Oxidation and Reduction in Organic Chemistry

In organic chemistry, oxidation usually means an increase in the number of bonds from carbon to oxygen (or a loss of hydrogen), and reduction means an increase in the number of bonds to hydrogen (or a loss of oxygen); an alcohol, an aldehyde or ketone and a carboxylic acid are successive oxidation states of carbon. In brief: ethanol is oxidized by acidified dichromate to ethanal and then to ethanoic acid (the orange dichromate turns green), and NaBH₄ reduces an aldehyde or ketone back to an alcohol.

Redox reactions are the way in which functional groups are interconverted, and they link organic chemistry with energy metabolism: the oxidation of glucose in the cell and the breathalyzer test both use the same ideas. This article shows how to follow oxidation states of carbon, lists the common oxidizing and reducing agents, explains the oxidation of the three classes of alcohol and gives worked examples including a stoichiometric calculation.

How do you recognize oxidation and reduction?

Organic chemists use a practical definition:

• Oxidation: gain of oxygen, loss of hydrogen, or an increase in the number of bonds to heteroatoms (O, N, halogens).
• Reduction: loss of oxygen, gain of hydrogen, or an increase in the number of C–H bonds.

This is consistent with the electron definition: when carbon forms a bond to a more electronegative atom, such as oxygen, the carbon loses electron density, and its oxidation number increases. To find the oxidation number of a carbon atom, count +1 for each bond to a more electronegative atom, −1 for each bond to hydrogen and 0 for a bond to another carbon.

Oxidation states of carbon in common compounds

How are alcohols oxidized?

• Primary alcohol → aldehyde → carboxylic acid. If the mixture is heated gently and the aldehyde is distilled off as it forms, it is the major product (the aldehyde boils at a lower temperature, as it cannot form hydrogen bonds). If the alcohol is heated under reflux with an excess of oxidant, the carboxylic acid is formed.
• Secondary alcohol → ketone. The reaction stops there, as further oxidation would require breaking a C–C bond.
• Tertiary alcohol → no reaction (under mild conditions), since there is no hydrogen on the carbon bearing the OH.

The dichromate ion is orange, and chromium(III) is green, so the color change shows that an alcohol or an aldehyde has been oxidized; a ketone or a tertiary alcohol leaves the solution orange.

Which reagents are used?

How are carbonyl compounds reduced?

Hydride reagents transfer H⁻ to the electrophilic carbon of the carbonyl group, and the resulting alkoxide is protonated by water or acid. CH₃CHO → CH₃CH₂OH (primary alcohol); CH₃COCH₃ → CH₃CH(OH)CH₃ (secondary). LiAlH₄ also reduces carboxylic acids and esters to primary alcohols, and nitriles and amides to amines. Catalytic hydrogenation reduces nitro groups to amines, such as nitrobenzene to aniline. A more drastic reduction, the Clemmensen reduction (Zn/Hg and HCl), converts C=O to CH₂.

Worked examples

Example 1: balancing the oxidation of ethanol

Ethanol to ethanoic acid with dichromate in acid. The half-reactions are: CH₃CH₂OH + H₂O → CH₃COOH + 4H⁺ + 4e⁻ and Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O. To equalize the electrons, multiply the first by 3 and the second by 2 (12 electrons each) and add: 3CH₃CH₂OH + 2Cr₂O₇²⁻ + 16H⁺ → 3CH₃COOH + 4Cr³⁺ + 11H₂O.

Example 2: mass of dichromate

How much potassium dichromate (M = 294.18 g/mol) is needed to oxidize 10.0 g of ethanol (M = 46.07 g/mol) to ethanoic acid? n(ethanol) = 10.0 / 46.07 = 0.2171 mol. From the equation, n(K₂Cr₂O₇) = 2/3 × 0.2171 = 0.1447 mol, which is 0.1447 × 294.18 = 42.6 g.

Example 3: predicting products

• Butan-1-ol, K₂Cr₂O₇/H⁺, reflux → butanoic acid.
• Butan-2-ol, K₂Cr₂O₇/H⁺ → butanone.
• Butanone + NaBH₄ → butan-2-ol.
• Toluene + hot KMnO₄ → benzoic acid.
• Ethanoic acid + LiAlH₄ (then H₂O) → ethanol.

Example 4: the breathalyzer

In the classic breath test, exhaled ethanol reduces orange dichromate to green chromium(III), and the extent of the color change, or the voltage in the fuel-cell version of the test, indicates the alcohol concentration. The same reaction, Example 1, applies, and each mole of ethanol consumes 2/3 mol of dichromate.

What happens in living cells?

Biological redox reactions use the coenzyme NAD⁺ as the oxidant and NADH as the reductant: ethanol + NAD⁺ → ethanal + NADH + H⁺ (alcohol dehydrogenase), and lactate ⇌ pyruvate. The NADH formed carries the electrons to the respiratory chain, where oxygen is reduced to water and the energy released is used to make ATP. The overall oxidation of glucose to CO₂ and H₂O, with carbon going from an average oxidation state of 0 to +4, is the main energy source of animals.

Who developed these methods?

Justus von Liebig described the oxidation of alcohol to aldehyde in 1835. Chromic acid oxidations were widely used from the 19th century, and the Jones reagent, with CrO₃ in aqueous sulfuric acid and acetone, was introduced by Ewart Jones in 1946. The hydride reagents LiAlH₄ and NaBH₄, discovered in 1947 by Anton Schlesinger and Herbert C. Brown, transformed the reduction of carbonyl compounds; Brown shared the Nobel Prize in Chemistry in 1979. The breathalyzer was invented by Robert Borkenstein in 1954 on the basis of the dichromate reaction.

Which mistakes are common?

• Thinking that ketones can be oxidized further under mild conditions: they cannot.
• Forgetting to distill or to use reflux: the conditions decide whether the product is the aldehyde or the acid.
• Using NaBH₄ to reduce acids: it is too weak.
• Adding water to LiAlH₄: it reacts violently, so dry ether is used.
• Confusing the color change: orange to green means that dichromate has been reduced, so the organic substrate was oxidized.