This process illustrates chemical engineering at its best. It merely involves converting two relatively cheap starting materials, benzene and propene into two more valuable ones, phenol and acetone. Other reactants required are oxygen from air and small amounts of a free radical initiator. Most of the worldwide production of phenol and acetone are now based on this method.
Benzene and propene are compressed together to a pressure of 30 standard atmospheress at 250°C (482°F) in presence of a catalytic Lewis acid. Phosphoric acid is often favored over aluminum halides. Cumene is formed in the Friedel Crafts Alkylation of benzene by propene:
C6H6 (g) + H2C=CHCH3 (g)----> C6H5-CH(CH3)2 (g)
Cumene is oxidized in slightly basic conditions in presence of a radical initiator, which removes the tertiary benzylic hydrogen from cumene and hence leaving a cumene radical:
C6H5-CH(CH3)2 (g) + radical initiator (g) ----> C6H5(CH3)2-C· (g)
It then bonds with an oxygen molecule to give cumene hydroperoxide radical, which in turn forms cumene hydroperoxide (C6H5C(CH3)2-O-O-H) by abstracting benzylic hydogen from another cumene molecule. This latter cumene converts into cumene radical and feeds back into subsequent chain formations of cumene hydroperoxides.
C6H5(CH3)2-C· (g) + O2 (g) ----> C6H5(CH3)2-C-O-O· (g)
C6H5(CH3)2-C-O-O· (g) + C6H5-CH(CH3)2 (g) ---->C6H5C(CH3)2-O-O-H (g) + C6H5—C·—(CH3)2 (g)
Cumene hydroperoxide is then hydrolysed in an acidic medium to give phenol and acetone. Loss of a water molecule from the hydroperoxide leaves an electron-deficient oxygen. Migration of the phenyl to the oxygen leads to a more stable resonance hybridized structure of tertiary benzylic radical, which in turn produce acetone and phenol after an attachment of a water molecule and rearrangement. Researches sugguest the loss of water molecule and phenyl migration may take place simultaneously, in other words the step of the mechanism is concerted.
C6H5C(CH3)2-O-O-H (aq) + H3O+ (aq) ----> C6H5(CH3)2C-O+ (aq) + 2H2O (l)
C6H5(CH3)2C-O+ (aq) ---> C6H5-O-(CH3)2C+ (aq)
C6H5-O-(CH3)2C+ (aq) + H2O (l) ----> C6H5-O-(CH3)2C—O+—H2 (aq) <----> C6H5—O+H—(CH3)2C-OH (aq)
C6H5—O+H—(CH3)2C-OH (aq) + H2O (l) ---> C6H5-OH (phenol) (aq) + CH3-CO-CH3 (acetone) (aq) + H3O+ (aq)
The products are extracted by distillation.
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