Rate-limiting benzene

Nitration at the encounter rate and nitrosation As has been seen ( 3.3), the rate of nitration by solutions of nitric acid in nitromethane or sulpholan reaches a limit for activated compounds which is about 300 times the rate for benzene imder the same conditions. Under the conditions of first-order nitration (7-5 % aqueous sulpholan) mesitylene reacts at this limiting rate, and its nitration is not subject to catalysis by nitrous acid thus, mesitylene is nitrated by nitronium ions at the encounter rate, and under these conditions is not subject to nitration via nitrosation. The significance of nitration at the encounter rate for mechanistic studies has been discussed ( 2.5). 

Even so, the results were claimed to show a greater resemblance to nitrations with nitronium salts than to nitrations in organic solvents. However, reaetion at the eneounter rate ( 3.3) imposes a limit to the rate of reaetion in these media, whieh deereases from 40 times the rate for benzene in 68 % sulphuric acid to 6 times the rate in 80 % sulphurie acid. Therefore it is reasonable to expeet that in stronger solutions even under homogeneous eonditions, the rates of these eompounds would approximate to that of benzene. 

Enough mutual polarisation can apparently result, in (8), for (9) to form, but polarisation of the bromine molecule may be greatly increased by the addition of Lewis acids, e.g. AlBr3 (cf. bromination of benzene, p. 138), with consequent rise in the rate of reaction. Formation of (9) usually appears to be the rate-limiting step of the reaction. 

These observations led the authors to propose the mechanism shown in Scheme 37. The acetic acid is mainly in the form of dimers when the solvent is 1% (v/v) acetic acid-benzene. As a result, the equilibrium concentration of Pb(OAc)3(OR) is small and the formation of Pb(OAc)2(OR)2 is favoured. Hence the reaction proceeds through the intermediate, Pb(OAc)2(OR)2, and the 2 step is rate-determining. At higher percent acetic acid, the increased concentration of the monomeric form of acetic acid prevents the formation of Pb(OAc)2(OR)2. As a result, the product is formed via the rate-limiting disproportionation of Pb(0Ac)30R, i.e. the k step is rate-determining. 

There is a very significant difference between the rate of aromatization of trans- and c/i-hexatriene (Table III), which shows that geometrical isomerization prior to cyclization may be rate limiting. Since this occurs via half-hydrogenated species (60), it is promoted by the presence of hydrogen, and so is benzene formation. It should be noted that cyelohexane and cyclohexene are produced from cw-triene. The hydrogenation of cyclohexadiene may explain their formation here and in other cases of stepwise Cg dehydro-cyclization. 

A kinetic smdy has been reported of substituent effects on the reactions of 2-phenoxy- and 2-(4-nitrophenoxy)-3-nitro-5-X-thiophenes with benzylamine and with A-methylbenzylamine in benzene as solvent. The intramolecularly hydrogen-bonded intermediate (14) is postulated. Reactions of the 5-unsubstimted thiophenes (X = H) are not base-catalysed, indicating that nucleophilic attack is rate limiting, and the more basic secondary amine shows higher reactivity than the primary... 

Kinetic smdies of the iodination of benzene and acetanilide by iodine, diiodine pentoxide, and sulfuric acid in acetic acid indicate that benzene is involved in an equilibrium reaction prior to the rate-limiting <7-complex formation." It is proposed that this equilibrium involves the formation of a itt-complex between iodine adsorbed on diiodine pentoxide and the benzene as it is adsorbed. In the case of acetanilide the a-complex is formed directly with activated iodine adsorbed on the diiodine pentoxide. 

Haloform reaction, 237, 296 Halogenation alkanes, 300, 323 alkenes, 179,186, 313 benzene, 138,316 ketones, 295 Hammett equation, 362 additional parameters, 374, 388, 395 derivation of, 362 deviations from, 375 empirical nature of, 395 implications of, 394 reaction pathway, and, 375 solvent effects and, 388 spectroscopic correlations, 392 standard reaction for, 362, 395 steric effects and, 361, 383 thermodynamic implications of, 394 Hammett plots, 359 change in rate-limiting step and, 383 change in reaction pathway and, 378... 

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