Alcohols, primary with sodium dichromate

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. 

The oxidation of secondary alcohols with sodium dichromate in dilute sulphuric acid gives acceptable yields of ketones since these do not normally undergo extensive further oxidation under the reaction conditions (cf. Section 5.7.1, p. 587, the oxidation of primary alcohols to aldehydes). 

Oxidation with sodium dichromate. Primary and secondary alcohols are oxidized to aldehydes and ketones in 80-90% yield by Na2Cr2 07 2H2O and sulfuric acid in DMSO at 70° (90 min.). DMSO is not involved and acts as a solvent. In its absence, considerable charring occurs with further oxidation of the carbonyl product. Cr03 can also be used as oxidant. ... 

So far, this section has covered only a few ways to make a C=0 bond. We saw that ketones can be made by treating a secondary alcohol with sodium dichromate (or the Jones reagent), and aldehydes can be made by treating a primary alcohol with PCC. We also saw that ketones and aldehydes can be made via ozonolysis. Let s get some practice with these reactions. 

Reaction of the C-0 and O-H Bonds Primary alcohols oxidize to carboxylic acids secondary alcohols oxidize to ketones with chromium trioxide or sodium dichromate. Tertiary alcohols do not oxidize under mild conditions. With pyridinium chlorochromate (PCC) the oxidation of primary alcohols can be stopped at aldehydes. 

Many of the preferred reagents for the oxidation of primary alcohols to aldehydes (secondary alcohols to ketones) contain the transition metal chromium in its highest oxidation state, VI. Upon reaction with an alcohol, the yellow-orange chromium(VI) species is reduced to the blue-green chromium(III) state. Normally the reaction is carried out in aqueous acid solution using the sodium dichromate salt, Na2Cr207, or the oxide, CrOs. A typical reaction is shown here ... 

Primary and secondary hydroxyl groups can be mildly oxidized to carbonyl groups (aldehydes or ketones) by reaction with pyridinium dichromate. Primary tosyl groups can also be oxidized to aldehydes by reaction with DMSO in collidine. Primary hydroxyl groups can be mildly and selectively oxidized, in the presence of secondary alcohols, to carboxyl groups by reaction with 2,2,6,6-tetram-ethyl-1-piperidine oxoammonium ion (TEMPO) to form uronic acids. Uronic acid carboxyl groups can be reduced to primary alcohols by reaction with car-bodiimide and sodium borohydride. 

Alcohols are often oxidized with chromium(VI) compounds. When the alcohol is oxidized, Cr(VI) is reduced in several steps to Cr(III). The specific Cr(VI) species used depends on the scale of the process, the cost of reagents, and limitations that result firom the presence of other functional groups in the reactant. For example, the inexpensive reagent sodium dichromate (Na2Cr207) oxidizes secondary alcohols to ketones. It also oxidizes primary alcohols to aldehydes and then to carboxylic acids. Sodium dichromate in acetic acid (HOAc) is used in large-scale reactions to convert secondary alcohols to ketones. 

Oxidation of 1-hexanol with chromic acid (sodium or potassium dichromate in aqueous sulfuric acid) yields hexanoic acid. Use of PDC or PCC in dichloromethane is not acceptable because those reagents yield aldehydes on reaction with primary alcohols. 

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