| Popis: |
The oxidation with oxygen of p-substituted isopropylbenzenes in butyric acid solution at 130-150° to the corresponding benzoic acids has been investigated, using manganese and cobalt acetates as catalysts. This reaction is slow compared with the oxidation of the intermediate acetophenones to benzoic acids, discussed in part I. Rates obey a Hammett-relationship with a p-value of –1.1. This result has a bearing on the oxidation behaviour of p-dialkylbenzenes to terephthalic acid, oxidation of the intermediate alkylbenzoic acids being the rate-determining step. However, p-alkylbenzoic acids alone appear to be much more refractory towards oxidation than would follow from the Hammett relationship; a study of the oxidation of p-toluic acid suggested that this is attributable to a short chain length, possibly due to decarboxylation of the acid followed by phenol formation. In the presence of compounds that are readily oxidized, such as p-xylene, p-toluic acid may be smoothly converted into terephthalic acid. The bis-p-tolyl ether of ethylene glycol (p-CH3-C6H4-O-CH2-)2 may be rapidly oxidized to the corresponding dicarboxylic acid, the electron-repelling substituent increasing the rate of attack at the methyl groups. |
