reactants in the micellar pseudophase (Table 3 and 4) and the same effect should be at work for reactions in functional micelles. The problem is simply that of estimating the concentration of functional groups in the micellar pseudophase. For the simplest case, that of a functional micelle, not involving deprotonation equilibria, with a nucleophilic head group denoted as N, there will be one reactive group per micellar head group, and if the substrate is fully micellar bound we can apply Eqn. 12, derived for reaction in non-functional surfactants, where ... 

Usually, the formation of radical cation from a neutral substrate is associated with the increase in its acidity [14,15] and, therefore, facile deprotonation processes may be expected as the common step from some of these intermediates. In majority of instances, the proton transfer takes place between radical cation/ radical anion pairs, with the net result being the bimolecular coupling product. However, during sensitized PET reactions, deprotonation from radical cation is associated either with unilateral radical reactions or their further oxidation to produce carbocationic species [7]. Normally, the rate of proton transfer depends on the kinetic acidity of the cation radical and the basicity of the anion radical. 

The rate constant for the deprotonation of NH4CI in anhydrous DMSO (pH = 4—5) is the same order of magnitude (10 1 mol s ) as that obtained in water and the reaction is promoted by a bimolecular reaction of NH4 with its conjugate base NH3. ... 

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. 

Figure 4. Variation of relative ionic abundances with reaction time, in a high-pressure source at 5-torr CH4, for negative ions derived from deprotonation of methanol. The exponential decay of rr /z 31 yields the bimolecular rate constant for formation of the proton-bound methoxide dimer, m/z 63. In addition, at this temperature (325 K) the subsequent reaction to generate the trimer anion (m/z 95) and attainment of equilibrium can be seen.

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

The time required to reach equilibrium very much depends on the pKd value of the acid. An acid with a pKa value of 4, for example, deprotonates with a rate of 106 s Thus, the equilibrium is established within a few microseconds. On the other hand, an acid with a pKa value of 7 dissociates with a rate of ca. 103 s"1, and the equilibrium becomes established only on the millisecond time range. In a pulse radiolytic experiment, a large part of the radicals will thus have disappeared in bimolecular termination reactions, before an equilibrium is reached. Buffers speed-up the protonation/deprotonation reactions, and their addition can overcome this problem. Yet, they deprotonate acids and protonate their corresponding anions typically two to three orders of magnitude more slowly than OH and H+ (for a DNA-related example, see Chap. 10.4 for potential artifacts in the determination of pKa values using too low buffer concentrations, see, e.g., von Sonntag et al. 2002). 

The H02- elimination at reaction (22) is often slow, and at a high concentration of peroxyl radicals this reaction may compete with the bimolecular decay of the peroxyl radicals (leading to chain scission Ulanski et al. 1994). However in the presence of base, deprotonation speeds up the 02 elimination [reactions (23)... 

Under hypoxic conditions, cellular enzymes reduce the benzotriazine di-N-oxide [(reaction (68) P450 reductase Cahill and White 1990 and NADPH may be involved Walton et al. 1992 Wang et al. 1993]. Upon microsomal reduction of tirapazamine the radical formed in reaction (68) has been identified by EPR (Lloyd et al. 1991). Using the pulse radiolysis technique, it has been shown that this radical has a pKd of 6 (Laderoute et al. 1988), and it is the protonated form that undergoes the DNA damaging reaction (Wardman et al. 2003). The rate constants of the bimolecular decay of the radical [reaction (70)] has been found to be 2.7 x 107 dm3 mol-1 s 1. The reaction with its anion is somewhat faster (8.0 x 108 dm3 mol-1 s 1), while the deprotonated radicals do not react with one another at an appreciable rate. From another set of pulse radiolysis data, a first-order process has been extracted (k = 112 s 1) that has been attributed to the water elimination reaction (72), and the tirapazamine action on DNA [reaction (74)] has been considered to be due to the resulting radical (Anderson et al. 2003). 

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