Michael Szwarc

The Addition of Methyl Radicals to Ethylene, Propylene, the Butenes and Higher 1-Olefines

The addition of methyl radicals to ethylene and its homologues in iso-octane solution was studied over the temperature range 55 to 85°C. The relative rate constants of addition to ethylene, propylene and iso-butene determined at 65°C are 34, 22 and 36 respectively. Taking into account the statistical factor of 2 for ethylene, we conclude that the increasing ease of addition for this series of olefines reflects the increasing stability of the produced radicals. The rates of addition to trans- and cis-butene-2 are significantly lower (6.9 and 3.4 respectively), indicating a steric hindrance resulting from the presence of a methyl group on the carbon atom on which the reaction takes place. Identical rates of addition were found for propylene, butene-1,pentene-1, 3 methyl butene-1, heptene-1, decene-1 and hexadecene-1, indicating that the rates of addition are not affected by the length and the shape of the hydrocarbon ‘tail’. The rates of abstraction of hydrogen atoms by methyl radicals were determined. It was found that the rate constants for active hydrogen (α to C=C double bond) fall into three distinctive classes characterizing the primary, the secondary, and the tertiary hydrogen atoms.

Addition of Methyl Radicals to Isolated, Conjugated and Cumulated Dienes

The rate of addition of methyl radicals to isolated, conjugated and cumulated dienes was investigated. The isolated dienes were found to behave like the corresponding mono-olefins. The conjugated dienes exhibit the expected high reactivity and the addition requires very low activation energy. The effect of methyl substituents on their reactivities was studied and the results explained quantitatively in terms of hyperconjugation and the steric ‘blocking’ effect. The effect of other substituents on their reactivities was also investigated. The reactivities of the cumulated dienes were found to be extremely low, lower than those of mono-olefins. The decrease in their reactivity seems to arise from a low A factor rather than from a high activation energy. It was shown that the addition to allene and its derivatives takes place on the central C atom and not on the terminal atoms.

Structures of ion-pairs derived from dihydroanthracenes I. Electronic spectra

The optical spectra of the 9-lithium and sodium salts of 9, 10-dihydroanthracene and its 10-alkyl substituted derivatives have been investigated in tetrahydrofuran over a wide temperature range (25°C to about −70°C). The spectra of the lithium salts indicate the coexistence of two species having a different degree of solvation, one absorbing at λmax. = 400 nm, the other at λmax. = 450 nm. Lower temperatures favour the latter. It has been found that the presence of an alkyl substituent on carbon 10 greatly increases the stability of that species which absorbs at the longer wavelength. Only one absorption peak is seen in the spectrum of the respective sodium salts. The absorption maximum of the unsubstituted ion-pair shifts with temperature, namely λmax. = 455 nm at −70°C and 425 nm at +20°C. Similar behaviour was observed for the substituted salt. These observations suggest the need for a ‘dynamic’ model of ion-pairs in preference to the ‘static’ one in which the energy of the pair is uniquely determined by the interionic distance. Such models are discussed.

Structure of ion-pairs derived from dihydroanthracenes II. Nuclear magnetic resonance studies

N. m. r. studies of 9, 10-dihydroanthracene, its 10-alkyl substituted derivatives and of the respective Li+ salts established the stereochemistry of these compounds. The spectra show that neither the molecules of these hydrocarbons nor of their lithium salts are planar, although in the anions the carbon 9 atoms possess a considerable amount of sp2 character. The anion of the unsubstituted salt vibrates rapidly through the planar configuration while the substituted anion prefers one conformation, apparently that in which the alkyl group is equatorial. The n. m. r. spectrum of the unsubstituted salt is compared with those of the lithium salts of some similar hydrocarbons, namely, fluorene and diphenylmethane.

Electron-Transfer and Proton-Transfer Equilibria in Systems Involving Different Ion-Pairs

Electron-transfer equilibria between radical-anions, say A1⋅¯ and A2⋅¯, and their parent compounds A1 and A2, may involve several distinct ionic species. Therefore, one should differentiate between equilibria such as A1⋅¯+A2⇌A1+A2⋅¯(K−),(A1⋅¯,Met+)tight+A2⇌A1+(A2⋅¯,Met+)tight(Ktight),(A1⋅¯,Met+)loose+A2⇌A1+(A2⋅¯,Met+)loose(Kloose),, etc. Symbols such as (A1⋅¯,Met+)tightor(A1⋅¯,Met+)loose denote distinct types of ion-pairs—• tight and loose, respectively. In a given solvent there are relations between K_, Ktight, Kloose, etc., e.g. Ktight, Kloose = K1/K2 where K1 and K2 are the equilibrium constants of the transformation of tight into loose pairs, i.e., (A1⋅¯,Met+)tight⇌(A1⋅¯,Met+)loose(K1)(A2⋅¯,Met+)tight⇌(A2⋅¯,Met+)loose(K2) We show that K _, lf Ktight and Kloose are independent of solvent, although K1, K2 or the dissociation constants of ion-pairs are profoundly affected by its nature. In a solvent where two or more kinds of ionic species coexist, the experimentally determined ‘equilibrium constant’, Kap, is a complex function of ‘true’ equilibrium constants, e.g. if tight and loose pairs coexist, Kap is given by Kap=[A1]{[(A2⋅¯,Met+)tight]+[(A2⋅¯,Met+)loose]}/[A2]{[(A1⋅¯,Met+)tight]+[(A1⋅¯,Met+)loose]} and Kap=Ktight(1+K2)/(1+K)=Kloose(1+K2−1)/(1+K1−1). Hence, determination of Kap leads to values of K1 and K2, provided Ktight and Kloose are determined by studies of the electron-transfer equilibria in solvents where only the tight or only the loose pairs are present. The validity of this method has been tested. The experimental and mathematical techniques developed in the course of these studies are directly applicable to studies of proton-transfer equilibria, e.g. A1H+A2−,Met+⇌A1−,Met++A2H..

IONS, ION-PAIRS, AND THEIR SIGNIFICANCE IN ELECTRON-AND PROTON-TRANSFER REACTIONS

Three somewhat interrelated topics are discussed in this paper: (1) Methods of differentiation between free ions and the various kinds of ion-pairs. After briefly mentioning the older techniques used in such studies, we shall stress the newer ones developed during the last few years. (2) Changes of the rates and equilibrium constants of electron- and proton-transfer reactions resulting from replacement of free ions by ion-pairs or tight ion-pairs by the loose pairs. The temperature dependence of such processes is particularly interesting when two or more different kinds of ionic species of a particular substrate simultaneously coexist in the studied system. (3) The role of dianions in proton-transfer and some electron-transfer reactions involving radical-anions.