Ionization potentials correlation with reactivities

Figure 20 Correlation of the reactivities of various enol ethers toward (p-ClCsH-OsCH at 20° C in acetonitrile [L mol-1 sec ] with their ionization potentials. (Reprinted with permission from Ref. 136. Copyright 1991 American Chemical Society.)...

The rates of attack of radicals on aromatic rings correlate with ionization potential,318 with localization energy319 and with superdelocalizability (p. 58),320 a picture reminiscent of the situation in aromatic electrophilic substitution. As in that field, there are evidently a number of related factors affecting reactivity. Frontier orbitals provide useful explanations for a number of observations in the field, as the following examples show. 

The exchange of a considerable number of linear, branched-chain, and cyclic alkanes have been studied (5i). The exchange rate increases with increase in the carbon chain length forn-alkanes (methane to hexane). It is found that there is a linear correlation between the logarithm of the exchange rate and the ionization potential of the alkane (Fig. 5 n-alkanes are plotted as circles). This correlation extends to aromatic compounds (Fig. 5 aromatic compounds are plotted as squares) and is evidence that alkanes and aromatic compounds react by a common mechanism. Indeed, the least reactive aromatic, benzene, is only about... 

Similarly to the low chemical reactivity of (simple) alkylsilanes devoid of functional groups, the electrochemical reactivity of simple alkylsilanes is quite low. Klingler and Kochi measured the oxidation potentials of tetraalkyl derivatives of group-14-metal compounds by using cyclic voltammetry3. These compounds exhibit an irreversible anodic peak in acetonitrile. The oxidation potential (7 p) decreases in the order of Si>Ge>Sn>Pb as illustrated in Table 1. This order is the same as that of the gas-phase ionization potentials (7p). The absence of steric effects on the correlation of Ev with 7p indicates that the electron transfer should take place by an outer-sphere mechanism. Since tetraalkylsilane has an extremely high oxidation potential (>2.5 V), it is generally difficult to oxidize such alkylsilanes anodically. 

Correlation of ionization potentials with reactivities or formation constants of reactions of alkenes with ArSCl, MeCOsH, Ag+ and Hg2+ revealed that additions whose first step is rate-determining are sterically independent, while those with the second step ratedetermining are sterically dependent411. 

Quantitative structure-activity relationships (QSARs) are important for predicting the oxidation potential of chemicals in Fenton s reaction system. To describe reactivity and physicochemical properties of the chemicals, five different molecular descriptors were applied. The dipole moment represents the polarity of a molecule and its effect on the reaction rates HOMo and LUMO approximate the ionization potential and electron affinities, respectively and the log P coefficient correlates the hydrophobicity, which can be an important factor relative to reactivity of substrates in aqueous media. Finally, the effect of the substituents on the reaction rates could be correlated with Hammett constants by Hammett s equation. 

Yoshimuzi 17>, using this method with different values for m, calculated the electronic distribution produced by substituting a heteroatom for hydrogen. He found that a value of the m parameter such that ma = 0.12 was necessary in order to reproduce the dipole moments of a set of linear paraffins. Fukui et al. 18>, using the positive square root of 0.12, i.e. m— +0.35, were able to correlate the ionization potentials, heats of formation, and bond energies in linear as well as cyclic hydrocarbons and their derivatives. It was also shown that the method permits a coherent interpretation of inductive effects to be made so that a relation exists between some calculated values and the reactivity. 

The yield of alcohol ROH depends on the nucleophility of the radical as well as on the degree of the delocalization of an unpaired dectron. If the unpaired electron is not ddocalized, the reactivity of the n-alkyl radical increases with its nucleophility which may be correlated with the ionization potential. On the other hand, if the unpaired spin is delocalized, the orbital overlap of the radical and the peroxidic oxygen becomes an important reactivity factor. A radical seemingly... 

In contrast to oxidations with Mn(III) acetate, the oxidation of alkylbenzenes by the stronger oxidant, Co(III) acetate, appears to involve only electron transfer. No competition from classical free radical pathways is apparent. Waters and co-workers,239,240 studied the oxidation of a series of alkylbenzenes by Co(III) perchlorate in aqueous acetonitrile. They observed a correlation between the reactivity of the arene and the ionization potential of the hydrocarbon which was compatible with the formation of radical cations in an electron transfer process. 

The proximity of the diffusion limit also inhibits a detailed discussion of the data in Table 7, but a significant difference to the substituent effects discussed in Section III.D.4 is obvious. Whereas the reactivities of terminal alkenes, dienes, and styrenes toward AnPhCH correlate with the stabilities of the new carbenium ions and not with the ionization potentials of the 7r-nucleophiles [69], the situation is different for the reactions of enol ethers with (p-ClC6H4)2CH+ [136]. In this reaction series, methyl groups at the position of electrophilic attack activate the enol ether double bonds more than methyl groups at the new carbocationic center, i.e., the relative activation free enthalpies are not controlled any longer by the stabilities of the intermediate carbocations but by the ionization potentials of the enol ethers (Fig. 20). An interpretation of the correlation in Fig. 20 has not yet been given, but one can alternatively discuss early transition states which are controlled by frontier orbital interactions or the involvement of outer sphere electron transfer processes [220]. 

There are a number of studies which indicate that certain TICT molecules undergo specific interactions with solvent or quencher molecules. One of the early proposals to explain the dual fluorescence of DM ABN was in fact that of a solute-solvent exciplex (excited complex) [26-28]. The first specific interaction documented as such, however, was that of DMABN (and model compounds) with saturated amines, where a mechanism necessitating a close approach was postulated. This was concluded from the lack of correlation of the quenching reactivity with the ionization potential of the amine quencher. However, a correlation was found between the reactivity pattern and the sterical demands of the saturated amine [140,141]. The complexes formed were non-emissive and were proposed to involve a two-center-three-electron bond between the aromatic and the saturated amino group. 

Hydrogen ion titration curves of proteins provide a powerful tool to reveal many aspects of the structures of individual proteins. The characteristic ionization constants of the acidic and basic groups in the amino acids and peptides may be profoundly modified when these groups are incorporated in a protein molecule. An increasing number of proteins have been found in which potentially reactive groups are inaccessible for titration in the native molecule, and become available only after denaturation. Such findings can, in the years ahead, be correlated with detailed knowledge of the three dimensional structure of proteins, as obtained by X-ray diffraction and other methods. The present state of the field is reviewed by Tanford, who has done so much to advance it over the last decade. 

This reaction depends on the type of nitrogen substitution and it is proposed that the reactivity may correlate with the ionization potential of the imine. Oxidized derivatives, i.e. the oxime (31 = OH) ... 

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