Substitution mechanism, acidic radicals

Anodic oxidation of methoxybenzenes in aqueous sulphuric acid also leads to loss of the methoxy substituent, this time through jju o-substitution on the radical-cation by water. Anisole and 4-niethoxyphenol are both converted to quinone [81]. The elimination of methanol is catalysed by protons by the mechanism illustrated in Scheme 6.8. Diphenyl derivatives have also been isolated from oxidation of some methoxybenzenes. They arise through the competitive reaction involving a... 

The oxidation of hydrocarbons by cobalt(lll) acetate has been thoroughly investigated, due to its relevance to industrial homolytic oxidation processes.56 361 547 Radical intermediates are produced from one-electron oxidation of hydrocarbons according to an electron transfer or an electrophilic substitution mechanism previously described in equations (200)-(203). These oxidations are dramatically accelerated by the presence of strong acids or halide salts. 

As a preparative method the direct decarboxylation of olefinic acids is almost limited to the formation of styrenes and stilbenes from substituted cinnamic acids. Thermal decomposition of cinnamic acid gives styrene (41%). The yield is nearly quantitative if the reaction is carried out in quinoline at 220° in the presence of a copper catalyst. The yields of substituted styrenes where the aryl radical contains halo, methoxyl, aldehyde, cyano, and nitro groups are in the range of 30-76%. cis-Stilbene and cis-p-nitrostilbene are prepared in this way from the corresponding a-phenylcinnamic acids (65%). One aliphatic compound worthy of mention is 2-ethoxypropene, prepared by heating -ethoxycro-tonic acid at 165° (91% yield). The mechanism of acid-catalyzed decarboxylations of this type has been studied. Isomerization of the double bond from the a,/5- to the /5, y-position before decarboxylation very likely occurs in many instances. ... 

Products of attack by OH radicals rather than hydrolysis by supercrihcal water occur when 4-nitrophenyl acetate is sonolysed in argon-saturated water. " Hydrolyses of substituted benzoic acid esters in near-critical water (250-300 °C) show autocatalytic kinetic behaviour and surprisingly give the same rate constant regardless of substituent, suggesting that an acid-catalysed mechanism predominates. ... 

The reaction mechanism involves superoxide anion radicals O, produced from L-cysteine Fe(ll) or L-cysteine Co(ll) complexes, which has an important role to attack on the carbonyl carbon and formed corresponding peroxoacid anions or peroxoacid intermediates which give oxidative, non-oxidative decarboxylated substituted cinnamic acid as well as some cyclized boizofuran. But for ferulic acid, no cyclized product was formed due to positive inductive effect of methoxy group in the benzene ring. 

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. 

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

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