Question

HELLO!

Could you explain to me how Terephthalic acid is produced by oxidation of p-xylene by oxygen in air..

And what is the promoters and co-ractants in this reaction????

thanks

Answer 1

The aerial oxidation of p-xylene to terephthalic acid is called the Amoco Process. This transformation of the two methyl groups on the phenyl ring to carboxylic acid is multi-step oxidations occuring in a one-pot reaction. Typically a cobalt-manganese-bromide catalyst is used. The bromide can be acquired from a wide variety of organic or inorganic bromides where the halogen is used as a source of free radical to initiate and sustain formation of oxygen radicals for oxidation.

The process involves oxidation of the methyl groups in two steps where the p-xylene is converted to p-toluic acid with the byproduct p-formylbenzoic acid. The reaction then proceeds through the p-toluic acid as the intermediate. In p-toluic acid, deactivation of the methyl by the electron-withdrawing carboxylic acid group makes the methyl less reactive compared to the methyls of xylene itself, making the second oxidation much more difficult than the first. Finally, the impurity - 4-formylbenzoic acid is removed by hydrogenation to yield pure terephthalic acid.

The process uses acetic acid as the solvent and as mentioned, oxygen in the air as the oxidant. So, the catalyst used are the multi-valent heavy metals with bromine radical as the promoter and oxygen as the co-reactant. The advantage of acetic acid as the solvent is the low solubility of the product alone in it, causing crystallization of the product as and when it is formed.