Decarboxylation rate constant

Most importantly, the careful kinetic analysis of the rise and decay of the transient species in equation (69) shows that the decarboxylation of Ph2C(OH)CO occurs within a few picoseconds (kc c = (2-8) x 1011 s-1). The observation of such ultrafast (decarboxylation) rate constants, which nearly approach those of barrier-free unimolecular reactions, suggests that the advances in time-resolved spectroscopy can be exploited to probe the transition state for C—C bond cleavages via charge-transfer photolysis. 

Another mechanistically useful nucleophile is acetate ion and related carboxylates. Acetate ion is difficult to oxidize (Eberson, 1963) and reacts with radical cations in a bond-forming reaction (Eberson and Nyberg, 1976). The oxidation product, the acetoxyl radical, has properties which make trapping it very unlikely in that its decarboxylation rate constant is 1.3 X 109 s 1 (Hillborn... 

Values of the Acid Dissociation Constant (K) and Ring-Opening Rate Constant (k2) of [L4Co(02COH)]2 + and the Decarboxylation Rate Constant (k2) of the Corresponding Monodentate Bicarbonato Complexes (at 25.0 °C,... 

These equations are in line with Eq. (30), such that kx denotes the ring-opening rate constant of the protonated carbonato complex and / 2 is the decarboxylation rate constant of the ring-opened bicarbonato complex. Values of these rate constants and the acid dissociation constants of some protonated carbonato complexes of cobalt(TII) (see Table III) reflect the ligand dependence with respect to charge variations, steric constraint, and donor properties of the non-labile ligands. 

With arylacetoxyl radicals, relatively slower decarboxylation rate constants have been measured (Table 11), pointing in this case towards higher activation barriers. Two main factors have been suggested to account for such behavior ... 

Evidence from Solvent Effects on Decarboxylation Rate Constants in Model... 

A comparison of the rate constants with those for the uncatalysed and ZvP -catalysed reactions reveals that the rates of enolization reflect the keto-enol equilibria, Cu(oxac)>Zn(oxac)>oxac ", and parallel the decarboxylation rate constants, 0.17 s 0.047 and 1.66x 10 s respectively for Cu(oxac)k, Zn(oxac)k, and oxack ". This latter observation is in agreement with the weakening effect of the metal on the C—C bond. ... 

Imidazole, 4-acetyl-5-methyl-2-phenyl-synthesis, 5, 475 Imidazole, 1-acyl-reactions, 5, 452 rearrangement, 5, 379 Imidazole, 2-acyl-synthesis, 5, 392, 402, 408 Imidazole, 4-acyl-synthesis, 5, 468 Imidazole, C-acyl-UV spectra, 5, 356 Imidazole, N-acyl-hydrolysis rate constant, 5, 350 reactions, 5, 451-453 synthesis, 5, 54, 390-393 Imidazole, alkenyl-oxidation, 5, 437 polymerization, 5, 437 Imidazole, 1-alkoxycarbonyl-decarboxylation, 5, 453 Imidazole, 2-alkoxy-l-methyl-reactions, 5, 102 thermal rearrangement, 5, 443 Imidazole, 4-alkoxymethyl-synthesis, 5, 480 Imidazole, alkyl-oxidation, 5, 430 synthesis, 5, 484 UV spectra, 5, 355 Imidazole, 1-alkyl-alkylation, 5, 73 bromination, 5, 398, 399 HNMR, 5, 353 synthesis, 5, 383 thermal rearrangement, 5, 363 Imidazole, 2-alkyl-reactions, 5, 88 synthesis, 5, 469... 

A few examples have been reported in which no steric parameter is involved in the correlation analysis of cyclodextrin catalysis. Straub and Bender 108) showed that the maximal catalytic rate constant, k2, for the (5-cyclodextrin-catalyzed decarboxylation of substituted phenylcyanoacetic acid anions (J) is correlated simply by the Hammett a parameter. 

Aliphatic acyloxy radicals undergo facile fragmentation with loss of carbon dioxide (Scheme 3,69) and, with few exceptions,428 do not have sufficient lifetime to enable direct reaction with monomers or other substrates. The rate constants for decarboxylation of aliphatic acyloxy radicals are in the range l 10xl09 M 1 s at 20 °C.429 lister end groups in polymers produced with aliphatic diacyl peroxides as initiators most likely arise by transfer to initiator (see 3.3.2.1,4). The chemistry of the carbon-centered radicals formed by (3-scission of acyloxy radicals is discussed above (see 3.4.1). 

For benzoyl and acetyl peroxides, loss of carbon dioxide occurs in a stepwise process. Estimates of the rate constants for step c in Scheme 1 are 7 x 10 sec (benzene, 60°). The corresponding process for acetyl peroxide has k = 2x 10 sec (n-hexane, 60°), so that the lifetime of radical pairs containing acetoxy radicals is comparable to the time necessary for nuclear polarization to take place (Kaptein, 1971b Kaptein and den Hollander, 1972 Kaptein et al., 1972). Propionoxy radicals are claimed to decarboxylate 15-20 times faster than acetoxy radicals (Dombchik, 1969). 

More recently Hand et al. (ref. 9) have studied the decomposition reaction of N-chloro-a-amino acid anions in neutral aqueous solution, where the main reaction products are carbon dioxide, chloride ion and imines (which hydrolyze rapidly to amine and carbonyl products). They found that the reaction rate constant of decarboxylation was independent of pH, so they ruled out a proton assisted decarboxylation mechanism, and the one proposed consists of a concerted decarboxylation. For N-bromoamino acids decomposition in the pH interval 9-11 a similar concerted mechanism was proposed by Antelo et al. (ref. 10), where the formation of a nitrenium ion (ref. 11) can be ruled out because it is not consistent with the experimental results. Antelo et al. have also established that when the decomposition reaction takes place at pH < 9, the disproportionation reaction of the N-Br-amino acid becomes important, and the decomposition goes through the N,N-dibromoamino acid. This reaction is also important for N-chloroamino compounds but at more acidic pH values, because the disproportionation reaction... 

Influence of ionic strength on the reaction rate constant. The influence of the ionic strength on the reaction rate constant was studied using KCl as electrolyte. The results obtained in this study are listed in Table 4, where we can see that the reaction rate constant for N-Br-alanine decomposition undergoes an increment of 40 % upon changing the ionic strength from 0.27M to IM, while in the case of N-Bromoaminoisobutyric acid the increment of the reaction rate constant is of about 12 %. This is an evidence of a non ionic mechanism in the case of the decomposition of N-Br-aminoisobutyric acid, as it is expected for a concerted decarboxylation mechanism. For the decomposition of N-Br-proline the increase on the reaction rate constant is about 23 % approximately, an intermediate value. This is due to the fact both paths (concerted decarboxylation and elimination) have an important contribution to the total decomposition process. 

Oxalic acid does not form intramolecular hydrogen bonds. The decarboxylation of oxalic acid occurs by the direct reaction of the peroxyl radical with the carboxylic group. The rate constants of the peroxyl radicals of the oxidized cyclohexanol in co-oxidation with oxalic acid was found to be pi2 = 7.4 x 107exp(—60.2/RT) L mol-1 s-1 in cyclohexanol at 348-368 K [106],... 

Another example of the use of transition state pKa values has been provided by Pollack (1978). From the rate constants for the decarboxylation of substituted a,a-dimethylbenzoylacetic acids ([37] — [38]) and their anions, he calculated pK for reaction of the acids (Table A6.2). The values vary significantly with the phenyl substituent (p = +1.7), much more so than the p/(a values of the substrate acids (p = +0.2). This difference is consistent with the proton being much closer to the phenyl group in the transition state than in the initial state, and it may even denote a relatively late transition state (Pollack, 1978). However, from the pKa values of the reactant acids (approximately 3.4), the transition states (approximately 4.4), and the enol product (11.8) (Pruszynski et al., 1986), the Leffler index... 

An early application of the Scheme 5 approach led to a rate constant for scavenging of t-butoxycarbonyl radicals by MNP (Perkins and Roberts, 1973, 1974). This depended upon the known rate of unimolecular decarboxylation of these radicals to give t-butyl radicals (Scheme 7). Both t-butyl and... 

The unusual rate enhancement of nucleophiles in micelles is a function of two interdependent effects, the enhanced nucleophilicity of the bound anion and the concentration of the reactants. In bimolecular reactions, it is not always easy to estimate the true reactivity of the bound anion separately. Unimolecular reactions would be better probes of the environmental effect on the anionic reactivity than bimolecular reactions, since one need not take the proximity term into account. The decarboxylation of carboxylic acids would meet this requirement, for it is unimolecular, almost free from acid and base catalysis, and the rate constants are extremely solvent dependent (Straub and Bender, 1972). 

Poly(vinylbenzo-18-crown-6) [54] dissolved in water also acts as an efficient catalyst for the decarboxylation through transfer of the substrate into the aromatic inner core of the tightly coiled polymer (Shah and Smid, 1978). The bound carboxylate decomposes 2300 times faster than in water, the rate constant (0.042 s-1) being the largest of those observed in aqueous media. 

Kunitake et al. (1980) subsequently studied the decarboxylation of [52] in a series of dialkylammonium bilayers. The rate constant increased with... 

The presence of a highly hydrophobic microenvironment is also reflected in the decarboxylation rate of [52] (see Table 4). The rate constant of decarboxylation in the presence of [7] (2 x 10 4 M) is at least 10 times larger than that in the presence of CTAB (1 x 10-3 M). 

---