Isotope exchange, kinetics

A more detailed study of the nitration of quinolinium (l) in 80-05 % sulphuric acid at 25 °C, using isotopic dilution analysis, has shown that 3-) 5-) 6-, 7- and 8-nitroquinoline are formed (table 10.3). Combining these results with the kinetic ones, and assuming that no 2- and 4-nitration occurs, gives the partial rate factors listed in table 10.4. Isoquinolinium is 14 times more reactive than quinolinium. The strong deactivation of the 3-position is in accord with an estimated partial rate factor of io for hydrogen isotope exchange at the 3-position in the pyridinium ion. It has been estimated that the reactivity of this ion is at least 10 less than that of the quinolinium ion. Based on this estimate, the partial rate factor for 3-nitration of the pyridinium ion would be less than 5 x io . 

Most of the chemical properties of tritium are common to those of the other hydrogen isotopes. However, notable deviations in chemical behavior result from isotope effects and from enhanced reaction kinetics induced by the ( -emission in tritium systems. Isotope exchange between tritium and other hydrogen isotopes is an interesting manifestation of the special chemical properties of tritium. 

Isotopic Exchange Reactions. Exchange reactions between the isotopes of hydrogen are well known and well substantiated. The equihbrium constants for exchange between the various hydrogen molecular species have been documented (18). Kinetics of the radiation-induced exchange reactions of hydrogen, deuterium, and tritium have been critically and authoritatively reviewed (31). The reaction T2 + H2 — 2HT equiUbrates at room temperature even without a catalyst (30). 

There have been numerous studies of the rates of deprotonation of carbonyl compounds. These data are of interest not only because they define the relationship between thermodynamic and kinetic acidity for these compounds, but also because they are necessary for understanding mechanisms of reactions in which enolates are involved as intermediates. Rates of enolate formation can be measured conveniently by following isotopic exchange using either deuterium or tritium ... 

A comparison of the kinetics of hydrolysis and isotopic exchange of amides A and B was carried out. Some of the data are given below. An interesting observation is that there is more C=0 exchange for A than for B. From this observation, and other data given, develop a stepwise mechanism for the hydrolysis of each amide and a qualitative comparison of the substituent effects on the various steps. 

Hyland, L., et al., 1991. Human immunodeficiency viru.s-1 protea.se 1 Initial velocity. studies and kinetic characterization of reacdon intermediates by Isotope exchange. Biochemistry 30 8441-8453. 

Only within the past few years have serious attempts been made to estimate quantitatively the differences in reactivity between thiophene and benzene and between the 2- and 3-position of thiophene. Careful investigation on the acid-induced exchange of deuterium and tritium have shown that the ratios of the exchange rates in the 2- and 3-positions are 1045 61 for deuterium and 911 60 for tritium in 57% by weight aqueous sulfuric acid at 24.6°C. A kinetic isotope effect in the isotopic exchange has been found to be k-r/kr, = 0.51 0.03 in the 2-position and kr/kjy — 0.59 0.04 in the... 

The kinetic isotope effect kD kT for hydrogen exchange of ferrocene in both trifluoroacetic acid-acetic acid and trifluoroacetic acid-dichloromethane has been measured558. In the former medium (1 1 molar ratio at 25 °C) kD kT was 1.2—1.3, which was less than that obtained for the 2 position of thiophene (D) = 1,200 xlO-7, ki(T) = 660xl0-7, kD kT = 1.9. It is also lower than that obtained for pentamethylbenzene fcx(D) = 3,300 x 10-7, Art(T) =... 

For the exchanges carried out in liquid ammonia, kinetic isotope effects kD kT of 2.3-2.5 have been obtained for reaction of benzene, toluene, and naphthalene and for the reactions of the 2 positions of furan and thiophene with -butoxide in dimethyl sulphoxide somewhat lower values, 1.5 and 1.3, respectively, were obtained591, but whether this was a solvent or a substituent effect is not apparent from the data. 

In this chapter it is clearly impossible to do more than sample the extensive literature on the carbon acidity of sulfinyl and sulfonyl compounds, as it illuminates the electronic effects of these groups, particularly in connection with linear free-energy relationships. There are three main areas to cover first, as already indicated, equilibrium acidities (pKa values) second, the kinetics of ionization, usually studied through hydrogen isotopic exchange and finally, the kinetics of other reactions proceeding via carbanionic intermediates. 

In our laboratory we have been investigating the mechanism of action of mandelic acid racemase from Pseudomonas putida (101), which catalyzes the racemization of either D or L-mandelic acid, 47. Evidence from kinetic and isotopic exchange studies indicates that the racemization proceeds via an... 

Is the paramagnetic adduct between CO and Cluster A a kinetically intermediate in acetyl-CoA synthesis Questions have been raised about whether this adduct is a catalytic intermediate in the pathway of acetyl-CoA synthesis 187, 188) (as shown in Fig. 13), or is formed in a side reaction that is not on the main catalytic pathway for acetyl-CoA synthesis 189). A variety of biochemical studies have provided strong support for the intermediacy of the [Ni-X-Fe4S4l-CO species as the precursor of the carbonyl group of acetyl-CoA during acetyl-CoA synthesis 133, 183, 185, 190). These studies have included rapid ffeeze-quench EPR, stopped flow, rapid chemical quench, and isotope exchange. 

Horne has studied the kinetics of exchange in aqueous perchlorate media at temperatures down to —78 °C by the isotopic method ( Fe) and dipyridyl separation. The same rate law in these ice media as in aqueous solution was observed, although the acid dependence was small. Horne concluded that the same exchange mechanism occurs in solid and liquid solvent. Evidence for a Grotthus-type mechanism has been summarised. ... 

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. 

The first term was found to correspond to the rate of enolisation (measured by an NMR study of hydrogen-deuterium exchange at the methylene group). The second term predominates at [Cu(II)] > 10 M and is characterised by a primary kinetic isotope effect of 7.4 (25 °C) and a p value of 1.24. Addition of 2,2 -bipridyl (bipy) caused an increase in 2 up to a bipy Cu(II) ratio of 1 1 but at ratios greater than this 2 fell gradually until the enolisation term dominated. The oxidation of a-methoxyacetophenone is much slower but gives a similar rate... 

This study presents kinetic data obtained with a microreactor set-up both at atmospheric pressure and at high pressures up to 50 bar as a function of temperature and of the partial pressures from which power-law expressions and apparent activation energies are derived. An additional microreactor set-up equipped with a calibrated mass spectrometer was used for the isotopic exchange reaction (DER) N2 + N2 = 2 N2 and the transient kinetic experiments. The transient experiments comprised the temperature-programmed desorption (TPD) of N2 and H2. Furthermore, the interaction of N2 with Ru surfaces was monitored by means of temperature-programmed adsorption (TPA) using a dilute mixture of N2 in He. The kinetic data set is intended to serve as basis for a detailed microkinetic analysis of NH3 synthesis kinetics [10] following the concepts by Dumesic et al. [11]. 

The kinetics of deuterium isotope exchange between diphenyl phosphine and t-butylthiol have been studied by H n.m.r. spectroscopy.274 A negative temperature coefficient was observed for the reaction of a perf1uoroalky1 phosphite with a fluorinated aldehyde.275 The kinetics for the reaction of alcohols with phosphoryl trichloride bore strong similarities to those of carboxylic acid derivatives.276 An interesting report desribed the solvolysis of ary 1 hydroxymethyl-phosphonates. It was shown that a phosphoryl group does not prevent carbocation formation on an immediately adjacent carbon atom.277... 

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