Solvent effects methyl halides

The effect of monomer concentration was studied using n-pentane solvent and maintaining the total volume of isobutylene plus n-pentane constant. Methyl halide concentration was kept constant so as to maintain constant medium polarity. Attempts were made to keep conversions below 20%. At -30 °C, due to almost explosive polymerizations, conversions could only be maintained below 40%. 

In previous papers1,2 we described reactivity studies of cationic isobutylene polymerization using r-butyl halide initiators, alkylaluminum coinitiators and methyl halide solvents. The effects of these reagents as well as temperature on the overall rate of polymerization and polyisobutylene (PIB) yield were studied and reactivity orders were established. These results were explained by a modified initiation mechanism based on an earlier model proposed by Kennedy and co-workers3,4. This paper concerns the effects of f-butyl halide, alkylaluminums and methyl halide, as well as temperature and isobutylene concentration on PIB molecular weights. 

Fig. 24 Evidence for the transition states in the hydrolysis of methyl halides from the different probes Marcus analysis, a 0.5 charge development, Z. Vi = 0.70, JZ, rjY = 0.46 solvent isotope effect, dotted lines (see Fig. 13) o-deuterium isotope effect, r 1.0 Hammett relations, shaded area (see Fig. 23)...

An analysis of these results in terms of solvent effects leads to the observation of similarities with Ritchie s work on the N+ relation. Thus the constant selectivities obtained in the solvolysis reactions of certain methyl derivatives (Table 9) may indicate the existence of a basic similarity between the rate-determining process in these reaction and in the electrophile-nucleophile combination reactions correlated by the IV+ relation. The failure of the methyl halides to conform to this pattern might suggest that their substitution reactions are fundamentally different, and that the free energy of activation is dependent on factors other than desolvation. 

Kennedy and coworkers (7) further studied the effect of solvent (MeCl, MeBr, Mel and cyclopentane) and the nature of the halogen (Cl, Br or I) in the t-butyl halide on the rate of neopentane formation in the reaction between f-butyl halide and MejAl. The findings of this work are 1. The rate of neopentane formation with different t-butyl halides follows the order t-BuCl > t-BuBr > t-BuI. 2. The nature of the solvent exerts a significant influence on the rate of alkylation, i.e., rate decreases in the order MeCl > MeBr > Mel > cyclopentane. 3. The rate of neopentane formation is first order in [t-BuX] and in [MejAl]. 4. The activation energies in the range — 20° to 80° are 11 kcal/mole for all the methyl halide solvents and 16 kcal/mole for cyclopentane. 

The solvent effects on oxidative addition reactions to square planar iridium(I) complexes have drawn some comment. The general acceleration of the addition reactions of hydrogen, oxygen and methyl iodide to trans-[lrX(CO)(FFhQ)2] (where Xis a. halogen) by polar solvents, such as dimethylformamide, is taken by Chock and Halpern to be evidence for a polar transition state. ° Perhaps more interesting is the stereochemical result of the addition of alkyl and hydrogen halides to these iridium(I) complexes. "... 

If the free energy of the reactants in a reaction is different from that in the transition state, the rate of the reaction in solution will be different from that in the gas phase. Such solvent effects on reactivity are indeed extremely important, as we have already seen in the discussion of the S 2 reaction in Section 5.3. The Sj 2 halide exchange reactions of methyl halides occur slowly in solution, via a transition state of the form (X CH3 Y). In the gas phase, however, the transition state becomes a stable species, formed exothermically from methyl halide and halide ion. Here solvent effects completely alter the course of the reaction. 

Quaternary salts, isolation of, 10 Quaternization, by alkyl halides, 2-7 by aryl halides, 7-9 on carbon, 53 definition of, 2 by dimethyl sulfate, 9 electronic effects in, 11 in JV-heterocycles, 16, 38 by heterocyclyl halides, 7—9 isotope effect on, 55 mechanism of, 53-56 by methyl euyl-sulfonates, 9, 10 on oxygen, 52 rates of, 55 reagents for, 2-10 by self-condensation, 8 solvent effect on, 10, 55 solvents for, 10 steric effects on, 12, 13 substituents, influence on, 11, 19, 23 on sulfur, 51 Quinaldine, 4-amino-, 4 Quinazolines, 2-alkyl-, salt formation of, 6... 

Quantum mechanical calculations have recently led to the suggestion that in the presence of Lewis acidic BF3 the reductive elimination reactions of trialkyl-copper(III) species may proceed via the formation of a Lewis acid-[P-cuprio(III) enolate] complex. DFT calculations have been employed to probe the mechanism for the SN2-substitution reaction of methyl halides and epoxides with lithium organocuprates(I) operate, with solvent effects, BF3 effects and trans-diaxial epoxide opening taken into account. Calculations have lately been combined with mass spectrometric results to probe equilibrium isotope effects for the coordination of C2H4 and C2D4 to otherwise bare coinage metal cations. ... 

Schrauzer and co-workers have studied the kinetics of alkylation of Co(I) complexes by organic halides (RX) and have examined the effect of changing R, X, the equatorial, and axial ligands 148, 147). Some of their rate constants are given in Table II. They show that the rates vary with X in the order Cl < Br < I and with R in the order methyl > other primary alkyls > secondary alkyls. Moreover, the rate can be enhanced by substituents such as Ph, CN, and OMe. tert-Butyl chloride will also react slowly with [Co (DMG)2py] to give isobutylene and the Co(II) complex, presumably via the intermediate formation of the unstable (ert-butyl complex. In the case of Co(I) cobalamin, the Co(II) complex is formed in the reaction with isopropyl iodide as well as tert-butyl chloride. Solvent has only a slight effect on the rate, e.g., the rate of reaction of Co(I) cobalamin... 

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