Oxidation Reactions of Alcohols

Simple 1º and 2º-alcohols in the gaseous state lose hydrogen when exposed to a hot copper surface. This catalytic dehydrogenation reaction produces aldehydes (as shown below) and ketones, and since the carbon atom bonded to the oxygen is oxidized, such alcohol to carbonyl conversions are generally referred to as oxidation reactions. Gas phase dehydrogenations of this kind are important in chemical manufacturing, but see little use in the research laboratory. Instead, alcohol oxidations are carried out in solution, using reactions in which the hydroxyl hydrogen is replaced by an atom or group that is readily eliminated together with the alpha-hydrogen. The decomposition of 1º and 2º-alkyl hypochlorites, referred to earlier, is an example of such a reaction.

RCH₂–OH   +   hot Cu    RCH=O   +   H₂

RCH₂–O–Cl   +   base    RCH=O   +   H–Cl

The most generally useful reagents for oxidizing 1º and 2º-alcohols are chromic acid derivatives. Two such oxidants are Jones reagent (a solution of sodium dichromate in aqueous sulfuric acid) and pyridinium chlorochromate, C₅H₅NH⁽⁺⁾CrO₃Cl^(–), commonly named by the acronym PCC and used in methylene chloride solution. In each case a chromate ester of the alcohol substrate is believed to be an intermediate, which undergoes an E2-like elimination to the carbonyl product. The oxidation state of carbon increases by 2, while the chromium decreases by 3 (it is reduced). Since chromate reagents are a dark orange-red color (VI oxidation state) and chromium III compounds are normally green, the progress of these oxidations is easily observed. Indeed, this is the chemical transformation on which the Breathalyzer test is based. The following equations illustrate some oxidations of alcohols, using the two reagents defined here. Both reagents effect the oxidation of 2º-alcohols to ketones, but the outcome of 1º-alcohol oxidations is different. Oxidation with the PCC reagent converts 1º-alcohols to aldehydes; whereas Jones reagent continues the oxidation to the carboxylic acid product, as shown in the second reaction. 

 For the first two reactions the mechanism diagram also shows the oxidation states of carbon (blue Arabic numbers) and chromium (Roman numbers). The general base (B:) used in these mechanisms may be anything from water to pyridine, depending on the specific reaction.

Mechanism

Two structural requirements for the oxidation to carbonyl products should now be obvious:
        1. The carbon atom bonded to oxygen must also bear a hydrogen atom.
            Tertiary alcohols (R₃C–OH) cannot be oxidized in this fashion.
        2. The oxygen atom must be bonded to a hydrogen atom so that a chromate ester intermediate (or other suitable leaving group) may be formed.
            Ethers (R–O–R) cannot be oxidized in this fashion.

The fourth reaction above illustrates the failure of 3º-alcohols to undergo oxidation. The second reaction mechanism explains why 1º-alcohols undergo further oxidation by Jones reagent. The aqueous solvent system used with this reagent permits hydration (addition of water) to the aldehyde carbonyl group. The resulting hydrate (structure shown below the aldehyde) meets both the requirements stated above, and is further oxidized by the same chromate ester mechanism. Water is not present when the PCC reagent is used, so the oxidation stops at the aldehyde stage.
Another chromate oxidizing agent, similar to PCC, is pyridinium dichromate, (C₅H₅NH⁽⁺⁾ )₂ Cr₂O₇^(–2), known by the acronym PDC. Both PCC and PDC are orange crystalline solids that are soluble in many organic solvents. Since PDC is less acidic than PCC it is often used to oxidize alcohols that may be sensitive to acids. In methylene chloride solution, PDC oxidizes 1º- and 2º-alcohols in roughly the same fashion as PCC, but much more slowly. However, in DMF solution saturated 1º-alcohols are oxidized to carboxylic acids. In both solvents allylic alcohols are oxidized efficiently to conjugated enals and enones respectively.