Isotopes, 69-70, Table kinetic isotope effect

Table Kinetic Isotope Effect for the Decomposition of Amalgams according to Electrochemical (I) and Chemical (II) Mechanisms...

Melander first sought for a kinetic isotope effect in aromatic nitration he nitrated tritiobenzene, and several other compounds, in mixed acid and found the tritium to be replaced at the same rate as protium (table 6.1). Whilst the result shows only that the hydrogen is not appreciably loosened in the transition state of the rate-determining step, it is most easily understood in terms of the S 2 mechanism with... 

The cases of pentamethylbenzene and anthracene reacting with nitronium tetrafluoroborate in sulpholan were mentioned above. Each compound forms a stable intermediate very rapidly, and the intermediate then decomposes slowly. It seems that here we have cases where the first stage of the two-step process is very rapid (reaction may even be occurring upon encounter), but the second stages are slow either because of steric factors or because of the feeble basicity of the solvent. The course of the subsequent slow decomposition of the intermediate from pentamethylbenzene is not yet fully understood, but it gives only a poor yield of pentamethylnitrobenzene. The intermediate from anthracene decomposes at a measurable speed to 9-nitroanthracene and the observations are compatible with a two-step mechanism in which k i k E and i[N02" ] > / i. There is a kinetic isotope effect (table 6.1), its value for the reaction in acetonitrile being near to the... 

Table 10.6. Kinetic Isotope Effects in Some Electrophilic Aromatic Substitution Reactions...

A kinetic isotope effect that is a result of the breaking of the bond to the isotopic atom is called a primary kinetic isotope effect. Equation (6-88) is, therefore, a very simple and approximate relationship for the maximum primary kinetic isotope effect in a reaction in which only bond cleavage occurs. Table 6-5 shows the results obtained when typical vibrational frequencies are used in Eq. (6-88). Evidently the maximum isotope effect is predicted to be very substantial. 

Table 6-5. Calculated Hydrogen/Deuterium Primary Kinetic Isotope Effects" ...

The kinetic isotope effect with 2-naphthol-8-sulfonic acid in Table 12-3 was reported only three years later (Ernst et al., 1958) based on a suggestion of P.D. Bartlett. That paper was publication no. 1 of the first-mentioned author. At that time he was an ETH student. He obtained the Nobel Prize 33 years later for his work on NMR spectroscopy ... 

The conclusions from the foregoing studies with phenol have been challenged by Challis and Lawson121, who find that rates of nitrosation (shown graphically) pass through a maximum at about 8 M perchloric acid, and also that the reaction shows a large primary kinetic isotope effect at 0.7 °C (Table 27). Hence loss of a... 

The kinetics of the decarboxylation of anthranilic acid have recently been examined. Earlier, an investigation of the decarboxylation of anthranilic acid in aqueous or acidic solution at 100 °C gave a C12 C13 value of 108.02 after 72 % reaction in 1.0 M sulphuric acid compared with 108.5 from complete decarboxylation, so that there is virtually no kinetic isotope effect for this compound646. First-order rate coefficients are given in Table 218 and from the variation of rate... 

Kinetic data exist for all these oxidants and some are given in Table 12. The important features are (i) Ce(IV) perchlorate forms 1 1 complexes with ketones with spectroscopically determined formation constants in good agreement with kinetic values (ii) only Co(III) fails to give an appreciable primary kinetic isotope effect (Ir(IV) has yet to be examined in this respect) (/ ) the acidity dependence for Co(III) oxidation is characteristic of the oxidant and iv) in some cases [Co(III) Ce(IV) perchlorate , Mn(III) sulphate ] the rate of disappearance of ketone considerably exceeds the corresponding rate of enolisation however, with Mn(ril) pyrophosphate and Ir(IV) the rates of the two processes are identical and with Ce(IV) sulphate and V(V) the rate of enolisation of ketone exceeds its rate of oxidation. (The opposite has been stated for Ce(IV) sulphate , but this was based on an erroneous value for k(enolisation) for cyclohexanone The oxidation of acetophenone by Mn(III) acetate in acetic acid is a crucial step in the Mn(II)-catalysed autoxidation of this substrate. The rate of autoxidation equals that of enolisation, determined by isotopic exchange , under these conditions, and evidently Mn(III) attacks the enolic form. 

Table 4-2. Computed and experimental primary 12C/13C and secondary 14N/15N kinetic isotope effects for the decarboxylation of N-methyl picolinate at 25 °C in water...

