How do primary, secondary and tertiary alcohols differ in their reactions?
Classification and properties of alcohols (hydrogen bonding), combustion, oxidation by acidified dichromate (primary to aldehyde and carboxylic acid, secondary to ketone, tertiary not oxidised), dehydration to alkenes, and substitution to haloalkanes.
An OCR H432 module 4 answer on alcohols: classification and hydrogen bonding, combustion, oxidation by acidified potassium dichromate to aldehydes, ketones and carboxylic acids, dehydration to alkenes, and conversion to haloalkanes.
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What this topic is asking
OCR specification point 4.2.1 wants you to classify alcohols as primary, secondary or tertiary, explain their physical properties from hydrogen bonding, and describe their reactions: combustion, oxidation by acidified potassium dichromate(VI), dehydration to alkenes, and substitution to haloalkanes. The oxidation products feed straight into the carbonyl and carboxylic-acid chemistry of Module 6.
Classification and properties
Combustion
Alcohols burn completely in plenty of oxygen to carbon dioxide and water, for example
This releases a lot of energy, which is why ethanol is used as a biofuel.
Oxidation by acidified dichromate
The product depends on the class of alcohol and the conditions:
- Primary alcohol, distilled off: stops at the aldehyde (distil out as it forms so it cannot be oxidised further).
- Primary alcohol, reflux with excess oxidant: goes on to the carboxylic acid.
- Secondary alcohol, reflux: gives a ketone (which resists further oxidation).
- Tertiary alcohol: not oxidised (no hydrogen on the carbinol carbon), so the dichromate stays orange.
Dehydration and substitution
Examples in context
Example 1. Ethanol as a renewable fuel. Ethanol made by fermentation of plant sugars, or by hydration of ethene from crude oil, burns cleanly and is blended into petrol. The complete-combustion equation above is the basis of its energy content.
Example 2. Breathalyser chemistry. Early breathalysers used acidified potassium dichromate, which oxidises ethanol in the breath and turns from orange to green; the colour change measured the alcohol concentration, a direct application of primary-alcohol oxidation.
Try this
Q1. State the reagent and the colour change when a primary alcohol is oxidised. [2 marks]
- Cue. Acidified potassium dichromate(VI); orange to green.
Q2. Name the product when propan-2-ol is heated under reflux with acidified potassium dichromate(VI). [1 mark]
- Cue. Propanone (a ketone, because propan-2-ol is a secondary alcohol).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksPropan-1-ol can be oxidised by acidified potassium dichromate(VI). (a) Give the reagent colour change. (b) State the conditions and product for partial oxidation. (c) State the conditions and product for full oxidation.Show worked answer →
(a) The dichromate changes from orange to green (chromium(VI) reduced to chromium(III)) (1).
(b) Distil the product off as it forms (warm and distil) to give the aldehyde, propanal (1).
(c) Heat under reflux with excess oxidising agent to give the carboxylic acid, propanoic acid (1)(1).
Markers reward the orange-to-green colour change, distillation giving the aldehyde, and reflux with excess giving the carboxylic acid.
OCR 20203 marksExplain why 2-methylpropan-2-ol is not oxidised by acidified potassium dichromate(VI), and describe what you would observe.Show worked answer →
2-methylpropan-2-ol is a tertiary alcohol: the carbon bearing the group has no hydrogen atom attached to it (1). Oxidation by dichromate requires the removal of a hydrogen from that carbon, which is impossible here, so no oxidation occurs (1).
Observation: the dichromate stays orange (no colour change to green), because the chromium(VI) is not reduced (1).
Markers reward identifying it as tertiary with no C-H on the carbinol carbon, the resulting inability to be oxidised, and the orange colour remaining.
Related dot points
- Formulae (empirical, molecular, general, structural, displayed and skeletal), homologous series and functional groups, IUPAC nomenclature, and isomerism (structural and E/Z stereoisomerism with Cahn-Ingold-Prelog priority).
An OCR H432 module 4 answer on the basics of organic chemistry: the six types of formula, homologous series and functional groups, IUPAC nomenclature, and structural and E/Z stereoisomerism using Cahn-Ingold-Prelog priority rules.
- Structure and bonding of alkenes (sigma and pi bonds, trigonal planar carbon), E/Z isomerism, electrophilic addition (hydrogen, halogens, hydrogen halides and steam), Markownikoff's rule, and addition polymerisation.
An OCR H432 module 4 answer on alkenes: pi bonding and trigonal planar shape, electrophilic addition with hydrogen, halogens, hydrogen halides and steam, Markownikoff's rule via the more stable carbocation, and addition polymerisation.
- Polarity of the carbon-halogen bond, nucleophilic substitution (with hydroxide, cyanide and ammonia), the trend in hydrolysis rate with carbon-halogen bond enthalpy, elimination to alkenes, and the role of CFCs in ozone depletion.
An OCR H432 module 4 answer on haloalkanes: the polar carbon-halogen bond, nucleophilic substitution with hydroxide, cyanide and ammonia, the hydrolysis-rate trend with bond enthalpy, elimination to alkenes, and CFCs and ozone depletion.
- Planning multi-step synthesis routes between functional groups using practical techniques (reflux, distillation, purification), and the analytical techniques of infrared spectroscopy (functional-group absorptions) and mass spectrometry (molecular ion and fragmentation).
An OCR H432 module 4 answer on organic synthesis and analysis: building multi-step reaction routes between functional groups, practical techniques, infrared spectroscopy for functional groups, and mass spectrometry for relative molecular mass and fragmentation.
- