Rate constants addition reactions

Miura (Y3) have studied the effect of the addition of glycerol to water on the reaction rate constant. The reaction in the liquid phase is second order. Their values for k" at 28°C, which indicate a slight increase with increasing viscosity, are given in Table I. 

In fact, Everett et al. (1995, 1996) have reported the scavenging of N02 by 3-CAR, and their results indicate that the reaction proceeds via electron transfer only and no radical addition occurs. The electron transfer was shown to proceed with a rate constant of 1.1 x 108 M 1 s 1 in tcrt-butanol/ water mixtures (50% v/v). This study was extended by the same workers (Mortensen et al. 1997) to include five other carotenoids, with canthaxanthin (CAN) having the lowest rate constant of reaction with N02 (1.2 x 107 M 1 s-1), and LYC having the second highest (1.9 x 107 M 1 s-1) after ZEA (2.1 x 107 M 1 s-1). All the rate constants obtained were an order of magnitude below that for 3-CAR. However, the experiments were carried out in 60 40%, v/v tert-butanol/water mixture (80 20%, v/v for LYC due to aggregation) rather than the 50% (v/v) mixture used for 3-CAR and the N02" was generated in a different way. 

The IR kinetic measurements (96) of the rate constants for reaction of Cr(CO)5(C6H12) with CO were very similar to those measured using uv-vis flash photolysis (30,33). In the presence of added ligands, Cr(CO)5(C6H12) decayed to give Cr(CO)5L products. For both L = CO and L = H20 the activation energy was 22 5 kJ mol-1 (96), but surprisingly the rate of addition of H20 was much faster than that of CO. Similar... 

Values of pA"R for the addition of water to carbocations to give the corresponding alcohols. The equilibrium constants KR (m) were determined as the ratio Hoh/ h> where fcHOH (s 1) is the first-order rate constant for reaction of the carbocation with water and H (m 1 s ) is the second-order rate constant for specific acid-catalyzed cleavage of the alcohol to give the carbocation.9,12 13... 

The products are formed in kinetically controlled reactions, except in those instances, considered in the next subsection, where ethers result from the addition of a hydroxyl group to an activated alkene. The analytical method of Spurlin266 has often been used in order to evaluate relative rate-constants for reaction at the hydroxyl groups. 

Steric effects on the nucleophile, aniline, were clearly evident. Rate constants for bimolecular attack of 2,6-dimethyl- 70a, 2,6-diethyl- 70b, and 3,5-dimethylaniline 70c at 308 K indicate that the ort/zo-substituted anilines react more than an order of magnitude slower at the same temperature (Table 7). Structure 70c must be able approach the reactive nitrogen more closely.42,43 A comparison of the rate constants for reaction of aniline 72c, /V-methyl- 71a and /V-phenylaniline 71b provides further evidence of steric effects although the very small rate constant for the diphenylamine could also be accounted for by reduced nucleophilicity on account of lone pair resonance into the additional phenyl ring. 

Transition State Theory [1,4] is the most frequently used theory to calculate rate constants for reactions in the gas phase. The two most basic assumptions of this theory are the separation of the electronic and nuclear motions (stemming from the Bom-Oppenheimer approximation [5]), and that the reactant internal states are in thermal equilibrium with each other (that is, the reactant molecules are distributed among their states in accordance with the Maxwell-Boltzmann distribution). In addition, the fundamental hypothesis [6] of the Transition State Theory is that the net rate of forward reaction at equilibrium is given by the flux of trajectories across a suitable phase space surface (rather a hypersurface) in the product direction. This surface divides reactants from products and it is called the dividing surface. Wigner [6] showed long time ago that for reactants in thermal equilibrium, the Transition State expression gives the exact... 

Several reactions of halogen-substituted carbon-centered radicals with silanes have been studied, but limited kinetic information is available for reactions of halogen-substituted radicals with tin hydrides. A rate constant for reaction of the perfluorooctyl radical with Bu3SnH was determined by competition against addition of this radical to styrenes, reactions that were calibrated directly by LFP methods.93 At ambient temperature, the n-C8F17 radical reacts with tin hydride two orders of magnitude faster than does an alkyl radical, consistent with the electron-deficient nature of the perflu-oroalkyl radical and the electron-rich character of the tin hydride. Similar behavior was noted previously for reactions of silanes with perhaloalkyl radicals. 

Andreasen et al. [86] also found that ball milling increased the rate constant, k, in the JMAK equation (Sect. 1.4.1), of reaction (Rib) in solid state but virtually had no effect on the rate constant of reaction (R2). They also showed that the reaction constant, k, of reaction (Rib) in solid state increases with decreasing grain size of ball-milled LiAlH within the range 150-50 mn. Andreasen et al. concluded that the reaction (Rib) in solid state is limited by a mass transfer process, e.g., long range atomic diffusion of Al while the reaction (R2) is limited by the intrinsic kinetics (too low a temperature of decomposition). In conclusion, one must say that ball milling alone is not sufficient to improve the kinetics of reaction (R2). A solution to improvement of the kinetics of reaction (R2) could be a suitable catalytic additive. 

