The Complex Synergy of Water in the Metal/Bromide Autoxidation of Hydrocarbons Caused by Benzylic Bromide Formation

One of the most active and selective catalysts in homogeneous liquid phase oxidation using molecular oxygen (O 2 ) is a mixture of cobalt, manganese and bromide salts in acetic acid. It has been used to produce hundreds of different carboxylic acids in high yield and purity including the commercial production of terephthalic acid from p ‐xylene. Water is normally a by‐product in these reactions and it is shown here that its concentration is an important reaction variable. In anhydrous acetic acid, with reagents with sufficiently strong electron‐withdrawing substitutents (toluene, 4‐carboxytoluene, 4‐chlorotoluene), all of the active bromide becomes inactive via benzylic bromide formation. The Co/Mn/Br catalyst is therefore converted to a Co/Mn catalyst which is dubbed ‘catalyst failure’ because of its undesirable characteristics of lower activity, decreased selectivity especially towards over‐oxidation and color formation. For 4‐chlorotoluene, increasing the water concentration to 5 weight % initially decreases the rate of reaction but eventually is more active and selective because the oxidation and hydrolysis of the benzylic bromide allows for sufficient active catalytic bromide. It is shown that benzylic bromides do not ‘promote’ the reaction and that both oxidation and solvolysis of the benzylic bromide occurs during autoxidation. During polymethylbenzene oxidation, benzylic bromide formation occurs only with the most reactive methyl group. The complex factors during metal/bromide autoxidation – some favored by increased water concentration and others detrimental – are outlined.