Elbs reaction mechanism

Another hydroxylation reaction is the Elbs reaction In this method, phenols can be oxidized to p-diphenols with K2S20g in alkaline solution. Primary, secondary, or tertiary aromatic amines give predominant or exclusive ortho substitution unless both ortho positions are blocked, in which case para substitution is found. The reaction with amines is called the Boyland-Sims oxidation. Yields are low with either phenols or amines, generally under 50%. The mechanisms are not clear, but for the Boyland-Sims oxidation there is evidence that the S20 ion attacks at the ipso position, and then a migration follows. ... 

The well-known Elbs reaction has been applied to thiophene syntheses, but the reaction may involve some isomerization. Thus pyrolysis of 3-o-toluoylbenzo[6]thiophene (347) for 3 hours at 340-360 °C gave naphtho[2,1 -b]benzothiophene (348) in 35% crude yield, instead of the expected naphtho[2,3-Z>]benzothiophene. A radical mechanism (Scheme 26) was proposed to explain this result (56JCS3435). 

This statement was first made in 1934 by Lewis and Elbe in the form of a hypothesis that the sum of the thermal and chemical energy is constant. They, however, associated it with the diffusion of active centers for a chain reaction whereas, in fact, variation of the sum of the thermal and chemical energy depends not on the reaction mechanism, but on the interrelation between the thermal diffusivity and the diffusion of the basic components participating in the reaction. 

The salt effect is small and positive, consistent with the attack of an ion on a neutral molecule. Allyl acetate has no effect on the rate or yield of product. Electron-releasing substituents accelerate the reaction. Thus the features of the reaction are similar to those of the Elbs reaction, and point to a mechanism involving nucleophilic attack of the amine on peroxide oxygen. In order to determine whether attack occurred by nitrogen or carbon, Behrman compared the rates of oxidation of amines such as 2-amino-5-methyl-pyridine and 2-amino-6-methyl-pyridine, and found that in all cases the order of reactivity is consistent with attack by nitrogen. Furthermore, the rate of oxidation of 3,5-dideutero-2-amino-pyridine is the same as for the normal material, and the yield of product is the same. Behrman proposed the reaction scheme... 

Dauben investigated the mechanism by employing a C-labeled carbonyl in an Elbs reaction. In this study, he and his co-workers showed that volatile by-products, which were trapped and analyzed accounted for 35% of the radioactivity taken into the reaction. The authors also suggested that optimum yield occurred after about 3 h of reaction time at elevated temperature for the conversion of diary Iketone 20 into dibenzanthracene 21. 

Recently, Behiman and coworkers discussed the mechanism of the Elbs oxidation reaction and explained why the para product predominates over the ortho product in this oxidation. According to the authors, semiempirical calculations show that the intermediate formed by the reaction between peroxydisulfate anion and the phenolate ion is the species resulting from reaction of the tautomeric carbanion of the latter rather than by the one resulting from the attack by the oxyanion. This is confirmed by the synthesis of the latter intermediate by the reaction between Caro s acid dianion and some nitro-substituted fluorobenzenes. An example of oxidative functionalization of an aromatic compound is the conversion of alkylated aromatic compound 17 to benzyl alcohols 20. The initial step in the mechanism of this reaction is the formation of a radical cation 18, which subsequently undergoes deprotonation. The fate of the resulting benzylic radical 19 depends on the conditions and additives. In aqueous solution, for example, further oxidation and trapping of the cationic intermediate by water lead to the formation of the benzyl alcohols 20 (equation 13) . ... 

Hydroxylation of arylamines with persulfate ion, or Boyland-Sims oxidation, gives ortho-substituted aminophenols in good yields [29]. As with the Elbs oxidation, the procedure is also carried out in two steps - first, treatment with the oxidant to obtain an aminophenyl sulfate ester and, second, hydrolysis to obtain the final product. Primary, secondary and tertiary amines can all be used in this reaction. The ortho product is formed, except when no ortho-positions are available, which leads to para-substitution. Electrophilic attack on the ipso-carbon is believed to be the most likely mechanism, although minor radical pathways also seem to be present. 

In addition to the small dependence of the second limit on vessel surface, Table 6 also shows a small dependence of the limit on the vessel size. Thus, Lewis and von Elbe [4] quote a variation in the second limit for 2H2 + O2 from 88 torr with a 10 cm diameter spherical KCl coated vessel at 530 °C to 68 torr for a 1.8 cm diameter vessel. They invoke a special mechanism for this, involving the postulated reaction H + O2 +... 

The molecules CI2O and O-CI-O are well known and need not be discussed. The preparation of CI2O3 has been reported only recently from the photolysis of OCIO. The products of the reaction are CI2, Oj, CI2O3 and CI2O6. The mechanism suggested by McHale and von Elbe is... 

These experimental data are already sufficient to indicate that the photooxidation of gaseous ethanal takes place via a chain reaction. Accepting the processes of propagation suggested by Backstrom, von Elbe and Lewis wrote out a kinetic scheme which seems to be a special case of the mechanism we have already referred to in dealing with photooxidation of ethanal in the liquid phase, i.e., the case where termination processes (t4 and ts) are negligible in comparison with termination process (ta). 

Methane and air in the proper proportions do not react until some other source ignites the rapid process. Possible mechanisms of chemical reactions involved in the ignition process are given by Lewis and Von Elbe [79]. A number of oxygenated intermediates are formed along with free radicals like OH in the process of forming C02 and H20. 

Elbs oxidation of 2- and 3-hydroxypyridine gives in each case mainly 2,5-dihydroxypyridine. In the first case some 2,3-dihydroxypyridine, and in the second, some 2,3- and 3,4-dihydroxypyridine are also formed. The kinetics and mechanism of the oxidation of 2-hydroxypyridine have been examined, and the reaction could involve attack by the anion of the hydroxy compound upon the peroxy bond of the persulphate ion, with the displacement of sulphate ioni sa, 2-Aminopyridine gives hydrogen 2-amino-3-pyridyl sulphate but 4-aminopyridine behaves differently (p. 359). 

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