Combustion and Oxidation of Alcohols

Alcohols can lose their hydrogen atoms to form new C=O bonds. In combustion this happens explosively with O₂ from the air, giving CO₂ and H₂O and a lot of energy. In chemical oxidation we use a milder oxidising agent — usually acidified potassium dichromate(VI) — to replace the C–H and O–H bonds on the alcohol carbon with one or two C=O bonds, producing aldehydes, ketones or carboxylic acids.

This lesson covers the OCR A-Level Chemistry A (H432) specification points 4.2.1 (b)–(c): combustion of alcohols; controlled oxidation of primary and secondary alcohols; the resistance of tertiary alcohols to oxidation; the use of acidified potassium dichromate(VI) as the oxidising agent; distinguishing aldehydes and ketones by Tollens' reagent and Fehling's solution.

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1. Combustion of Alcohols

Alcohols burn in a plentiful supply of oxygen to give carbon dioxide and water. The general equation for a saturated alcohol C_nH_{2n+1}OH is:

$C_nH_{2n+1}OH + \tfrac{3n}{2}\,O_2 \rightarrow n\,CO_2 + (n+1)\,H_2O$Cn​H2n+1​OH+23n​O2​→nCO2​+(n+1)H2​O

For example, ethanol:

$C_2H_5OH + 3\,O_2 \rightarrow 2\,CO_2 + 3\,H_2O \quad \Delta H = -1367 \text{ kJ mol}^{-1}$C2​H5​OH+3O2​→2CO2​+3H2​OΔH=−1367 kJ mol−1

Combustion is exothermic and releases enough energy for alcohols — particularly ethanol and methanol — to be used as fuels.

1.1 Biofuels: Ethanol from Fermentation

Ethanol is produced industrially by two routes:

1. Hydration of ethene (a fossil-fuel route): C₂H₄ + H₂O → C₂H₅OH, catalysed by H₃PO₄ on silica at ~300 °C and 60 atm.
2. Fermentation of sugars (a biofuel route): C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂, catalysed by yeast enzymes at ~35 °C, anaerobic.

Feature	Hydration of ethene	Fermentation
Feedstock	Crude oil (ethene from cracking)	Sugar cane, sugar beet, corn
Conditions	300 °C, 60 atm, H₃PO₄ catalyst	~35 °C, 1 atm, yeast
Process type	Continuous	Batch
Rate	Fast	Slow
Purity	High (~95%)	Low (~15%), needs fractional distillation
Atom economy	100%	51%
Renewable?	No	Yes (carbon neutral in theory)

graph TD
  A[Sugar cane / beet] --> B[Crush & extract sugars]
  B --> C[Aqueous glucose]
  C --> D[Add yeast, 35 C, anaerobic]
  D --> E[Fermentation<br/>C6H12O6 -> 2 C2H5OH + 2 CO2]
  E --> F[Dilute ethanol 12-15%]
  F --> G[Fractional distillation]
  G --> H[Concentrated ethanol 95%]
  H --> I[Use as biofuel / blend with petrol e.g. E85]

1.2 Carbon Neutrality — A Closer Look

Ethanol from fermentation is often claimed to be carbon neutral because the CO₂ released when it burns was originally absorbed from the atmosphere by the sugar crop during photosynthesis.

In reality:

• Farming uses fossil-fuel powered machinery and nitrate fertilisers.
• Fractional distillation requires significant energy input.
• Transporting the fuel adds further CO₂.
• Clearing rainforest to grow biofuel crops releases stored carbon.

Ethanol biofuel typically reduces lifecycle CO₂ by 30–80% compared with petrol, depending on the feedstock and process — still a reduction, but not truly zero.

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2. Controlled Oxidation — The Big Picture

The "combustion" route takes the alcohol all the way to CO₂ and H₂O. The "controlled oxidation" route stops at one of several useful organic products, depending on:

• The class of alcohol (primary, secondary, tertiary).
• The conditions (distillation vs reflux).
• The amount of oxidising agent used.

The oxidising agent is almost always acidified potassium dichromate(VI), K₂Cr₂O₇ / H₂SO₄, which contains the orange Cr₂O₇²⁻ ion. During the reaction it is reduced to the green Cr³⁺ ion:

$Cr_2O_7^{2-} + 14\,H^+ + 6\,e^- \rightarrow 2\,Cr^{3+} + 7\,H_2O$Cr2​O72−​+14H++6e−→2Cr3++7H2​O

Observation: Acidified potassium dichromate(VI) changes from orange to green when a primary or secondary alcohol is oxidised. Tertiary alcohols give no colour change.

The symbol [O] is conventionally used in equations to represent one oxygen atom donated by the oxidising agent, to save balancing half-equations.

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3. Oxidation of Primary Alcohols

A primary alcohol has two oxidisable positions: the C–H and O–H on the C–OH carbon. Both can be removed, so it can be oxidised twice:

1. First oxidation removes the C–H and O–H to give an aldehyde (R–CHO). One [O] is added effectively.
2. Second oxidation adds an O–H to the aldehyde carbon to give a carboxylic acid (R–COOH).

graph LR
  A[Primary alcohol<br/>R-CH2-OH] -->|+ O<br/>distil| B[Aldehyde<br/>R-CHO]
  B -->|+ O<br/>reflux| C[Carboxylic acid<br/>R-COOH]
  A -->|+ 2 O<br/>reflux excess| C

3.1 Stopping at the Aldehyde: Distillation

To stop at the aldehyde, you use:

• A limited amount of oxidising agent.
• Distillation apparatus, so the aldehyde boils out of the flask as soon as it forms and is condensed into a collecting flask before it can be oxidised further.

This works because aldehydes have lower boiling points than the parent alcohols (no hydrogen bonding in the aldehyde, because there is no O–H).

Example: Ethanol (bp 78 °C) → ethanal (bp 20 °C). The ethanal distils off immediately.

$CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O$CH3​CH2​OH+[O]→CH3​CHO+H2​O

3.2 Going to the Acid: Reflux

To oxidise all the way to the carboxylic acid, you use:

• Excess oxidising agent.
• Heating under reflux, so that any volatile intermediate is condensed back into the reaction flask and exposed to more oxidising agent.

Example: Ethanol → ethanoic acid.

$CH_3CH_2OH + 2\,[O] \rightarrow CH_3COOH + H_2O$CH3​CH2​OH+2[O]→CH3​COOH+H2​O

3.3 Reflux Apparatus — What It Is