Alcohols, Phenols and Ethers · Lesson 7 of 12
Oxidation and Commercially Important Alcohols
“Use carbon-bound hydrogen and reaction conditions to predict oxidation products, then connect methanol and ethanol chemistry with production and use.”
• Predict oxidation products of primary, secondary and tertiary alcohols. • Select conditions for stopping at an aldehyde or continuing to an acid. • Distinguish oxidation by an oxidant from copper-catalysed dehydrogenation. • Explain methanol synthesis and ethanol fermentation with balanced equations.
Inspect the carbon bearing OH before choosing an oxidant
Oxidation in organic chemistry commonly increases bonding from carbon to oxygen or decreases bonding from carbon to hydrogen. Converting an alcohol carbon to a carbonyl carbon requires loss of the hydroxyl hydrogen and a hydrogen on that same carbon. A primary alcohol has two such carbon-bound hydrogens, a secondary alcohol has one, and a tertiary alcohol has none. This structural check predicts why their ordinary oxidation behaviour differs.
In the first stage an aldehyde forms without changing the number of carbon atoms. In aqueous conditions, an aldehyde can hydrate and be oxidised further to the carboxylic acid. Thus reagent identity, water content and removal of product determine whether an aldehyde can be isolated. An equation showing only “oxidation” is insufficient when product selection matters. Oxygen must not be inserted at a random carbon simply because the reagent is strong.
| Alcohol and target | Typical choice | Expected product and qualification |
|---|---|---|
| Primary → aldehyde | PCC in anhydrous medium; suitable anhydrous CrO₃-based conditions | RCHO; limit further oxidation |
| Primary → carboxylic acid | Excess acidified dichromate or suitable permanganate, heating/reflux | RCOOH; aldehyde is an intermediate |
| Secondary → ketone | Acidified dichromate, CrO₃ or suitable oxidant | RC(=O)R′ |
| Tertiary → ordinary carbonyl product | No simple mild oxidation retaining the full skeleton | No H on the OH-bearing carbon; harsh oxidation may cleave C–C bonds |
PCC means pyridinium chlorochromate. In a standard dry oxidation it can convert an allylic primary alcohol CH₃CH=CHCH₂OH to CH₃CH=CHCHO, but-2-enal, while preserving the C=C bond. This illustrates useful selectivity; it does not make every chromium reagent interchangeable. Aqueous strongly oxidising conditions can give different products, and harsh permanganate conditions may also attack double bonds.
Problem
Oxidise butan-1-ol to butanal or butanoic acid; oxidise butan-2-ol under ordinary conditions.
- 1.To isolate butanal, use a controlled anhydrous reagent such as PCC. CH₃CH₂CH₂CH₂OH + [O] → CH₃CH₂CH₂CHO + H₂O.
- 2.To obtain butanoic acid, use a suitable excess aqueous oxidant and heating: CH₃CH₂CH₂CH₂OH + 2[O] → CH₃CH₂CH₂COOH + H₂O. Both stages retain four carbons.
- 3.Butan-2-ol has one H on its OH-bearing carbon, so it gives butan-2-one: CH₃CH(OH)CH₂CH₃ + [O] → CH₃COCH₂CH₃ + H₂O. Oxidation does not move the functional group to the chain end.
A tertiary alcohol cannot make a C=O bond at its OH-bearing carbon while retaining all three C–C bonds: that would give carbon an impossible bond count. Under strong oxidising conditions and high temperature, C–C cleavage can occur and generate smaller products, often mixtures. Therefore “tertiary alcohols resist ordinary oxidation” is appropriate; “tertiary alcohols can never react with an oxidant” is not.
In an alkaline oxidation, report the salt before acidification
Oxidation of a primary alcohol by alkaline permanganate gives a carboxylate under the reaction conditions. Acidification then gives the carboxylic acid. For propan-1-ol, write propanoate in the alkaline mixture, rather than pretending neutral propanoic acid remains fully protonated in strong base. Balancing oxidation and reduction half-reactions checks charge as well as atoms.
Each alcohol-to-carboxylate oxidation releases four electrons; three alcohol molecules release twelve. Each Mn(VII)-to-Mn(IV) reduction accepts three electrons; four permanganate ions accept twelve. The overall left charge is −4 and the right is −3−1=−4. After acid work-up, CH₃CH₂COO⁻ + H⁺ → CH₃CH₂COOH. The protonation step is not an additional oxidation.
Copper removes hydrogen rather than supplying oxygen
The two hydrogens lost now leave as molecular hydrogen. Although this is still oxidation of the organic carbon, there is no external [O] in the net equation. In the corresponding tertiary-alcohol comparison, dehydration gives an alkene and water instead: (CH₃)₃COH → (CH₃)₂C=CH₂ + H₂O. Inspect the coproduct, H₂ versus H₂O, to distinguish dehydrogenation from dehydration. Both words contain “hydrogen”, but the bond changes differ.