Cyclizations of amidyl radicals have been studied both synthetically and kinetically. A detailed study on the rates of a variety of amidyl radical reactions was determined by both LFP and indirect competition methods (Table l) In addition, the rate constants for reactions with BusSnH and PhSH were also reported (thus giving a range of simple amidyl radical clocks). The results obtained will be useful in synthetic sequenceplanning involving amidyl radicals. 

In this paper the rate expressions have all been corrected for nitrogen evolution from the azo initiator, oxygen absorption by initiator radicals, and oxygen evolution in termination. It is assumed that the initiator which decomposes without starting oxidation chains does not react with oxygen (21). This correction involves the addition of (l-e)Ri/2e to the measured rate, where e is the efficiency of chain initiation, found to be 0.5 at 30 °C. and 0.6 at 56 °C. The rate constant for Reaction 7 has been written as 4ktCT in order that the three termination constants may be comparable (26, footnote 27).]... 

Addition of sodium polyphosphate appreciably altered the rate constants for reactions (19)—(21) and stabilized the small non-metallic silver clusters [512, 513]. Advantages of the steady-state and pulse-radiolytic approaches to silver-cluster formation are manifold. Firstly, experimental conditions can be precisely adjusted such that the reactive species is exclusively e or, alternatively, that it is a known alcohol radical. Secondly, the concentration of the reducing species (the number of reducing equivalents generated) is readily calculable. Thirdly, in time-resolved experiments, rate constants for the individual reaction steps can be determined by monitoring absorption and/or conductivity changes. These latter determinations permitted the assessment of agglomeration numbers [512,513]. 

Franzen34 photolyzed CH2N2-butadiene mixtures in the pressure range 31-335 mm., with butadiene in excess by a factor of 2-15. Franzen also observed cyclopentene as a product, the ratio of cyclopentene to vinyl cyclopropane decreasing from 0.25 at 35 mm. to 0.095 at 335 mm. Franzen proposed that some of the cyclopentene resulted from 1,4 addition of methylene to butadiene, on the grounds that all excited vinyl-cyclopropane should be collisionally deactivated at pressures as high as 335 mm. However, the ratio of cyclopentene to vinylcyclopropane obtained by Franzen at 335 mm. is close to that predicted by the ratio of rate constants for reactions (63) and (64) calculated by Frey.44... 

Irradiation in the presence of MDEA completely inhibits the formation of products. The amine quenches the fluorescence of Eosin with a rate constant of 8 x 108 M-1s-1 and quenches the Eosin triplet with a rate two orders of magnitude lower. A summary of rate constants for the decay of the triplet is presented in Table 8. In addition to the reactions shown in Scheme 3, with Am = (V-methyl diethanolamine, the rate constants for reaction of PDO with Eosin triplet and semioxidized Eosin radical in aqueous solution (Eqs. 19 and 20) are included in the table. 

Further studies367 on the hydrolysis of (107) have shown that a reduction in the chloride ion concentration leads to a marked increase in rate. The approximate relative reactivities of the different mercury(Il) species in solution are Hg" (l) = HgCl+ (l) = HgCl2 (1)> HgCV" (0.1) HgCl42 (CO.001). The rate constant for reaction via Hg11 is ca. 106-fold larger than for hydrolysis in the presence of HjO+ alone. Addition of HC1 leads to a reduction in rate, rather than an increase, presumably due to the formation of less reactive chloro complexes. Hydrolysis of (107)... 

With R = benzyl and in the absence of 02, the major product (73%) is the de-carbonylation product [reaction (209) possible formed to a large extent within the solvent cage], and the dimer of the allylic radical [reaction (207)] is formed only in small amounts. Addition of a thiol increases the yield of Thd [reaction (208)]. If an evaluation of the data reported for the reduction of the allylic OH-adduct to 1,3-cylohexadiene by a thiol (Pan et al. 1988), estimated at 104 dm3 mor1 s"1, is a good guide the rate constant for reaction (208) should be similar. This would revise an assumed rate constant of 106 dm3 mol-1 s-1 and the conclusions as to the repairability of allylic Thy in DNA radicals by cellular thiols (Anderson et al. 2000). 

As rate constants of reactions of benzenoid hydrocarbons can be related to differences in energy of reactants and products (or intermediates) Hess-Schaad resonance energy differences can also be used as reactivity indices. Hess-Schaad resonance energies are defined as the difference between HMO ji energy and the additive contribution obtained by summing individual bond energies [42, 43]. Correlations of Hess-Schaad resonance energies with other HMO parameters have been discussed [44, 45],... 

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