Heterolytic Ionic Reactions

In acid-catalyzed reactions of unsaturated hydrocarbons (alkenes, alkynes, arenes) positive hydrocarbon ions—carbocations—are formed, which are then responsible for the electrophilic transformations 93b 

The carbocations involved in these reactions are trivalent carbenium ions, of which CH3+ is parent. It was Whitmore in the 1930s who first generalized their importance in hydrocarbon transformations based on fundamental studies by Meerwein, Ingold, Pines, Schmerling, Nenitzescu, Bartlett, and others. 

More recently it was realized that hydrocarbon ions (carbocations) also encompass five (or higher) coordinate carbonium ions for which CH5+ is parent.94 Alkanes having only saturated C—H and C—C bonds were found to be protonated by very strong acids, specifically, superacids, which are billions of times stronger than concentrated sulfuric acid.21 

Protolytic reactions of saturated hydrocarbons in superacid media21 were interpreted by Olah as proceeding through the protonation (protolysis) of the covalent C—H and C—C single bonds. The reactivity is due to the electron donor ability of the 7 bonds via two-electron, three-center bond formation. Protolysis of C—H bonds leads via five-coordinate carbocations with subsequent cleavage of H2 to trivalent ions, which then themselves can further react in a similar fashion  

The reverse reaction of carbenium ions with molecular hydrogen, can be considered as alkylation of H2 through the same pentacoordinate carbonium ions that are involved in C—H bond protolysis. Indeed, this reaction is responsible for the long used (but not explained) role of H2 in suppressing hydrocracking in acid-catalyzed 

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Heterolytic (ionic) reactions occur with both electrons remaining with one of the atoms. 

Supercritical fluid (SCF) solvents are unique in that their densities can be varied continuously from gas-like to liquid-like values simply by varying the thermodynamic conditions. Because many of a fluid s solvating properties are strongly dependent on the fluid density, such large changes in density can have dramatic effects on solute reactivity [1,2]. For example, at low pressures supercritical water supports homolytic, free radical reactions, whereas at higher pressures, heterolytic, ionic reactions dominate [3,4]. Thus, thermodynamic control of SCF solvent densities promises to enable us to control reaction outcome and selectively produce desired products. 

In contrast to the abundant examples of radical cleavage, only a few proposals of ionic cleavage of carbon-carbon a bonds have been put forward in the long saga of mechanistic studies on heterolytic cleavage reactions. 

Ionic reactions are those in which covalent bonds break heterolytically, and in which ions are involved as reactants, intermediates, or products. 

All data sets are from V. Pakti (Ed.), Tables of Rate and Equilibrium Constants of Heterolytic Organic Reactions, Vol. 1(1), VINTTI, Moscow, 1975, unless otherwise noted. Superscript numbers indicate temperature other than 25° C. I is ionic strength. 

It would be useful to classify reactions as to their type, heterolytic (ionic) or homolytic (free radical), radical cation, or radical anion. Below are a number of examples of these types of reaction. 

Heterolytic cleavage of X-Y => X+ + Y" ion pair, stabilized by resonance or polar solvent. Characteristic of ionic reactions involving nucleophiles and electrophiles. 

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

If a bond breaks in such a way that both electrons remain with one fragment, the mechanism is called heterolytic. Such reactions do not necessarily involve ionic intermediates, although they usually do. The important thing is that the electrons are never unpaired. For most reactions, it is convenient to call one reactant the attacking reagent and the other the substrate. In this book. 

Table 1.13). In order to classify photochemical reactions, one defines first the electronic transition (a-electron, -electron, or n-electron (non-bonding electron pair)) from which a given reaction occurs. In other words, we have to start by identifying the correspondence of photochemical reactions to the electronic states of the molecular fields. Then, one determines whether a photochemical reaction proceeds via the excited singlet state (S) or the excited triplet state (T). Finally, one has to clarify the type of bond cleavages heterolytic (ionic type) or homolytic (free-radical type). The names of the phenomena or reactions are systematically arranged as a consequence of these considerations.17-23 25)... 

Reactions in which bonds are broken in a homolytic manner (that is, by the movement of single electrons) are called radical reactions. Reactions in which bonds are broken in a heterolytic manner (that is, by the movement of pairs of electrons) are called ionic reactions. 

Other reactions proceed by ionic mechanisms. In ionic reactions, bonds are broken heterolytically. The hydrocarbon framework is unreactive, and the functional groups provide the possible sites of reaction. Whether or not reaction occurs depends on the choice of substrate and reagent. 

