Hydrogen exchange rates, comparison with

Comparison of the rate constants for hydrogen exchange in naphthalene and biphenyl with a KNH2 solution in NH3 and with HBr also shows that the first medium is very effective in suppressing differences in the reactivity of non-equivalent CH bonds in the molecule of the hydrocarbon (p. 163), while in the second medium they are clearly differentiated. 

The operation of the addition-elimination route and its details are inferred from the use of several criteria which will be discussed in the order below (a) The element effect, i.e. comparison of the substitution rates of compounds which differ only in the leaving group, extended also to study of the competition of two leaving groups attached to the same or to different carbon atoms, of the same molecule, (b) The stereochemistry of the substitution, (c) The reactivity of various systems as a function of the structural parameters. The hydrogen-exchange criterion will be discussed in connection with the elimination-addition route. 

As in hydrogen exchange (Chapter 2, Section 1. G), evaluation of standard nitration rates permits comparison of substrate reactivities under the same conditions. Previous comparisons between nitration rates of compounds of widely differing reactivity have required a stepwise procedure (71 Mil), which carries the implicit assumption that partial rate factors do not vary appreciably with acidity. 

Fig. 30. Comparison of ESR and enzyme activity changes with hydration. Effect of hydration on lysozyme dynamic properties. (Curve f) Log rate of peptide hydrogen exchange. (Curve g) , Enzyme activity (log uo) O, rotational relaxation time (log t ) of the ESR probe TEMPONE. From Rupley et al. (1983).

A kinetic study has been made of the oxidation of ascorbic acid (H A) by neptunium(vi). The rate law is of the form — d[NpVi]/dr=A 2[H2A] [Np i] with no evidence for a hydrogen-ion dependence over the range 0.05 < [H+] < l.OOM. In aqueous media Np i exists as the [O—Np—0] + ion with between four and six equatorial water molecules. A comparison with the reactions with aquo-ions, e.g. or where inner-sphere reactivity is established and with [Fe(phen)3] + indicates that the preferred pathway is outer sphere. The exchange rate for [(5-nitrophen)gFe] +/ + and the redox potential are both greater than for the Np VNp " couple and these features are considered- the reason for the rate of the present reaction being 40 times slower than that of the iron system. 

---