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Relative rates of arylation of pyridine with o-tolyl and o-nitrophenyl radicals produced by the Gomberg-Hey reaction have been measured by means of competitive experiments. The ratios of isomers, and total and partial rate factors have been compared with those obtained with the corresponding para-substituted aryl radicals and the unsubstituted phenyl radical. o-Tolyl radicals gave less a-isomer, and o-nitrophenyl radicals more a-isomer than expected. A new type of " ortho-effect " is suggested to rationalise these results. As expected, tolyl radicals are nucleophilic and nitrophenyl radicals are electrophilic when compared with the unsubstituted phenyl radical. The ratios of isomers formed were relatively unaffected (within experimental error) by the presence of oxygen. The significance of this observation in terms of a possible mechanism for the reaction is discussed. THIS Paper reports a study of the orientation of the entering substituent when pyridine is attacked by ortho-substituted phenyl radicals, and attempts to unravel factors which determine the observed isomer ratios. The latter can only be used meaningfully if they reflect the reactivities of the various nuclear positions in pyridine towards free-radical attack.There are conflicting opinions concerning the effects of the ortho-substituents on the mechanism and orientation in the reactions of aromatic compounds with ortho-substituted phenyl radicals, and in a number of instances apparently anomalous products have been obtained in preparative reactions (for summary see Hey, Moulden, and Williams 3). Hey and Walker isolated methyl benzoate and two isomeric methyl o-pyridylbenzoates from the reaction of o-methoxycarbonylbenzoyl peroxide and pyridine. This reaction was later re-investigated by Hey and Williams who isolated only methyl o-3-pyridylbenzoate from the reaction of diazotised methyl anthranilate with pyridine. Molecular-orbital calculations predict that when pyridine is substituted by a free-radical the proportion of isomers formed will be in the order cc > p > y,6 and this order has been observed in a number of reactions, e.g., the phenylation of pyridine.7 On the other hand, preparative o-nitrophenylation of pyridine 8 gave the three isomeric o-nitrophenylpyridines in the ratio p : a : y :: 52 : 45 : 3. No abnormality was noted, however, in the reactions of diazotised 9-and m-nitroanilines with pyridine. Hey, Moulden, and Williams recently studied quantitatively the relative rates of arylation of nitrobenzene, chlorobenzene, and toluene with o-bromo-and o-nitrophenyl radicals and determined the isomer distributions in some of these reactions. Toluene was found to be less reactive towards o-nitrophenylation than towards p-nitrophenylation, contrary to expectations based on the relative electrophilicity of these two radicals. It was assumed that this was due to some steric hindrance to the attachment of the o-substituted radical at the ortho-positions of toluene leading to a reduction of the total reactivity of that molecule (isomer ratios were n...
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