Addition mechanisms are broadly defined to be heterolytic, homolytic, or cyclic, which are processes that involve ionic or radical intermediates or are concerted, respectively. Concerted reactions will be discussed in Chapter 11. The emphasis here will be heterolytic (ionic) additions, although we will also consider some aspects of radical reactions. Addition reactions may be categorized further as being electrophilic or nucleophilic. In an electrophilic addition, a compoimd with a multiple bond reacts with an electrophilic reagent to produce an intermediate that subsequently reacts with a nucleophile. In nucleophilic addition, the unsaturated substrate reacts with a nucleophile to produce an intermediate that subsequently reacts with an electrophile to produce the final product. ... 

Organic reactions can be divided mechanistically by the formation of either heterolytic ionic or homolytic radical intermediates. 

In Section 6.1, we mentioned that a bond can be broken in two different ways heterolytic bond cleavage forms ions, while homolytic bond cleavage forms radicals (Figure 11.1). Until now, we have focused mostly on ionic reactions—that is, we have been exploring mechanisms that involve ions. This chapter wiU focus exclusively on radicals. Look carefully at the curved arrows... 

Reactions of Chlorinated Hydrocarbons. The energy of dissociation of the C-Cl bond is relatively low, the manner of dissociation can be heterolytic (ionic) (a), and homolytic (radical) (b). 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

Diacyl peroxides may also undergo non-radical decomposition via the carboxy inversion process to form an acylcarbonate (Scheme 3.27).46 The reaction is of greatest importance for diaroyl peroxides with electron withdrawing substituents and for aliphatic diacyl peroxides (36) where R is secondary, tertiary or ben/,yl.157 The reaction is thought to involve ionic intermediates and is favored in polar solvents 57 and by Lewis acids.158 Other heterolytic pathways for peroxide decomposition have been described.150... 

FIGURE 2.3. The energetics of a heterolytic bond cleavage reaction in a polar solvent. The specific example shown corresponds to the CH3OCH3— CH3 + CH30 reaction in water. The energy of the covalent state does not include the effect of the solvent on this state, but a more consistent treatment (e.g., eq. (2.21) should account for the polarization of the solvent toward the charges of the ionic state. This would result in destabilization of H31. 

In the course of the salt synthesis, it was found that a hydrocarbon [3-2], which was formed by an unfavourable cation-anion combination reaction, dissociates into the original carbocation and carbanion in a polar aprotic solvent (Okamoto et ai, 1985) (1). This was the first example of ionic dissociation of the carbon-carbon a bond in genuine hydrocarbons, although a few cases of heterolytic dissociation of carbon-carbon tr bonds had been reported by Arnett (Arnett et al., 1983 Troughton et al., 1984 Arnett and Molter, 1985) for compounds bearing cyano and nitro groups, e.g. [4-6] and [5-6] as in (2). 

Early attempts to fathom organic reactions were based on their classification into ionic (heterolytic) or free-radical (homolytic) types.1 These were later subclassified in terms of either electrophilic or nucleophilic reactivity of both ionic and paramagnetic intermediates - but none of these classifications carries with it any quantitative mechanistic information. Alternatively, organic reactions have been described in terms of acids and bases in the restricted Bronsted sense, or more generally in terms of Lewis acids and bases to generate cations and anions. However, organic cations are subject to one-electron reduction (and anions to oxidation) to produce radicals, i.e.,... 

The reverse emulsion stabilized by sodium dodecylsulfate (SDS, R0S03 Na+) retards the autoxidation of dodecane [24] and ethylbenzene [21,26,27]. The basis for this influence lies in the catalytic decomposition of hydroperoxides via the heterolytic mechanism. The decay of hydroperoxides under the action of SDS reverse micelles produces olefins with a yield of 24% (T=413 K, 0.02mol L 1 SDS, dodecane, [ROOH]0 = 0.08 mol L 1) [27], The thermal decay gives olefins in negligible amounts. The decay of hydroperoxides apparently occurs in the ionic layer of a micelle. Probably, it proceeds via the reaction of nucleophilic substitution in the polar layer of a micelle. 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

The ESR signal due to 02 demonstrates that the heterolytic activation of R-H has occurred. An interesting feature of the H-D exchange reactions over the MgO surface is the low activation energy, i.e., Ea 2 kcal/mol. This is lower than the gas phase for the reaction H + D2 —> 2 D + HD. The high activity of the ionic oxides has been attributed to the presence of basic sites and in particular to defect sites that are formed during the oxide preparation that persist even at elevated temperatures.41 42... 

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