The oxidation of cyclohexene with molecular oxygen catalyzed by solid
Pd/C or Pd(II) acetate-hydroquinone-iron phthalocyanine gives cyclohex
ane, benzene and oxygenated products. Oxygen pressure and solvent used
influence significantly the distribution of the products and at the p
ressure above 2 atm no cyclohexane is formed. Over Pd/C catalyst in th
e absence of oxygen, disproportionation to cyclohexane and benzene (th
e ratio is nearly 2:1) proceeds exclusively. Under comparable conditio
ns 2-cyclohexenol is disproportionated to cyclohexanol and phenol and
some of it rearranges to cyclohexanone. The explanation for the dispro
portionation of cyclohexene under the Wacker conditions is that Pd0 ce
ntres intermediately formed after stoichiometric oxidation of cyclohex
ene by Pd(II) are not completely reoxidized, but depending on the reac
tion conditions, they can partly aggregate and then, similarly to meta
llic surfaces, dehydrogenate cyclohexene to benzene. The hydrogen spec
ies formed migrate on the palladium surface and hydrogenate cyclohexen
e or at sufficient oxygen pressure they are oxidized to water.