Photochemical Ionic Reactions

Cyclisation of malonates is one of the few reliable ionic reactions giving four-membered rings. Hence when the ais and trans ketones (38) were wanted for a photochemical study, acids (39) were the obvious starting materials as these could be made by cyclisation of (40). Analysis... 

The original work was on ionic reactions in normal micelles in water, but subsequently there has been extensive work on reactions in reverse micelles (O Connor et al., 1982, 1984 Kitahara, 1980 O. A. El Seoud et al., 1977 Robinson, et al., 1979). There also has been a great deal of work on photochemical and radiation induced reactions in a variety of colloidal systems, and microemulsions have been used as media for a variety of thermal, electrochemical and photochemical reactions (Mackay, 1981 Fendler, 1982 Thomas, 1984). 

This section gives tabulated examples of recent work on micellar effects upon chemical and photochemical reactions. In general the examples given in this section do not duplicate material covered elsewhere in the chapter for example micellar effects on some photochemical reactions and reactivity in reversed micelles are listed here although they are neglected in the body of the text. For many ionic reactions in aqueous micelles only overall rate effects have been reported, in many cases because the evidence did not permit estimation of the parameters which describe distribution of reactants between aqueous and micellar pseudophases. These reactions are, nevertheless, of considerable chemical importance, and they are briefly described here. 

The intermediacy of a cation, formed by electron transfer within a photochemically created radical pair, was also invoked to explain the results obtained upon photolysis of 2-bromo-, 2-chloro- and 2-iodopyridine in methanol, ethanol and acetonitrile-water376. The major products are 2-methoxypyridine, 2-ethoxypyridine and 2-acetamidopyridine + 2-hydroxypyridine. In all cases pyridine was the minor reaction product, in contrast with the 3- and 4-halopyridines which produce pyridine exclusively, via a radical process. It is proposed that the unshared electron pair on the nitrogen atom assists in the formation of the 2-pyridyl cation. The presence of cupric salts increases the relative amounts of products formed via ionic reactions because Cu2+ can accept an electron from the 2-pyridyl radical. 

A detailed study of the photochemical behaviour of the alkyl halides (196) has been reported.Measurement of the photophysical data of the compounds (197) has been carried out. The photoisomers of Dieldrin, Aldrin and Endrin (198) are photoreactive in the presence of triethylamine. For example, the irradiation of photoaldrin (198a) affords the dehalogenated compounds (198b) and (198c) in a ratio of 1 5.° Ionic reactions in photochemistry have been reviewed by Chow and Wu.° ... 

Introduction Molecular Orbitals and Frontier Orbitals Ionic Reactions Thermal Pericyclic Reactions Radical Reactions Photochemical Reactions Exceptions. 

In our analysis of the synthesis of rings of various sizes, we concluded (Chapter 29) that four-membered rings are uniquely difficult. For this reason, a special method, the photochemical 2 + 2 cycloaddition is often used to make four-membered rings. Some 2 + 2 thermal cydoadditions, particularly of ketenes (Chapter 33), and some ionic reactions (page 272) are also useful. 

Cyclobutene (3) was used on page 269 in a photochemical cycloaddition. It is made by ionic reactions from readily available adipic acid (27) (see Chapter 27). This is something of an exception and cyclisation by carbonyl condensations is not normally a recommended route to four-membered rings... 

There are in principle three possibilities for reaction of halogens with aromatic hydrocarbons, namely, addition, substitution in the nucleus, and substitution in a side chain. The last of these is discussed on pages 152 and 157. Substitution of benzene by chlorine or bromine is an ionic reaction,114 whereas photochemical or peroxide-catalyzed addition of these halogens involves a radical-chain mechanism.115 Substitution in the side chain also proceeds by a radical mechanism,116 addition rather than side-chain substitution being favored by higher chlorine concentrations.115... 

While in pure alkanes the lifetime of the ions is very short, this is not necessarily the situation in the presence of solutes. In some cases, solute anions and cations formed by electron capture and charge transfer, respectively, may have considerably long lifetimes. If this occurs, it is possible that ionic reactions take place and therefore the free radical mechanism of the chain reaction has to be established. This can be done by various methods such as the addition of electron, charge and radical scavengers, and the determination of the effect of these additives on the chain reaction. Another method is to show that the same reaction can also be initiated thermally or photochemically. 

The solid state ionic chiral auxiliary method of asymmetric synthesis is not limited to Yang photocychza-tion and can, in principle, be applied to any chemical reaction — photochemical or thermal — that... 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

The initial discussion in this chapter will focus on addition reactions. The discussion is restricted to reactions that involve polar or ionic mechanisms. There are other important classes of addition reactions which are discussed elsewhere these include concerted addition reactions proceeding through nonpolar transition states (Chapter 11), radical additions (Chapter 12), photochemical additions (Chapter 13), and nucleophilic addition to electrophilic alkenes (Part B, Chi iter 1, Section 1.10). 

The radical X is formed by homolysis of the X—R bond either thermally or photolytically. In the reactions of alcohols with lead tetraacetate evidence suggests that the X—R bond (X = 0, R = Pb(OAc)3) has ionic character. In this case the oxy radical is formed by a one electron transfer (thermally or photochemically induced) from oxygen to lead. 

The thermal (or photochemical) decomposition of the azo group gives rise to a radically initiated polymerization. The reactive site F, the transformation site, however, can, depending on its chemical nature, initiate a condensation or addition type reaction. It can also start radical or ionic polymerizations. F may also terminate a polymerization or even enable the azo initiator to act as a monomer in chain polymerizations. 

Boddington and Iqbal [727] have interpreted kinetic data for the slow thermal and photochemical decompositions of Hg, Ag, Na and T1 fulminates with due regard for the physical data available. The reactions are complex some rate studies were complicated by self-heating and the kinetic behaviour of the Na and T1 salts is not described in detail. It was concluded that electron transfer was involved in the decomposition of the ionic solids (i.e. Na+ and Tl+ salts), whereas the rate-controlling process during breakdown of the more covalent compounds (Hg and Ag salts) was probably bond rupture. 

Irradiation of an ITIES by visible or UV light can give rise to a photocurrent, which is associated with the transfer of an ion or electron in its excited state. Alternatively, the photocurrent can be due to transfer of an ionic product of the photochemical reaction occurring in the solution bulk. Polarization measurements of the photoinduced charge transfer thus extend the range of experimental approaches to... 

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