Hydrocarbons photochemical addition reactions

Additions to Aromatic Hydrocarbons. A variety of photochemical additions to aromatic hydrocarbons have been reported. Benzene and its derivatives add to maleic anhydride74-76 as well as to simple olefins,77-80 isoprene,81 acetylene derivatives,79,82 and alcohols.83 The mechanism of the maleic anhydride-benzene reaction is discussed in Section IV. A.4. Naphthalene forms a photoadduct with dimethyl acetylenedicarboxylate62 and with acrylonitrile8211 while anthracene behaves similarly with maleic anhydride84 and with 1,2-benzanthracene.85 The photoaddition of several aromatic amines to anthracene has been reported to proceed via a charge transfer complex86,87 in fact, the majority of these addition reactions may proceed in this manner. 

A study of die addition reactions of radicals to fiillerenes (C60/C70) by EPR has appeared and die dynamic effects in the EPR spectra of fidlerenyl radicals due to hindered rotation and the multi-addition of radicals to fidlerenes are described.9 Other review articles which have appeared this year include recent advances in the radical substitution reactions of alkyl, aryl, and vinyl halides10 and the substitution and photochemical reactions of heterocyclic A -oxidcs.11 The mechanisms for the oxidation of hydrocarbons, lipids, and low-density lipoproteins have been reviewed.12... 

In the photoaddition of a saturated hydrocarbon to ethyl propiolate (equation 19) it is likely that the excited state of the acetylenic ester initiates reaction by abstracting a hydrogen atom from the hydrocarbon. The addition of cyclic ethers to an alkyne seems similar (equation 20), although a ketone sensitizer is required for addition of tetrahydropyran or dioxan . When reactions of this type involve a conjugated acetylenic ester, the first-formed a,p-unsaturated ester can normally undergo further photochemical reaction to produce the p,y isomer (see equations 19 and 20). 

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... 

Compared with 2, cyanoacetylene (1) has hardly been used so far as a dienophile. Since from the reactivity viewpoint it should not differ very much from 2, but since its adducts should be easier to convert into the corresponding hydrocarbons, we have started a comprehensive program aimed at elucidating the scope and limitations of the use of 1 and its derivatives as a partner in thermal, photochemical and catalyzed addition reactions. 

For most unsaturated hydrocarbons, addition of OH is the first and rate-limiting step of the photochemical reaction chain. In the case of aromatics, which are emitted from automobiles, forest fires and fuel wood burning [11], the addition reaction is reversible at atmospheric temperatures. The effective rate constant for removal of the aromatic depends on consecutive reactions of the adduct. Prior to LACTOZ, consecutive reactions with O2 had not been detected for benzene -OH... 

Fluorescence spectroscopy plays an important function in modern food analysis as can be seen from its wide use in the determination of numerous food components, contaminants, additives, and adulterants. This technique has made available very sensitive and selective methods that satisfy the requirements of food analysis, which are usually very complex, taking into account the large number of species to be determined, frequently at very low concentrations, and the wide variety of foodstuffs available. Initially, the use of fluorescence spectroscopy in food analysis was limited to the determination of species with intrinsic fluorescence (e.g., vitamins, aflatoxins, and some polycyclic aromatic hydrocarbons (PAHs)), but now it is widely applied to nonfluorescent species, using several physicochemical means such as chemical or photochemical derivatization reactions. Numerous techniques involve fluorescence detection in liquid chromatography (LC), frequently using pre- or postcolumn derivatization. In addition to conventional fluorime-try, which is commonly chosen for this purpose, other fluorimetric techniques such as laser-induced... 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

Cox (246) estimates the concentration of HN02 to be 109 molec cm"3 in the daytime natural troposphere. The photolysis of HN02 may be an important source of OH in the troposphere, since HN02 absorbs the sun s radiation above 3000 A. The reactions of OH with hydrocarbons (either hydrogen abstraction from paraffins or addition to the double bond in olefins) in the troposphere are known to be the initial steps for photochemical smog formation [see Section VIII-2, p. 333],... 

Atmospheric molecules such as 02, Os, NO and NOz are inherently reactive because of the free radical nature of their electronic structures. In addition, there are literally hundreds of free radical species produced in the atmosphere via either photochemical or dark reactions of various hydrocarbons [1,2,27]. Clearly, an important prerequisite to laboratory studies of atmospheric chemistry is the ability to generate key free radical species in a clean fashion. Some representative techniques for generating the major free radical reactants, i.e., HO, HOO, R, RO and ROO (R = alkyl or other organic group), in combination with a long path IR absorption cell-chemical reactor are described below. 

Kinetic studies have recently been reported in an effort to assess systematically the effects of substituents at silicon and carbon on silene reactivity, and put the earlier theoretical work on a firm experimental basis111,117. Absolute rate constants for addition of methanol served as the diagnostic indicator of silene reactivity in these studies, since methanol addition is mechanistically the best understood reaction of transient silenes (vide infra) and can be studied under the widest possible range of conditions in photochemical experiments. Both studies were carried out in hydrocarbon solution at 23 °C, so as to provide a standard set of experimental conditions for the analysis and to minimize the effects of solvation on Si=C reactivity. 

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