Methanol: synthesis and chemical identity
Methanol, CH₃OH, was historically obtained from destructive distillation of wood, giving the common name wood spirit. Industrial synthesis uses carbon oxides and hydrogen with catalysts at elevated temperature and pressure. A representative catalytic route uses CO and H₂ with a ZnO–Cr₂O₃ catalyst at about 573–673 K and 200–300 atm. Modern industrial catalyst systems vary; these conditions illustrate the stated route rather than a universal factory specification.
Methanol is a colourless liquid with a normal boiling point near 337 K. It is used as a solvent and as a feedstock for methanal and other chemicals. Its chemistry must be distinguished from ethanol despite their similar appearance. Metabolism converts methanol through methanal to methanoic acid/formate; the toxic metabolites can damage vision and cause severe poisoning. This is chemical background, not an identification test based on smell, colour or taste.
Ethanol: fermentation and hydration are different routes
Sucrose can first be hydrolysed by invertase to glucose and fructose. Yeast enzyme systems, conventionally described by the collective name zymase, convert a hexose sugar to ethanol and CO₂ under oxygen-limited conditions. The familiar fermentation equation is an overall biochemical balance rather than one elementary reaction. Enzyme activity, temperature, nutrient availability and the yeast strain affect the process.
CO₂ evolution is a visible observation. Increasing ethanol concentration inhibits ordinary yeast, so fermentation alone does not create an arbitrarily concentrated ethanol solution. Values near 14% are a useful traditional description, not an exact biological cutoff for every strain. In the presence of suitable microorganisms and oxygen, ethanol can instead be oxidised to ethanoic acid: C₂H₅OH + O₂ → CH₃COOH + H₂O. Simply writing air above an equation must not imply that uncatalysed oxidation is instantaneous.
Ethanol, C₂H₅OH, boils near 351 K and is widely used as a solvent and chemical feedstock. Industrial ethanol can also be made by catalytic hydration of ethene, an addition reaction rather than fermentation. Denatured alcohol is ethanol deliberately made unsuitable for drinking by additives; formulations vary. Denaturation is not a chemical transformation that gives a new single molecular formula for ethanol. Ethanol affects the nervous system, and methanol contamination adds a distinct toxic-metabolite hazard.
Problem
Find the theoretical ethanol mass from 180 g glucose, using molar masses 180 g mol⁻¹ for glucose and 46 g mol⁻¹ for ethanol.
- 1.Moles of glucose = 180 g ÷ 180 g mol⁻¹ = 1.00 mol. The balanced equation gives 2.00 mol ethanol per mole glucose.
- 2.Theoretical ethanol mass = 2.00 mol × 46 g mol⁻¹ = 92 g. The other 88 g appears as 2 mol CO₂ at 44 g mol⁻¹, checking the mass balance.
- 3.This is the ideal stoichiometric yield. Real fermentation diverts material into biomass and by-products, so an actual recovered mass requires measured efficiency.
Quick Quiz
Quiz
Which reagent is commonly chosen to stop a primary alcohol at an aldehyde?
Explanation: A controlled dry oxidation limits subsequent aldehyde-to-acid oxidation.
Why does a tertiary alcohol resist ordinary carbonyl-forming oxidation?
Explanation: Forming C=O without C–C cleavage would otherwise exceed carbon valence.
What distinguishes copper-catalysed dehydrogenation from dehydration?
Explanation: Compare the atoms removed and the resulting carbonyl versus alkene.
What is produced with 2 mol ethanol per mole glucose in ideal fermentation?
Explanation: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ balances carbon, hydrogen and oxygen.
What is the immediate organic product of alkaline primary-alcohol oxidation to the acid oxidation level?
Explanation: The basic medium deprotonates the acid; acid work-up gives the neutral form.
Practice Problems
- Choose conditions for benzyl alcohol to benzaldehyde or benzoic acid.
- Compare propan-1-ol, propan-2-ol and 2-methylpropan-2-ol on heated Cu at 573 K.
- Calculate the ideal ethanol yield from 45.0 g glucose and the mass at 80% yield.
- Explain why NaBH₄ and PCC accomplish opposite changes on a carbonyl/alcohol pair.
- Give the alkaline permanganate oxidation product of propan-1-ol and explain acid work-up.
- Distinguish methanol synthesis, ethanol fermentation and alcohol denaturation.
For C₆H₅CH₂OH → C₆H₅CHO use a controlled dry reagent such as PCC. To continue to C₆H₅COOH use a suitable excess aqueous oxidant with heating, then acidify if the oxidation medium was alkaline. The benzylic carbon remains present; the choice is oxidation extent, not loss of the side-chain carbon.
Key Takeaways
• Primary alcohols can oxidise to aldehydes and then acids; reagent and medium control the endpoint. • Secondary alcohols give ketones under ordinary oxidation. • Tertiary alcohols lack the hydrogen required for skeleton-preserving carbonyl formation. • Alkaline oxidation gives carboxylates before acidification. • Heated copper can dehydrogenate primary and secondary alcohols with H₂ release. • Methanol is made catalytically from carbon oxides and hydrogen. • Ideal hexose fermentation gives two ethanol and two CO₂ molecules. • Denaturation changes a mixture’s suitability for drinking, not ethanol’s molecular identity.