Hammett aromatic nucleophiles with

We interpreted the constancy of p for the reaction of substituted aromatic nucleophiles with methylating agents of varying reactivity as evidence of lack of curvature in a log k versus log K plot and hence an invariability of a (the slope of this plot) with reactivity of the methylating agent. Two entirely different factors may result in this one factor is an inherent part of the Marcus treatment, and the other is a problem of the relation between the position of the transition state along the reaction coordinate and the slope of LFERs, such as used to define the Hammett p. 

While we can make substitutions on the aromatic ring of the nucleophile or the leaving group for methyl transfers, it is clearly impossible to do the same for the transferring group. The solvolysis of benzyl halides (Bennett and Jones, 1935) gives curved Hammett plots. The interaction of the aromatic ring with the reaction centre makes this a more complicated reaction. Hence we will not consider these reactions in detail. Thus the simplest system we can consider is... 

Coordination compounds with organonitrile ligands are of considerable interest due to the increased susceptibility of the coordinated nitrile to nucleophilic attack by reagents such as hydroxide ion. Rate accelerations of 10 in base hydrolysis (nitrile-> amide) have been observed. Two new aromatic organonitrile complexes of [Co(NH3)5] with 2- and 4-nitroben-zonitrile have been prepared, and their base hydrolysis to coordinated carboxamides studied. The second-order rate constants for base hydrolysis are 180 4 and 510 90 M s at 25°C and / = 1.0 M for the ortho and para isomers, respectively. Rate constants for the hydrolysis of 11 coordinated aromatic nitriles follow a Hammett-type correlation with log koH = l-93o- + 1.30 at 25 C and / = 1.0 M. Carbon-13 NMR studies of the free and coordinated nitriles indicate similar chemical shifts, so that the variation in the slope of the Hammett plots for free- and coordinated-nitrile hydrolysis is a transition-state effect rather than a ground-state phenomenon. 

Just as electrophilic aromatic substitutions were found more or less to follow the Hammett relationship (with a" " instead of o see p. 692), so do nucleophilic substitutions, with cr instead of a for electron-withdrawing groups. ... 

Similarly, those reactions that are strongly assisted by withdrawal of electrons from the reaction site, such as nucleophilic aromatic substitution, give a poor fit to a Hammett plot for the substituents that are capable of withdrawing electrons by delocalization (—N02, —N2 , —C=N, and so on). An example is Reaction 16 in Table 26-7. To correlate reactivity data with structures where strong resonance effects operate, different sets of substituent constants are required.1... 

In Table 19, the relative rates, obtained in the alkylation of 3-substituted pyridines by f-butyl radical, are reported. The exceptional positional selectivity (only the position 6 is attacked) results from combined polar and steric effects. A satisfying Hammett correlation was observed with Op the value of p = 5.5 is of the same order of magnitude of those of nucleophilic aromatic substitutions, indicating a high degree of charge development in the transition state. 

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