A kinetic study of the title reactions has allowed an interpretation of the higher efficiency of an o-carboxamido group with respect to an o-carbomethoxy group in activating the benzenethiolate-dehalogenation reactions in methanol (k(CONH)()2/k(CO)()2(Me) 2.2-3.0) as due to an interaction between the anionic nucleophile and the hydrogen atoms of the carboxamido group. An inversion of the activating power of the two groups (k(CONH)()2/k(CO)()2(Me) 0.14) in the reactions with the same nucleophile has been observed when they are in a para-position. Moreover, for piperidino-dehalogenation reactions in methanol k(CONH)()2/k(CO)()2(Me) ratios less than unity (0.2-0.6) have been observed independently of the position (ortho or para) of the carboxamido and carbomethoxy groups with respect to the reaction center.

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The rates of anilino-debromination of 2-bromo-3,5-dinitrothiophen (I) by some ortho-substituted anilines (I1 ; X = OMe, Me, Et, CI, Br, and F) have been measured in methanol. The kinetic results have been treated according to the multiparameter analysis of ortho-effects proposed by Fujita and Nishioka. This treatment of data has also been applied to acid-base equilibria of anilinium ions. The susceptibility constants obtained in the two series of reactions (respectively p -2.96 and -2.90, 6 1.31 and 0.56, f -1.83 and -1.70) have been explained by taking into account the structure of the transition states involved.RECENTLY we examined the reaction of 2-bromo-All the reactions were first order both in (I) and ( 11). On 3,5-dinitrothiophen (I) with 14 metaor paya-substituted changing from aniline to ortho-substituted anilines, irrespecanilines in methanol and showed that the kinetic data tive of the nature Of the electron-withdrawing or -repelling

The I3C n.m.r. chemical shifts of the title compounds and of the corresponding anions have been measured in methanol. The observed substituent chemical shifts have been analysed by means of crosscorrelations and of single and dual substituent parameter linear free energy relationships. Different results have been obtained for thiophene and benzene derivatives using DSP analysis: for the first ring system the results have been rationalized in terms of separate resonance and inductive contributions of substituent effects (pointing out the occurrence of an apparent reverse polar effect); in contrast the same treatment of the data for the benzene system gave poor results, confirming once again the different proximity effects in six-and five-membered ring derivatives.From studies of the reactivity of many classes of ortho-dior poly-substituted thiophene derivatives and the application of linear free energy (l.f.e.) ortho-correlations to reactivity and equilibrium data, we have shown that in thiophene compounds it is possible to provide evidence for many examples of good 1.f.e. ortho-correlations, both in reactions on the ring'"" and on the side-chain,Id-' at variance with the results obtained for six-membered ring derivatives (e.g. benzene, na~hthalene).~The aim of the present work is to ascertain whether an ortholike substituent induces on the carbon of the side-chain in thiophene series a 3C n.m.r. substituent chemical shift (SCS) which could give linear correlations by dual substituent parameter (DSP) treatment of data, duplicating the situation observed in reactivity and equilibrium studies. It must be noted that SCSs induced on side-chain carbon atoms in benzene derivatives by ortho-substituents are usually excluded from DSP treatment due to steric and/or magnetically anisotropic effects. We have chosen as probe compounds a series of 3substituted thiophene-2-carboxylic acids (1) t and the corresponding anions (1-), whose 13C chemical shifts have been measured in deuteriated methanol.In order to enable a comparison between the effects of orthosubstituents in thiophene-and benzene-carboxylic acids we have also collected ' 3C n.m.r. data of ortho-substituted benzoic acids (2) and the corresponding anions (2-) under similar experimental conditions.Recently: we measured the 13C n.m.r. chemical shifts of some