Reactions of aryl diazonium salts and alkyl arylazo ethers. X. General acid and intramolecular electrostatic catalysis in the ionization of methyl (E)-2-Organyl-5-nitrophenylazo ethers in alcoholic solvents

Abstract

The ionization of the (E)-methyl ethers derived from 2-carboxy-5-nitrobenzenediazonium salt and some related compounds have been studied in alcoholic solvents. In the presence of buffers prepared from aliphatic carboxylic acids and their salts, intermolecular general acid catalysis is observed. The observed rate of ionization in buffered solution depends on the pH of the solution. The carboxy group of 2-carboxy-5-nitrophenylazo methyl ethers is ionized to a different extent in each of the carboxylic acid buffers depending on the pH of each solution. Furthermore, it has been found that the carboxylate (COO^-) form of this compound ionizes more rapidly than the protonated (COOH) form.

For these reasons, the calculated catalytic rate constants cannot be used to determine a Bronsted α-value for the reaction and the uncatalysed rates determined in buffer solutions of each carboxylic acid are different.

Buffers of cyanoacetic acid are found to behave anomalously when compared to all the other carboxylic acid buffers used.

The greater rate of ionization of the carboxylate form is shown to be the result of different electronic effects of the COO^- and CO₂H substituents. By comparisons of estimated electronic and steric effects it is possible to show that the COO^- form also exerts intramolecular electrostatic catalysis in the ionization of the azo ether group.

For the ionization of the 2-carboxy-5-nitrophenylazo methyl ether in the more acidic buffer solutions, the plots of absorbance against time are linear for up to 20% of reaction. This effect is explained by the small amount of reactive COO- form of the substrate in these buffer solutions. As the reaction proceeds the reactive form is replenished by the equilibrium between the COOH and COO^- forms of the substrate. Thus, the linear portion of the absorbance-time plot is greater than normal.

Ionization of the 2-methoxycarbonyl-5-nitrophenylazo methyl ether in both basic methanol and in the presence of buffers of dichloroacetic acid occurs by two concurrent reactions. The slower of these reactions is believed to be the normal ionization of the arylazo ether. A number of possibilities for mechanism of the faster reaction have been eliminated and a possible mechanism is suggested.