Polar reactions involving ionic intermediates

Addition reactions occur when two starting materials add together to form only one product. 

The mechanism of these reactions can involve an initial electrophilic or nucleophilic attack on to the key functional group. 

Elimination reactions are the opposite of addition reactions. One starting material is converted into two products. 

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Such radicals or ion pairs are formed transiently as reactive intermediates in a very wide variety of organic reactions, as will be shown below. Reactions involving radicals tend to occur in the gas phase and in solution in non-polar solvents, and to be catalysed by light and by the addition of other radicals (p. 300). Reactions involving ionic intermediates take place more readily in solution in polar... 

As befits their status as compounds well-known to be in equilibrium with carbonium ions in suitable solvents, triphenylmethyl halides and related compounds give particularly unambiguous evidence of reaction involving ionic intermediates. In polar solvents they give... 

When a functional group (X) is attached to a carbon atom, the bond polarity is synthetically important out to about the third bond, as indicated in 24. If we make the assumption that most reactions involve ionic intermediates or highly polarized species, then three bonds become very important the C-X bond (called the a... 

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... 

Effect of Solvent on El versus E2 versus ElcB. With any reaction a more polar environment enhances the rate of mechanisms that involve ionic intermediates. For neutral leaving groups, it is expected that El and ElcB... 

The solvent may have a positive effect on the rate and or selectivity of the reaction. The general rule is reactions that involve ionic intermediates will be faster in polar solvents. For example, SN1 substitutions proceed best in polar solvents while SN2 substitutions, which involve a covalent intermediate, fare better in apolar solvents. The solvent may effect the position of an equilibrium, e.g. in a keto-enol mixture, thereby influencing selectivity in a reaction which involves competition between the two forms. 

Some indication of the importance of the solvent interactions is afforded by the observation that of the thousands of reactions which have been studied in solution, less than 20 have been capable of comparative study in the gas phase. The study of ionic reactions has been almost completely restricted to solutions for reasons which are quite understandable ionic processes are virtually nil in the gas phase at temperatures below 1000 K. However, this accounts for most of the solution reactions studied, since, as we shall see, most reactions between polar molecules involve ionic species as intermediates. Thus such common reactions as the hydrolysis of alkyl halides or of esters do not proceed at measurable rates in the gas phase (at least not at temperatures at which other, competing reactions are not dominant). The only large class of reactions which proceed conveniently in both gas and liquid states is the free radical class, and undoubtedly as... 

If most reactions involve ionic chemistry, as a first step, disconnect those bonds that lend themselves to formation of ionic or polarized intermediates. A polarized bond results from the presence of a heteroatom, as discussed in Chapter 3. Therefore, it is reasonable to assume that many reactive intermediates arise from a functional group that contains one or more heteroatoms. Therefore, when a disconnection is made, that disconnection is probably adjacent or close to a functional group. Disconnection of a polarized bond in a retrosynthesis should lead to a polarized or ionic fragment, and such fragments will form that bond in the synthesis via ionic or highly polarized chemical reactions. 

Ionic reactions, also called polar reactions, involve the participation of ions as reactants, intermediates, or products. In most cases, the ions are present as intermediates. These reactions represent most (approximately 95%) of the reactions that we will encounter in this text. The other two major categories, radical reactions and pericycHc reactions, occupy a much smaller focus in the typical undergraduate organic chemistry course but will be discussed in upcoming chapters. The remainder of this chapter will focus on ionic reactions. 

Ionic reactions, also called polar reactions, involve the participation of ions as reactants, intermediates, or products—in most cases, as intermediates. 

Addition of Br2 or CI2 can of course also take place by the electrophilic mechanism. Since this involves ionic intermediates, it occurs much faster in polar solvents. Ionic addition therefore prevails in solvents such as water or acetic acid. Radical reactions take place almost equally well in solvents of all kinds. Radical addition is therefore favored by nonpolar solvents such as carbon tetrachloride and by the presence of radicals or atoms produced, e.g., by light. 

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. ... 

It has been demonstrated that ionic intermediates are not involved in the epoxidation reaction. The reaction rate is not very sensitive to solvent polarity.71 Stereospecific syn addition is consistently observed. The oxidation is therefore believed to be a concerted process. A representation of the transition structure is shown below. 

In agreement with the involvement of ionic intermediates for electrophilic halogenation of alkenes, an important role is also exerted by the solvent. Not only the reaction rate is strongly solvent-dependent, but also the stereochemical course of the addition process may be affected by the polarity of the medium. Solvent properties determine the reaction rate the overall kinetic order the nature of the products the stereochemistry of the products... 

Evidence for the polar character of the transition state is that electron-withdrawing groups in the para position of toluene (which would destabilize a positive charge) decrease the rate of hydrogen abstraction by bromine while electron-donating groups increase it,10 However, as we might expect, substituents have a smaller effect here (p -1,4) than they do in reactions where a completely ionic intermediate is involved, e.g., the SnI mechanism (see p. 344). Other evidence for polar transition states in radical abstraction reactions is mentioned on p. 685. For abstraction by radicals such as methyl or phenyl, polar effects are... 

The rates of Diels-Alder reactions are little affected by the polarity of the solvent. If a zwitterionic intermediate were involved, the intermediate would be more polar than either of the starting materials, and polar solvents would solvate it more thoroughly. Typically, a large change of solvent dipole moment, from 2.3 to 39, causes an increase in rate by a factor of only 10. In contrast, stepwise ionic cycloadditions take place with increases in rate of several orders of magnitude in polar solvents. This single piece of evidence rules out stepwise ionic pathways for most Diels-Alder reactions, and the only stepwise mechanism left is that involving a diradical. 

In principle, macrocyclic complexing agents may be used to enhance any reaction in which ions, ionic intermediates or highly polar species are involved. Important examples include ... 

Recently, trans insertion of hexafluorobutyne into one of the M—H bonds in some metallocene hydrides, Cp2MH , was studied in some detail (47). Experiments carried out in the presence of various radical-sensitive reagents such as TV-phenyl-a-naphthylamine suggested that a free radical mechanism was unlikely. A stepwise ionic mechanism, involving a zwitter-ionic intermediate, Cp2(H2)M+—C(CF3)==CCF3, is improbable, since (i) the stereochemistry and the apparent rate are not influenced by the polarity of the solvents, (ii) no deuterium is incorporated in the reaction in EtOD, and (iii) the trend in reactivity (Mo > W) does not reflect the trend in v-basicity or M—C bond stability (W > Mo). An essentially concerted trans-insertion mechanism is inferred, which is supported inter alia by the low kinetic deuterium isotope effect (kH/k0 = 1). 

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