The controlled oxidation or dehydrogenation of primary alcohols

Simple aldehydes may be obtained in reasonably good yield by oxidation of the corresponding primary alcohol with sodium dichromate in dilute sulphuric acid solution (e.g. butyraldehyde, Expt 5.74). To avoid further oxidation to the corresponding acid, the aldehyde is removed as rapidly as possible by distillation through a fractionating column. The main by-product is an ester which arises as the result of the oxidation of an intermediately formed hemiacetal. 

Satisfactory yields of simple aldehydes are also usually obtained when the vapour of the primary alcohol is dehydrogenated by passage over a heated catalyst of copper-chromium oxide deposited on pumice (Expt 5.75). 

PTC methods for the oxidation of water-insoluble add-sensitive alcohols have been described which use an aqueous sulphuric acid solution of potassium dichromate, dichloromethane and catalytic amounts of tetrabutylammonium hydrogen sulphate.103 

Cognate preparation. Propionaldehyde (propanal). Use 34 g (42.5 ml, 0.567 mol) of propan-l-ol and a solution containing 56g (0.188 mol) of sodium dichromate dihydrate, 300 ml of water and 40 ml of concentrated sulphuric acid. The experimental details are identical with those for butyraldehyde, except that the temperature at the top of the column is not allowed to rise above 70-75 °C, and during the subsequent heating for 15 minutes the liquid passing over below 80 °C is collected the receiver must be cooled in ice. The yield of propionaldehyde, b.p. 47-50 °C, is 12 g (36%). 

(1) If the high boiling residue is transferred to a smaller flask and fractionally distilled, some hexanol passes over first, followed by hexyl hexanoate (Me(CH2)4C02(CH2)5-Me) (2g) at 240-250 °C (mainly at 245 °C). 

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The controlled oxidation or dehydrogenation of primary alcohols (Expts 5.74 to 5.76). 

The reaction is an oxidation since the carbon that previously held the OH group goes to a higher oxidation state, i.e., it has two bonds to oxygen instead of one. The product is referred to as aldehyde if either R or R or both are H and a ketone if neither R nor R is H. Primary and secondary alcohols yield aldehydes and ketones, respectively. This type of oxidation, also referred to as a dehydrogenation, is carried out in living systems under the control of a dehydrogenase enzyme such as nicotinamide adenine dinucleotide (NADH). 

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