Deprotonation bimolecular

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. 

This bimolecular mechanism also applies to cycloalkanes which can be activated by intermolecular hydride transfer to small carbenium ions to form cyclohexyl cations prior to cracking. Alternately, the cyclohexyl cations can deprotonate and form cyclohexene. With two similar intermolecular hydride transfers an aromatic can also form [46]. 

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] ... 

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). 

A transient absorption spectrum with a peak at 575 nm assigned to DMP was observed immediately after e during PR of DMP (5.0 x 10 M) in DCE. The time-resolved transient absorption spectra showed a slow growth of a new peak at 500 nm assigned to 3,5-dimethoxyphenoxy radical (DMP ) (Fig. 22). The peak at 575 nm due to DMP decreased with time, and most of DMP disappeared at 2 psec after e to yield DMP through deprotonation of DMP It has been shown that bimolecular deproto-... 

Excess ground state MB deprotonates the amine cation radical (eq. 11) to afford the a-amino radical. Bimolecular coupling with MB- yields a reduced, leuco dye eq. 12. 

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). 

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. 

This is an example of the first step of an E2 (bimolecular elimination) reaction mechanism. Note the base-mediated deprotonation of the diester converting the ferf-butoxide anion to ferf-butanol. For clarity, the anion was repositioned and the bond was lengthened. Arrow pushing is illustrated below ... 

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). 

Deprotonation reaction of dipyrrole 179 was studied by nanosecond laser photolysis. The bimolecular rate constants of proton transfer to heterocyclic bases were determined. The inhibiting of the radical cation reaction by bases occurs during the radical cation formation (09CHE554). 

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... 

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