Combustion and Oxidation of Alcohols

Spec Mapping — OCR H432 Module 4.2.1(b)-(c) — Combustion and oxidation of alcohols, covering complete combustion of alcohols as fuels; controlled oxidation of primary alcohols to aldehydes (distillation) and to carboxylic acids (reflux); oxidation of secondary alcohols to ketones; the lack of reactivity of tertiary alcohols towards acidified potassium dichromate(VI); use of [O] as a shorthand for oxidising agents in organic equations; the orange-to-green colour change of dichromate as a diagnostic for primary or secondary alcohols; Tollens' and Fehling's tests to distinguish aldehyde from ketone (refer to the official OCR H432 specification document for exact wording).

Alcohols are one of the most chemically versatile homologous series because the -OH group can be removed in three different ways. Combustion strips every C-H and O-H bond, taking the molecule all the way to CO₂ and H₂O with a large release of energy. Controlled chemical oxidation removes some hydrogens but leaves the carbon skeleton intact, giving aldehydes, ketones or carboxylic acids depending on the class of alcohol and the apparatus chosen. Biological oxidation (alcohol dehydrogenase in the liver) follows the same chemistry but uses an enzyme cofactor (NAD⁺) instead of a chromium oxidising agent. This lesson develops the controlled-oxidation framework that you will need for synthetic-route questions throughout the rest of the A-Level course — primary → aldehyde or carboxylic acid (distillation vs reflux choice), secondary → ketone, tertiary → no reaction — and the analytical tests (Tollens', Fehling's) that confirm which class of carbonyl you have produced.

Key Mechanism: controlled oxidation by acidified dichromate(VI) proceeds by removing a hydrogen from the C-OH carbon (which becomes the carbonyl carbon) AND a hydrogen from the -OH (which goes off as water with the oxygen of the new carbonyl). A primary alcohol has two oxidisable α-hydrogens (one on the C-OH carbon and one on the -OH), giving aldehyde then carboxylic acid. A secondary alcohol has one α-hydrogen, giving a ketone. A tertiary alcohol has none — no reaction.

Combustion of alcohols

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

$C_nH_{2n+1}OH + \tfrac{3n}{2}\,O_2 \rightarrow n\,CO_2 + (n+1)\,H_2O$

Worked stoichiometry for ethanol ( $n = 2$ ):

$C_2H_5OH + 3\,O_2 \rightarrow 2\,CO_2 + 3\,H_2O \quad \Delta_cH^\ominus = -1367 \text{ kJ mol}^{-1}$

Combustion is strongly exothermic, releasing enough energy for alcohols — particularly ethanol and methanol — to be used as fuels. In a limited supply of oxygen, incomplete combustion produces CO and unburnt soot (C), with associated toxicity and reduced energy yield.

Ethanol as a biofuel — fermentation route

Ethanol is produced industrially by two complementary routes:

Hydration of ethene (a fossil-fuel route): $C_2H_4 + H_2O \rightarrow C_2H_5OH$ , catalysed by H₃PO₄ on silica at ~300 °C and ~60 atm. Continuous process, fast, ~100 % atom economy, but ethene comes from cracking crude oil.
Fermentation of sugars (a biofuel route): $C_6H_{12}O_6 \rightarrow 2\,C_2H_5OH + 2\,CO_2$ , catalysed by yeast at ~35 °C, anaerobic. Batch process, slow, ~51 % atom economy, but feedstock is renewable.

Feature	Hydration of ethene	Fermentation
Feedstock	Crude oil (ethene from cracking)	Sugar cane, sugar beet, corn
Temperature / pressure	300 °C, 60 atm	~35 °C, 1 atm
Catalyst	H₃PO₄ on silica	Yeast (zymase enzymes)
Process type	Continuous	Batch
Rate	Fast (minutes)	Slow (days)
Initial purity	~95 %	~15 % (toxicity limit of yeast)
Atom economy	100 %	51 %
Renewable?	No	Yes (in principle)

flowchart TD
  A[Sugar cane / beet / corn] --> B[Crush and extract sugars]
  B --> C[Aqueous glucose / sucrose]
  C --> D[Add yeast, 35 C, anaerobic, sealed]
  D --> E[Fermentation: 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 or blend with petrol e.g. E85]

Carbon-neutrality — a closer look

Ethanol from fermentation is often described as carbon neutral because the CO₂ released on combustion was previously absorbed from the atmosphere by the sugar crop during photosynthesis. In practice the picture is more nuanced:

Farming uses fossil-fuel-powered machinery and nitrate fertilisers (Haber-Bosch ammonia → nitric acid → ammonium nitrate).
Fractional distillation to concentrate the ethanol requires significant heat input.
Transporting the fuel to filling stations adds CO₂.
Clearing rainforest to grow biofuel crops (palm-oil and sugar-cane plantations) releases stored soil carbon and reduces future photosynthetic capacity.

