Electrophilic Aromatic Substitution of Groups other than Hydrogen. Part I: Ipso Factors and Rate-Limiting Steps in Dehalogenations by Diazonium Ions. 26th communication on diazo coupling reactions

The rates of substitution of the group X in 1-X-2-naphthol-6-sulfonic acids (X = H, Cl, Br, and I) by p-chlorobenzenediazonium ions in aqueous solution have been measured. The rates of the halogenated naphthols relative to that of the parent compound (X = H) are 0.0070:0.0089:0.149 for X = Cl, Br, and I respectively. The reaction of 1-bromo-2-naphthol-6-sulfonic acid is catalysed by thiosulfate ions; the relative rate observed for this compound does not, therefore, represent the ipso factor. It is postulated that in its substitution the release of the electrofugal leaving group (Br^⊕) is rate-limiting.     

The Rôle of Solvent Participation in Electrophilic Aromatic Substitution: Rates of a diazo coupling reaction in water and in aprotic polar solvents. 24th Communication on diazo coupling reactions

The rates of the diazo coupling reaction of 4-toluenediazonium salts with N, N-dimethylaniline in tetramethylene sulfone, acetonitrile, water, and nitromethane at 30°C are the same within a factor of 5. No significant influence of ‘gegenions’ (HSO or BF) was found. The results are explained by postulating little solvent reorganisation in the transition state. They are discussed comparatively with solvent effects in other electrophilic aromatic substitutions, particularly with regard to the reactions of nitronium salts.     

Stereochemistry of the σ-Complex Intermediate in Sterically Hindered Electrophilic Aromatic Substitutions. (20th communication on diazo coupling reactions)

The kinetics of the diazo coupling reactions of diazotized sulfanilic acid with 9 derivatives of 2-naphthol, each containing a substituent in the 8-position, have been measured. The reactions proceed by general base catalysis. The rate constants (k₁) for the formation of the steady-state intermediate in the presence of varying concentrations of a base (pyridine) have been evaluated from rate measurements. Similarly the ratios k₂/k₁ (where k₂ is the rate constant for proton transfer from the steady-state intermediate to the base, and k_?1 is the rate constant for the dissociation of the intermediate) have been determined. The dependence of logk₁ on σ⁺_m and of the ratio k₂/k_?1 on a steric parameter R^f (defined in this paper) is interpreted as evidence for the steady-state intermediate existing as a benzodienone σ-complex with the sp³-bound hydrogen in a pseudo-equatorial position and the electrophile pseudo-axial. A sterically caused destabilisation of the intermediate can be excluded. The steric influence on the ratio k₂/k_?1 is therefore due to the steric influence on k₂ only. Diazo coupling of 8-(2′-pyridyl)-2-naphthol is subject to intramolecular base catalysis, as demonstrated by kinetic hydrogen isotope effects.