The kinetic isotope effect proves the attack of ozone on the C—H bond and consequently the C—D bond of the oxidized compound. The values of the kinetic isotope effect (kH/kD) are collected in Table 3.6. 

The rate constants of the reaction of PINO with hydrocarbons and alcohols, and the values of kn/kn are given in Table 5.7. A very high kinetic isotope effect is seen in this reaction. 

TABLE 9. The primary hydrogen-deuterium kinetic isotope effects for the hydrogen transfer reactions between alkyl radicals and tributyltin hydride (deuteride)... 

TABLE 11. The primary hydrogen-tritium kinetic isotope effects found in the reactions between various alkyl radicals and tributyltin hydride and tributyltin hydride-t... 

TABLE 16. The hydrogen-deuterium kinetic isotope effects measured for the oxidation of mandelic acid" by Pb(OAc)4 in benzene and in benzene-pyridine... 

If the reaction rates of a specific carbene with various quenchers are studied in the same solvent, and with small concentrations of Q, K will be constant. Relative reactivities for the singlet state of a spin-equilibrated carbene can thus be derived. However, few researchers have varied the acidity of ROH, estimated kinetic isotope effects, and compared alcohols with ethers (Table 4). The data indicate proton transfer to diarylcarbenes (139d, 139k, 205, 206)112-117 and diadamantylcarbene (207).118... 

The time-resolved, chemical behavior of FL depends on the solvent. Irradiation of DAF in cyclohexane gives FLH The lifetime of FL in cyclohexane is 1.4 ns, and the ratio of products obtained (26) indicates that both direct insertion and abstraction-recombination mechanisms are operating (Griller et al., 1984b). Replacement of the cyclohexane by its deuteriated counterpart reveals a kinetic isotope effect of ca 2 (Table 5). 

According to Scheme 11, the isomeric ortho para) product ratios are established during the collapse of the radical pair in (64) (most probably at the positions of AN+- with the highest electron density). Furthermore, the absence of a measurable kinetic isotope effect in the decay of the deuterated analogue (C6D5OCH.v) in Table 3 is predicted from Scheme 11 since the proton loss occurs in a subsequent, rapid step (65). The absence of a deuterium kinetic isotope effect also indicates that the presence of pyridine in the triad in (63) does not lead to the nitroanisoles by an alternative... 

Table 3 collects the results obtained by different authors on the reaction of 0s04(NH3) with ethylene. Though the methods were not identical, the results are very similar, and in all cases there is a clear preference of the [3+2] over the [2+2] pathway. Additional confirmation was provided by a combined experimental and theoretical study using kinetic isotope effects (KIEs) to compare experiment and theory. Kinetic isotope effects were measured by a new NMR technique [25] and compared to values, which are available from calculated transition states. It showed that indeed only the [3+2] pathway is feasible [10]. 

TABLE 1. The nitrogen, carbon-13 and secondary hydrogen-deuterium kinetic isotope effects found for the one- and two-proton benzidine rearrangements... 

TABLE 2. The nitrogen and carbon-13 kinetic isotope effects for the acid-catalyzed and for the thermal benzidine rearrangement of 2,2 -hydrazonaphthalene in 70% aqueous dioxane at 0°C and in 95% ethanol at 80 °C, respectively... 

TABLE 3. The nitrogen, the carbon-13 and carbon-14 kinetic isotope effects found for the acid-catalyzed ortho,ortho1-rearrangement of iV-naphthyl-iV-phenyl-hydrazine in 60% aqueous dioxane at 0°C... 

The most recent addition to Shine s extensive study of the benzidine-type rearrangements41 involved remeasuring the nitrogen and the carbon-13 and carbon-14 kinetic isotope effects at the 4- and at the 4- and 4 -carbons as well as determining the carbon-13 and carbon-14 isotope effects at the 1- and at the 1- and l -carbons in the benzidine rearrangement of hydrazobenzene (equation 30). The reaction, which was carried out in 75% aqueous ethanol that was 0.1 M in hydrochloric acid and 0.3 M in lithium chloride at 0°C, gave an 86% yield of benzidine (11) and a 14% yield of diphenyline (12). The kinetic isotope effects found for the formation of benzidine and diphenyline under these reaction conditions are presented in Table 5. 

It is worth noting that Yamataka and coworkers47 also found large (near the theoretical maximum) alpha carbon kinetic isotope effects for the Menshutkin reactions between 3,5-disubstituted pyridines and methyl iodide (equation 38, Table 6). 

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