Oxidation of Alcohols

Oxidation of alcohols, typically with acidified sodium dichromate (Na2Cr2O7), gives carbonyl compounds. Primary alcohols oxidise first to an aldehyde and then, on further oxidation, to a carboxylic acid (ethanol to ethanal to ethanoic acid); to stop cleanly at the aldehyde stage, the milder reagent pyridinium chlorochromate (PCC) is used instead (propan-1-ol to propanal with PCC). Secondary alcohols oxidise to ketones (propan-2-ol to propanone) and do not oxidise further under these conditions. Tertiary alcohols resist oxidation under normal conditions, since there is no alpha-hydrogen on the -OH-bearing carbon to remove; only under forcing conditions with a strong oxidant does the C-C bond cleave, giving a mixture of smaller carboxylic acids. In SWERN OXIDATION, dimethyl sulfoxide (DMSO) activated by oxalyl chloride, followed by triethylamine, oxidises a secondary alcohol to a ketone (or a primary alcohol to an aldehyde) under mild, non-acidic conditions (e.g. propan-2-ol to propanone, with (CH3)2S, CO2, CO and HCl as by-products). BIOLOGICAL OXIDATION: the liver enzyme alcohol dehydrogenase (ADH), using NAD+ as the oxidising cofactor, converts toxic ethanol from fermented food into the less-toxic ethanal (NADH + H+ is the reduced by-product), part of the body's detoxification. CATALYTIC DEHYDROGENATION: vapours of a primary or secondary alcohol passed over heated copper at 573 K lose H2 to give an aldeh …