Isotopic substitution effects, on group

This reaction proceeds via the transition state illustrated in Fig. 10.2. An Sn2 reaction (second order nucleophilic substitution) in the rate limiting step involves the attack of the nucleophilic reagent on the rear of the (usually carbon) atom to which the leaving group is attached. The rate is thus proportional to both the concentration of nucleophile and substrate and is therefore second order. On the other hand, in an SnI reaction the rate limiting step ordinarily involves the first order formation of an active intermediate (a carbonium ion or partial carbonium ion, for example,) followed by a much more rapid conversion to product. A sampling of a and 3 2° deuterium isotope effects on some SnI and Sn2 solvolysis reactions (i.e. a reaction between the substrate and the solvent medium) is shown in Table 10.2. The... 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

In order to assess the validity of such an approach, within the assumptions of the model outlined, we have recently undertaken an examination of deuterium isotope effects on both the radiative and unimolecular dissociation rates. One such case is that of the protonated dimer of acetone in which either the methyl groups, the protonated oxygen, or both, are deuterium substituted. Results for these four systems are shown in Figure 14 and the rate data derived are summarized in Table 1. 

The trifluoromethyl group is found in a wide variety of pharmaceuticals. Besides its effect on the electron density, its high lipophilicity improves transport characteristics in vivo and facilitates lower dosage rates. F for F isotopic exchange reaction has been used to synthesize [l- F]l,l-difluoro-2,2-dichloroethyl-arylethers. The substitution proceeds in good to excellent yields (> 85 %) and is strongly dependent on the reaction conditions. Substitution occurs even at room temperature in DMSO or in acetonitrile containing up to 20% of water (Scheme 61) [247]. 

The rate of polymerization in emulsion polymerization is proportional to kg-, where kg is the fhain transfer step on the vinyl group (10). Substituting trideuterovinyl acetate for vinyl acetate raised the rate by a factor of 1.76. When the calculation for the isotope effect on rate is done accurately, taking into account the 3% H on the trideuterovinyl, we find that if the effect is purely on k3, the rate should rise by a factor of 1.69 as compared to 1.76 . 02. This is almost within the experimental error. There may be a very slight secondary isotope effect (23,24) on the propagation and re-ini tation rate constants k2 and k, but it cannot be decided from these data. 

---