Independent lifecycle analyses typically show ethanol biofuel cuts well-to-wheel CO₂ by 30-80 % relative to petrol, depending on feedstock, distillation energy source, and land-use change. The lower end (30 %) is for first-generation corn ethanol; the upper end (80 %) is for sugar-cane ethanol in Brazil using bagasse as the heat source for distillation. Second-generation cellulosic ethanol (from agricultural waste, not food crops) avoids the food-vs-fuel tension and is the active research frontier.

Controlled oxidation — the big picture

The "combustion" route strips every C-H and O-H bond, taking the molecule all the way to CO₂ and H₂O. The "controlled oxidation" route stops at a useful synthetic intermediate by removing only a specific subset of hydrogens. The product depends on three choices:

Class of alcohol (primary, secondary, tertiary).
Apparatus — distillation (removes volatile product as it forms) versus reflux (keeps everything in the flask).
Amount of oxidising agent — limited (gentle, stops early) versus excess (forcing, goes all the way).

The oxidising agent is almost always acidified potassium dichromate(VI), K₂Cr₂O₇ in dilute H₂SO₄, in which the active species is the orange Cr₂O₇²⁻ ion (Cr in oxidation state +6). During the oxidation Cr₂O₇²⁻ is reduced to the green Cr³⁺ ion (Cr in oxidation state +3):

$\text{Cr}_2\text{O}_7^{2-} + 14\text{H}^+ + 6\text{e}^- \rightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}$

The observable orange-to-green colour change is therefore a diagnostic test for the presence of an oxidisable alcohol. Tertiary alcohols give no colour change — the dichromate stays orange.

The symbol [O] is used by convention in organic equations to represent one oxygen atom supplied by the oxidising agent, saving you from balancing the full chromium half-equation each time.

Primary R-CH₂-OH [O] / distil K₂Cr₂O₇/H₂SO₄ R-CHO [O] / reflux excess R-COOH

Secondary R₂CH-OH [O] / reflux R₂C=O no further oxidation

Tertiary R₃C-OH [O] No reaction — solution stays orange

flowchart TD
  A[Unknown alcohol] --> B[Warm with K2Cr2O7 / H2SO4]
  B --> C{Colour change?}
  C -->|Orange to green| D[Primary or secondary]
  C -->|Stays orange| E[Tertiary - 3 degrees]
  D --> F[Reflux to completion; distil off carbonyl]
  F --> G[Add Tollens reagent]
  G --> H{Silver mirror?}
  H -->|Yes| I[Aldehyde - original was primary]
  H -->|No| J[Ketone - original was secondary]

Oxidation of primary alcohols

A primary alcohol $R\text{-CH}_2\text{-OH}$ has two oxidisable hydrogens on the C-OH carbon: the H on carbon and the H on oxygen. Both can be removed in stages, giving aldehyde then carboxylic acid.

Stage 1 — Stopping at the aldehyde (distillation)

The aldehyde has a lower boiling point than the parent alcohol because the aldehyde has no O-H and so no hydrogen-bond donor — it cannot form hydrogen bonds (only dipole-dipole). If you set up a distillation apparatus, the aldehyde boils out of the flask as soon as it forms and is condensed into a separate collecting flask before it can be exposed to more dichromate.

Conditions: limited oxidising agent, distillation apparatus, gentle heating.

$\text{CH}_3\text{CH}_2\text{OH (b.p. 78 °C)} + [O] \rightarrow \text{CH}_3\text{CHO (b.p. 20 °C)} + \text{H}_2\text{O}$

The 58 °C boiling-point gap is the reason this works.

