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  Oxidation of Alcohols Homework Help - K-12 Grade Level, College Level Chemistry

Introduction to Oxidation of Alcohols

In the gaseous state Simple 1º and 2º-alcohols lose hydrogen when showing to a hot copper surface. This catalytic dehydrogenation reaction generates ketones and  aldehydes, and because the carbon atom bonded to the oxygen is oxidized, such type of alcohol to carbonyl conversions are normally considered as oxidation reactions. Gas phase dehydrogenations of this type are significant in chemical manufacturing, but observe little use in the research laboratory. In its place, alcohol oxidations are carried out in solution, using reactions wherein the hydroxyl hydrogen is substituted by an atom or group that is readily eliminated together with the alpha-hydrogen. Decomposition of 1º and 2º-alkyl hypochlorites, considered to previous, is an instance of such type of a reaction.

RCH2-OH   +   hot Cu    RCH=O   +   H2

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

The most commonly helpful reagents for oxidizing 1º and 2º-alcohols are chromic acid derivatives. Two such type of oxidants are Jones reagent (a solution of sodium dichromate in aqueous sulfuric acid) and pyridinium chlorochromate, C5H5NH(+)CrO3Cl(-), usually termed by the acronym PCC and employed in methylene chloride solution. Within the each case a chromate ester of the alcohol substrate is believed to be an intermediate, which goes through an E2-like elimination to the carbonyl product. The oxidation state of carbon increments by 2, whereas the chromium decreases by 3 (it is reduced). Because chromate reagents are a dark orange-red color (VI oxidation state) and chromium  III  compounds are generally green, the progress of these oxidations is simply observed. Certainly, this is the chemical transformation on which the Breathalyzer test is based. The following equations demonstrate some oxidations of alcohols, employing the two reagents described here. Both reagents influence the oxidation of 2º-alcohols to ketones, but the product of 1º-alcohol oxidations is change. Oxidation with the PCC reagent converts 1º-alcohols to aldehydes; where Jones reagent keeps on the oxidation to the carboxylic acid product, as displayed in the second reaction. For the first two reactions the technique diagram also displays the oxidation states of chromium (Roman numbers) and carbon (blue Arabic numbers). The general base (B:) employed in these techniques may be anything from water to pyridine, depending on the purticular reaction.

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

The fourth reaction above demonstrates the failure of 3º-alcohols to go through oxidation. The second reaction technique depicts why 1º-alcohols go through further oxidation by Jones reagent. The aqueous solvent system employed with this reagent allows hydration (addition of water) to the aldehyde carbonyl group. The resultant hydrates meets both the needs stated above, and is further oxidized by similar chromate ester technique. Water is not exists when the PCC reagent is employed, so the oxidation stops at the aldehyde stage.
Other chromate oxidizing agent, identical to PCC, is pyridinium dichromate, (C5H5NH(+) )2 Cr2O7(-2), identified by acronym PDC. Both PCC and PDC are orange crystalline solids which are soluble in many organic solvents. Because PDC is less acidic than PCC it is frequently employed to oxidize alcohols that might be sensitive to acids. PDC oxidizes 1º- and 2º-alcohols in roughly the same fashion as PCC, but much more slowly, In methylene chloride solution. Though, in DMF solution saturated 1º-alcohols are oxidized to carboxylic acids. In both of the solvents allylic alcohols are oxidized efficiently to conjugated enones and enals and correspondingly.

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