Ortho-distributed

Recently, we investigated the associative alkylation reaction of toluene with methanol catalyzed by an acidic Mordenite (see Figures 13 and 14) by means of periodic ab initio calculations." We observed that for this reaction some transition selectivity occurred, and induced sufficiently large differences in activation energies to explain the small changes in the para/meta/ortho distribution experimentally observed on large pore zeolites. Thepara isomer is the more valuable product as it is an important intermediate for terphthalic acid, an important polymer monomer." The steric constraints obtained for the transition state structures could be estimated from local intermediates for which the orientations of the toluene molecule were similar as the ones observed for the transition states (see Figure 14). 

Lewis acid halide toluene enzene Iscmer ortho distribution, % meta para 0/ ... 

Noncatalytic ring chlorination of toluene in a variety of solvents has been reported. Isomer distributions vary from approximately 60% ortho in hydroxyhc solvents, eg, acetic acid, to 60% para in solvents, eg, nitromethane, acetonittile, and ethylene dichloride (49,50). Reaction rates are relatively slow and these systems are particularly appropriate for kinetic studies. 

Organic Halides. Alkyl halides and aiyl halides, activated by election withdrawing groups (such as NO2) in the ortho or para positions, react with alkyleneamines to form mono- or disubstituted derivatives. Product distribution is controlled by reactant ratio, metal complexation or choice of solvent (16,17). Mixing methylene chloride [75-09-2J and EDA reportedly causes a mnaway reaction (18). 

Many other definitions of an ortho substituent constant have been made Shorter has reviewed these. Charton analyzed Oo in terms of Oi and CTr, i.e., = a(Ti -I- fpoR, finding that the distribution of inductive and resonance effects (the ratio a/b) varies widely with the substituent and, therefore, that no general Oo scale is possible. Charton also subjected to analysis according to Eq. (7-47),... 

There is some debate in the literature as to the actual mechanism of the Beirut reaction. It is not clear which of the electrophilic nitrogens of BFO is the site of nucleophilic attack or if the reactive species is the dinitroso compound 10. In the case of the unsubstituted benzofurazan oxide (R = H), the product is the same regardless of which nitrogen undergoes the initial condensation step. When R 7 H, the nucleophilic addition step determines the structure of the product and, in fact, isomeric mixtures of quinoxaline-1,4-dioxides are often observed. One report suggests that N-3 of the more stable tautomer is the site of nucleophilic attack in accord with observed reaction products. However, a later study concludes that the product distribution can be best rationalized by invoking the ortho-dinitrosobenzene form 10 as the reactive intermediate. 

This activation of the ortho position is most strikingly illustrated in the reactivity of 2,5-dimethylthiophene, which competitive experiments have shown to undergo the SnCb-catalyzed Friedel-Crafts reaction more rapidly than thiophene and even 2-methylthiophene. The influence of the reagent on the isomer distribution is evident from the fact that 2-methoxythiophene is formylated and bromi-nated (with A -bromosuccinimide) only in the 5-position. Similarly, although 3-bromo-2-methylthiophene has been detected in the bromi-nation of 2-methylthiophene with bromine, only the 5-isomer (besides some side-chain bromination) is obtained in the bromination of alkylthiophenes with A -bromosuccinimide. ° However, the mechanism of the latter type of bromination is not established. No lines attributable to 2-methyl-3-thiocyanothiophene or 2-methyl-3-chIoro-thiophene could be detected in the NMR spectra of the substitution products (5-isomers) obtained upon thiocyanation with thiocyanogen or chlorination with sulfuryl chloride. 2-Methyl- and 2-ethyl-thiophene give, somewhat unexpectedly, upon alkylation with t-butyl chloride in the presence of Feds, only 5-t-butyl monosubstituted and... 

The most accurate data on isomer distributions in alkylation of heterocycles have been obtained from the reaction of 3-n-butylpyridine with methyl radicals in acetic aeid. The ratio of the monomethyl products was determined by infrared spectroscopy and gas chromatography and is showm in (27). A small amount of 2,6-dimethyl-3-n-butylpyridine was also obtained. These ratios again show a high proportion of ortho substitution. 

Broekhoven, E. V, and Wijngaards, H., Investigation of the Acid Site Distribution of FCC Catalysts with Ortho-xylene as a Model Compound, 1988 Akzo Chemicals FCC Symposium, Amsterdam, The Netherlands. 

Equilibrium distribution of ortho and para states. Defined value. 

The isomer distribution is normal even when the low relative reactivity of substrates is observed. If reaction occurred at every collision then the statistical isomer distribution (40 % ortho, 40 % meta, and 20 % para for monoalkylbenzenes for example) would be expected, but this is not the case. 

Early kinetic work127 showed that the formation of both ortho and para products was a first-order process and that the rates of reaction were insensitive to added acid or base and to change of solvent. The activation parameters were of the same order of magnitude for both reactions and the suggestion was made that both had a similar rate-determining step. Schmid et a/.128 showed that the formation of a dienone intermediate in the para rearrangement was also reversible since the radioactivity from allyl 2,6-dimethyl-4-allyl-y-14C phenyl ether LXXXVII became uniformly distributed in the y carbon atoms of the O- and C-allyl groups... 

The thermodynamic product distribution in the Friedel-Crafts methylation (Scheme 20) is in contrast to the kinetic distribution. The reaction kinetically shows the ortho and para orientations. Thermodynamic stabilities of the products prefer the meta isomer as a major product. 

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