Stage 2 — Going to the carboxylic acid (reflux)

If you instead heat the alcohol with excess dichromate under reflux, any aldehyde that forms is condensed back into the flask and oxidised again, this time with the addition of an -OH to give the carboxylic acid.

Conditions: excess oxidising agent, reflux apparatus, heat for several hours.

$\text{CH}_3\text{CH}_2\text{OH} + 2[O] \rightarrow \text{CH}_3\text{COOH} + \text{H}_2\text{O}$

The two-step net equation combines stage 1 and stage 2.

Reflux apparatus

flowchart TD
  A[Round-bottomed flask: alcohol + K2Cr2O7 + H2SO4] --> B[Anti-bumping granules in flask]
  A --> C[Vertical Liebig condenser]
  C --> D[Cold water in at bottom; out at top]
  D --> E[Vapour condenses on inner glass]
  E --> F[Returns as liquid to flask]
  A --> G[Heat with water bath or electric mantle]
  G --> H[Open top of condenser - never sealed]

Key features (examined frequently):

Vertical Liebig condenser. A horizontal condenser is distillation.
Open top. Sealing the apparatus would let pressure build and the flask could explode.
Anti-bumping granules. Prevent superheating and uneven boiling.
Water bath or electric mantle. Never a naked flame for flammable organic liquids.
Counter-current water flow. Water enters at the bottom, exits at the top, to keep the condenser jacket full.

Oxidation of secondary alcohols

A secondary alcohol $R_2\text{CH-OH}$ has only one oxidisable C-H on the C-OH carbon (the other two positions are C-C bonds, which dichromate cannot break). So it can be oxidised once only, to a ketone:

$R_2\text{CHOH} + [O] \rightarrow R_2\text{C=O} + \text{H}_2\text{O}$

Worked example for propan-2-ol → propanone:

$(\text{CH}_3)_2\text{CHOH} + [O] \rightarrow (\text{CH}_3)_2\text{CO} + \text{H}_2\text{O}$

You can use either distillation or reflux for a secondary alcohol — both give the ketone, because the ketone has no further oxidisable hydrogen on the carbonyl carbon and so is inert to further dichromate attack. Reflux is usually preferred because it gives a higher yield (no product lost as vapour escaping out of the condenser).

Oxidation of tertiary alcohols

A tertiary alcohol $R_3\text{C-OH}$ has no C-H on the C-OH carbon — all three non-OH positions are C-C bonds. There is nothing for dichromate to remove, so tertiary alcohols are not oxidised by acidified K₂Cr₂O₇ under standard conditions. (Forcing conditions like hot chromic acid or KMnO₄ at high temperature will eventually break a C-C bond, but these are not on the A-Level syllabus.)

Key observation: adding acidified K₂Cr₂O₇ to a tertiary alcohol gives no colour change — the dichromate stays orange.

This is the basis of the standard A-Level test for a tertiary alcohol:

Warm a few drops of the alcohol with acidified K₂Cr₂O₇.
Orange stays orange → tertiary.
Orange goes green → primary or secondary; proceed to Tollens'/Fehling's.

Distinguishing aldehydes from ketones — Tollens' and Fehling's

Once you have done the oxidation, you can test the product to confirm whether it is an aldehyde (so the original alcohol was primary) or a ketone (so the original alcohol was secondary).

Test	Reagent and conditions	Aldehyde	Ketone
Tollens'	$[\text{Ag(NH}_3)_2]^+$ in aqueous NH₃, warm in a water bath at ~50 °C	Silver mirror forms on the inside of the test tube as $\text{Ag}^+$ is reduced to Ag metal	No reaction; solution stays clear
Fehling's	$\text{Cu}^{2+}$ in alkaline tartrate (sodium potassium tartrate keeps Cu(II) in solution at high pH), warm	Blue solution turns to a brick-red precipitate of Cu₂O	No reaction; solution stays blue

Both tests exploit the easy oxidation of aldehydes to carboxylic acids. The electrons released by the aldehyde reduce $\text{Ag}^+ \rightarrow \text{Ag}$ (Tollens') or $\text{Cu}^{2+} \rightarrow \text{Cu}^+$ (Fehling's). Ketones have no H on the carbonyl carbon and cannot be oxidised by these mild reagents — hence the negative result.

Lesson 2: Combustion and Oxidation of Alcohols