Search

Absolute rate constants for hydrogen abstractions from carbon by the t-butoxyl radical are reported for several hydrocarbons and tertiary amines in solution. The analysis of the available data reveals that most hydrogen abstractions from carbon are entropy controlled.

A substance achieves supercritical state when taken above its critical temperature and pressure. A supercritical fluid (SCF) has intermediate bulk properties between those of a gas and a liquid. Well known reactions in conventional solvents were used to seek divergent behaviour in an SCF solvent, based on the examination of the free radical chlorination of several alkanes in supercritical carbon dioxide (SC-CO(sub2)). There was no evidence of enhanced cage effect in SC-CO(sub2), and its magnitude was as anticipated.

Solvent effect highly influences the chemoselectivity of the free radical chlorination of cyclopropane. Internal pressure of the solvent is related to the ratio between ring-opened and hydrogen-abstraction products. Gas and condensed-phase chlorination of cyclopropane differ due to the solvent pressure effect of the phase. Results obtained from extrapolation techniques are in good agreement with the experimental data.

Analysis of the effect of phenyl and vinyl radicals on the ring opening of various cyclopropylcarbinyl radicals and structurally similar cyclopropylketyl anions reveals that placement of a phenyl group on the cyclopropane ring compensates for the resonance stabilization loss. The radical anion-based resonance and losses are not fully compensated for by substitution and ring strain relief, implying that radical anions are useless in single-electron transfer probes.

A study shows a connection between the internal pressure of solvent and selectivity in free radical brominations. An analysis of 18 solvents and solvent mixtures shows that the rate constant ratio for SH2 ring opening of cyclopropylbenzene (kc) versus hydrogen abstraction from toluene (kH) by bromine atom differed by almost a factor of two as a function of solvent. The observed rate constant ratio for the examined solvents is almost the same as the internal pressure. The volume of stimulation related to kc is more negative than for kH by ca. 20 cubic centimeters per mol, and this probably causes the difference in selectivity with solvent pressure.

Research describing applications of the method of tert-Butoxyl for exploring radicals found in biological systems is presented. Particular attention is given to measurements of hydrogen abstraction constants at absolute rates and to intrinsic reactivities.

The possibility of using the chlorine atom cage effect as a highly sensitive probe for cage effects in supercritical fluid solvent was investigated. The chlorine atom cage effect was, therefore, studied in supercritical carbon dioxide. Related experiments were also conducted in conventional solvents. The photoinitiated free radical chlorination of cyclohexane was studied in various conventional solvents and in supercritical carbon dioxide. Findings failed to show the existence of an enhanced cage effect near the critical point in the supercritical carbon dioxide solvent.