Isotope exchange

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 are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

Pyridinium sulfate, bis-l,2,4,6-tetramethyl-hydrogen isotope exchange reactions, 2, 194 Pyridinium-1 -sulfonates reactions, 2, 34... 

Another example is the absence of oxygen exchange with solvent in the hydrolysis of gluconolactone. Simple acyclic esters usually undergo isotopic exchange at a rate that is conqietitive with hydrolysis. This occurs through the tetrahedral addition intermediate. 

If it is assumed that ionization would result in complete randomization of the 0 label in the caihoxylate ion, is a measure of the rate of ionization with ion-pair return, and is a measure of the extent of racemization associated with ionization. The fact that the rate of isotope exchange exceeds that of racemization indicates that ion-pair collapse occurs with predominant retention of configuration. When a nucleophile is added to the system (0.14 Af NaN3), k y, is found to be imchanged, but no racemization of reactant is observed. Instead, the intermediate that would return with racemization is captured by azide ion and converted to substitution product with inversion of configuration. This must mean that the intimate ion pair returns to reactant more rapidly than it is captured by azide ion, whereas the solvent-separated ion pair is captured by azide ion faster than it returns to racemic reactant. 

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 ... 

It is also possible to measme the rate of enolization by isotopic exchange. NMR spectroscopy provides a very convenient method for following hydrogen-deuterium exchange, and this is now the preferred method. Data for several ketones are given in... 

When X=CN, the isotopic exchange occurs with 99% retention of configuration, but when X=CPh, only about 30% net retention is observed. Explain this result. 

Althoi h the equilibrium constant for hydration is unfavorable, the equilibrium between an aldehyde or ketone and its hydrate is established rapidly and can be detected by isotopic exchange, using water labeled with 0, for example ... 

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. 

Radioactive isotopes of the halogens have found use in the study of isotope-exchange reactions and the mechanisms of various other reactions.The properties of some of the most used isotopes are in Table 17.5. Many of these isotopes are available commercially. A fuller treatment with detailed references... 

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... 

In general, isotopic exchange is both expensive and difficult. In the case of many room-temperature ionic liquids, however, the manufacture of deuterated ionic liquids is relatively easily achievable. For example, the general synthesis of l-allcyl-3-methylimidazolium salts is shown in Scheme 4.1-1 [2]. This methodology allows maximum flexibility in the deuteration on the imidazolium cation that is, it can be either ring or side chain deuteration or both. 

Caraculacu et al. [48] also quantitatively determined allylic chlorines in PVC by isotopic exchange with SO Cl2. The selective exchange of chlorine in the polymer was verified by experiments with model compounds. The number of allylic chlorines in PVC was found to be between 0.12 and 0.16 for 100 monomer units. 

This enzyme is of wide occurrence in bacteria where it is concerned with the reduction of nitrate and CO2 as well as sulphur. Methods for its estimation depend on measuring some activity of hydrogenase by (a) dye reduction (benzyl viologen or methylene blue), (b) isotopic exchange and (c) evolution of molecular hydrogen. Interpretation of quantitative results is difficult due to the complex relationship between the enzyme cell structure and the particular method selected. ... 

The rate of isotopic exchange in the solid state, between cobalt in the cation and in the anion of [60Co(H2O)6] [Co(edta)]2 4 H20, was increased [1144] by irradiation (100 Mrad) of the reactant. It was concluded that exchange occurred via vacancies, rather than through motion of a ring of cobalt atoms, one from a cationic site and the other from a neighbouring anionic site. 

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