Reactivity of hydrated electrons toward

On the Reactivity of Hydrated Electrons Toward Inorganic Compounds... 

According to the Marcus theory [64] for outer-sphere reactions, there is good correlation between the heterogeneous (electrode) and homogeneous (solution) rate constants. This is the theoretical basis for the proposed use of hydrated-electron rate constants (ke) as a criterion for the reactivity of an electrolyte component towards lithium or any electrode at lithium potential. Table 1 shows rate-constant values for selected materials that are relevant to SE1 formation and to lithium batteries. Although many important materials are missing (such as PC, EC, diethyl carbonate (DEC), LiPF6, etc.), much can be learned from a careful study of this table (and its sources). 

Anbar, M. and Hart, E.J., Reactivity of aromatic compounds toward hydrated electrons, /. Am. Chem. Soc., 86, 5633-5636, 1964. 

Hydrogen atoms and hydroxyl radicals react with aliphatic compounds mainly by H-abstraction from the chain, although reactions with certain substituents are also important. With hydrated electrons the functioned group is the only site of reaction and its nature determines the reactivity. The reactions of hydrated electrons are by definition electron transfer reactions. The rate of reaction of a certain substrate will depend on its ability to accommodate an additional electron. For example, in an unsaturated compound the rate may depend on the presence of a site with a partial positive charge. Thus acrylonitrile and benzonitrile are three orders of magnitude more reactive toward e q than are ethylene and benzene. On the other hand, this large difference does not exist in the case of addition of H and OH. 

EJ Hart and M Anbar have detailed the characteristics and the chemistry of the solvated electron in water, otherwise known as the hydrated electron and denoted by e] y or e. A number of reviews on the solvated electron are also available.In this article, we will recall briefly the main steps of the discovery and the principal properties of the solvated electron. We will then depict its reactivity and focus on recent results concerning the effect of metal cations pairing with the solvated electron. At last, we will present results on the solvation dynamics of electron. Due to the development of ultrashort laser pulses, great strides have been made towards the understanding of the solvation and short-time reactivity of the electron, mainly in water but also in polar solvents. However, due to the vast and still increasing literature on the solvated electron, we do not pretend for this review to be exhaustive. 

The formation of hydrated electrons (in the water-saturated zeolites X and Y) has been identified through their absorption spectra, their short lifetimes distinct from the long-lived cation cluster-trapped electrons, and their reactivity towards typical hydrated electron quenchers such as methylviologen. Based on these spectra (Fig. 5), yields between 4 x 10 mol and 6 x 10 mol were measured for electrons in fully hydrated NaY. These high radiolytic yields were also explained by electron transfer from the ionized zeolitic skeleton to the water clusters [Eqs. (1) and (7)]. 

The study of the reactivity of towards an extensive series of monosubstituted benzene derivatives CjHsX revealed the first linear firee-energy relation for reactions of hydrated electrons (Anbar and Hart, 1964b). The specific rates of these reactions ranges over four orders of magnitude from 4 x 10 sec for phenol to 3 x 10 sec for nitrobenzene. 

This results in a net lower reactivity of dinitrogen monoxide towards the hydrated electron in acid solution. 

The reactivity of organic compounds towards the hydrated electron depends on the availability of a low-lying vacant electron orbital. Thus saturated hydrocarbons and the corresponding amines and alcohols are unreactive. 

Anbar and Hart [63] have correlated the reactivities of aromatic compounds towards the hydrated electron in terms of the Hammett a function and have shown that the rates are a function of the tt electron density. The results are shown in Fig. 4. Hammett s equation can be written as... 

Oxidative reactions of 2AP(-H) have been studied in oxygen-saturated solutions because 2AP(-H) radicals do not exhibit observable reactivities towards O2 [10]. In contrast, O2 rapidly reacts with hydrated electrons (rate constant of 1.9x10 ° s ) [51] and hydrated electrons do not interfere in... 

The Reactivity of Different Chemical Species toward Hydrated Electrons and the Products of these Reactions... 

Table V. Comparison of Optical Hydrated Electron Production with Reactivity toward Hydrated Electrons...

It is suggested, therefore, that hydrated electrons are not likely to be formed in the intracellular fluid and that the formation of solvated electrons is also of low probability in the presence of solutes that are capable of accommodating electrons. On the other hand, it should be remembered that electrons are being formed in radiolysed living systems and are finally incorporated in certain functional groups of the molecules involved. A qualitative, and perhaps a semiquantitative, correspondence is expected between the tendency of the constituents of the living cell to incorporate an electron and their reactivity towards hydrated electrons in dilute solutions. From this standpoint only, it may be beneficial to acquire qualitative as well as quantitative information on the reactions of biopolymers and their functional groups with hydrated electrons. 

Carbohydrates are practically non-reactive towards hydrated electrons, and upper limits of specific rates of the order of 106 M-1 sec-1 have been reported (Davis et al., 1965b). The result is consistent with the non-reactivity of alcohols and ethers. 

Purines and pyrimidines are highly reactive towards hydrated electrons, as has been described in Section I, and most of them react at... 

The reactivity of an organic compound toward eaq depends on its functional groups because the main hydrocarbon chain is non-reactive. Aliphatic alcohols, ethers, and amines are also nonreactive (k 106 M 1 s-1), although alkylammonium ions show a slight reactivity and can transfer a proton to the hydrated electron. Isolated double bonds are practically nonreactive, for ethylene k <2-5 X 106 M -1 s-1, but conjugated systems or double bonds with an electron withdrawing group attached to them are very reactive. For example, butadiene and acrylic acid react with practically diffusion controlled rates ( 10 0 M -1 s-1). 

The hydrated electron is obviously a nucleophile and its reactions are affected by substituents correspondingly. The hydroxyl radical is expected to behave as an electrophile and this behaviour was, indeed, demonstrated with aromatic compounds. The low reactivity of O toward aromatic and olefinic ir-systems suggests that this species behaves as a nucleophile because of its charge. The behaviour of hydrogen atoms is not easily predictable the effect of substitution in benzene demonstrated a slight electrophilicity. 

The same structural features that favour or disfavour hydrate formation are important in determining the reactivity of carbonyl compounds with other nucleophiles, whether the reactions are reversible or not. Steric hindrance and more alkyl substituents make carbonyl compounds less reactive towards any nucleophile electron-withdrawing groups and small rings make them more reactive. 

---