What reactions do alcohols undergo and how do we classify them?
Classify alcohols and describe their oxidation, dehydration, substitution and combustion reactions.
Classifying primary, secondary and tertiary alcohols and their reactions: oxidation, dehydration to alkenes, substitution and combustion.
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What this dot point is asking
You must classify alcohols and describe their characteristic reactions with equations.
Classifying alcohols
The classification depends on how many carbon atoms are attached to the carbon that carries the hydroxyl group.
- Primary: the hydroxyl carbon is attached to one other carbon.
- Secondary: attached to two other carbons.
- Tertiary: attached to three other carbons.
This matters because it controls the oxidation products.
Oxidation
Oxidising agents such as acidified potassium dichromate or acidified potassium permanganate oxidise alcohols.
The orange dichromate ion turns green as it is reduced, and the purple permanganate ion fades, so a colour change is a useful test for an oxidisable alcohol.
Dehydration
Heating an alcohol with a concentrated acid catalyst removes a molecule of water to form an alkene. For example, ethanol dehydrates to ethene:
This is an elimination reaction, the reverse of the hydration of an alkene.
Substitution and combustion
Alcohols react with hydrogen halides to replace the hydroxyl group with a halogen, forming a haloalkane. They also burn in oxygen; complete combustion gives carbon dioxide and water and releases energy, which is why ethanol is used as a fuel.
In the exam, classify the alcohol first, then state the oxidation product for its class, write the dehydration product as an alkene, and use the dichromate or permanganate colour change as evidence of oxidation.
Exam-style practice questions
Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2023 TASC6 marksTwo alcohols used in hand sanitisers are ethanol and propan-2-ol. Identify each as a primary, secondary or tertiary alcohol. A student reacted each with acidified potassium permanganate to tell them apart; name the organic product of each reaction.Show worked answer →
Classification (2 marks). Ethanol, CH3CH2OH, has its -OH on a carbon bonded to only one other carbon, so it is a primary alcohol. Propan-2-ol, CH3CH(OH)CH3, has its -OH on a carbon bonded to two other carbons, so it is a secondary alcohol.
Oxidation products (4 marks). Acidified potassium permanganate is a strong oxidiser. A primary alcohol is oxidised first to an aldehyde and then, with excess oxidiser, to a carboxylic acid, so ethanol gives ethanoic acid (CH3COOH). A secondary alcohol is oxidised to a ketone, so propan-2-ol gives propanone (CH3COCH3). The fact that one forms an acid and the other a ketone lets the student distinguish them.
2024 TASC2 marksAlkyl halides have higher boiling points than the alkanes from which they are synthesised. Explain why.Show worked answer →
Both alkanes and alkyl halides are covalent molecules, but they differ in polarity.
The carbon-halogen bond (for example C-Cl) is polar because the halogen is more electronegative than carbon, giving the alkyl halide molecule a permanent dipole. This means alkyl halides have dipole-dipole attractions in addition to dispersion forces, whereas non-polar alkanes have only dispersion forces.
Adding the halogen also increases the molar mass and the number of electrons, which strengthens dispersion forces as well. The stronger overall intermolecular forces require more energy to separate the molecules, so the boiling point is higher. (2 marks: identify the extra dipole-dipole/dispersion forces and link to boiling point.)