Electron affinity ethylene

Whether the second step does take place depends on a number of factors. The electron affinity of the M ion must be sufficiently great, and this point can be appreciated by considering a few examples. Electron transfer to stilbene or tetraphenyl ethylene leads to the formation of negative ions which in turn rapidly ac-... 

One of the earliest measurements of the gas-phase equilibrium acidity of propene involved measuring the rates of reaction of propene with hydroxide ion in both directions33. The resulting equilibrium constant gave A//acid = 391 1 kcalmol-1. In the case of ethylene, the acidity and independently measured electron affinity of vinyl radical were used to determine the bond dissociation energy, a quantity difficult to obtain accurately by other means8. 

It should be pointed out that the predicted higher ionization potential and lower electron affinity of a cis 1,2-disubstituted ethylene relative to the corresponding trans isomer is based upon the assumption that substantial pi nonbonded inter-... 

Linear correlations between the energy of the LUMO of a molecule and its electron affinity are known39. Since the energy of the LUMO of the disubstituted ethylenes varies, according to our model, in the order, molecular electron affinities will follow the order ... 

Experimental electron affinity data of disubstituted ethylenes is not abundant but Table 25 lists the A s for some typical examples. As can be seen, the A of the 1,1-isomer is larger than the A s for the 1,2-isomers, in accord with our prediction. 

It is tempting to relate the thermodynamics of electron-transfer between metal atoms or ions and organic substrates directly to the relevant ionization potentials and electron affinities. These quantities certainly play a role in ET-thermo-dynamics but the dominant factor in inner sphere processes in which the product of electron transfer is an ion pair is the electrostatic interaction between the product ions. Model calculations on the reduction of ethylene by alkali metal atoms, for instance [69], showed that the energy difference between the M C2H4 ground state and the electron-transfer state can be... 

Any substituted benzyl- ions formed in the course of the reduction will yield eventually a polystyrene, and indeed, a small amount of polymer was found in the reduction products of styrene (17). However, the reduction of compounds which give radicals of higher electron affinity leads to a substantial amount of carbanions. i.e. with those compounds the electron-transfer to a radical competes efficiently with a hydrogen transfer from NH2, e.g. 1,1-diphenyl ethylene gives Pl C-CH3 ion under conditions which yield ethyl benzene from styrene (17). 

It seems to this writer that the first alternative is the correct one. A proton transfer from NHS to styrene- ion is unlikely to be faster than a proton transfer from NH3 to poly-styryl- ion, and it was shown that the latter reaction is not too rapid. Hence, if an electron transfer does take place one might expect dimerization of styrene ions and eventually initiation of polymerization. This might be an alternative explanation for the formation of a small amount of polymer during the reduction, but nevertheless this still remains to be only a minor reaction. On the other hand, in the reduction of 1,1-diphenyl ethylene, the electron affinity of which is higher than that of styrene, the dimeric di-ion, Ph2 C. CH2. CH 2. C. Ph2 is formed in comparable amounts with the monomeric Ph2 C. CH3ion (17). 

Bicycio[m. .0]alkenes (25 in. n) with the double bond linking the bridgehead carbons are another class of relevant compounds. Enthalpy-of-formation data are available for the m = n = 1 case through analysis of gas-phase acidity and electron-affinity measurements76, and for m = 2, n = 2, 3 and 4 from hydrogenation data54 from reaction in a thermochemically innocuous hydrocarbon solvent. Parallelling the earlier transalkylation reactions (equation 14), perhaps the simplest comparison we can make is to use the enthalpy of the reaction of the bicyclic species with ethylene to form two mono-cyclic ones. 

In 1983 the HPMS TCT method for determining molecular electron affinities was introduced. It is based on the same fundamental concepts as the ICR method, but the absolute value was anchored to the electron affinity of benzophenone obtained from the ECD. Later, the scale was anchored to the electron affinity of SO2. At present about 300 electron affinities of organic molecules have been determined by the TCT and/or ECD methods [1,3, 55-59]. The TCT method has been used to measure Ea between 0.50 eV for nitromethane and 3.2 eV for tetracyano-ethylene, as shown in Chapter 10. 

The relation is different when the initial reaction is followed by a virtually irreversible process. For example, reduction of 1,1-diphenyl-ethylene yields radical-anions which subsequently dimerize. The dimerization is virtually irreversible its rate constant was recently determined by the flash-photolysis technique82 and shown to vary from 1 x 108 for the Li+ salt to 30 x 108 M-1 s-1 for the Cs+ salt. The irreversibility of dimerization makes the conversion quantitative in spite of the relatively low electron affinity of the ethylene derivative. 

The electron affinity of trinitrobenzene which characterizes the ability of the compound to form charge-transfer complexes is not very high and is estimated to be equal to 0.6 cV, whereas stronger electron acceptors such as tetracyan-ethylene and chloranil show values of 1.6 and 1.35 eV respectively [86]. 

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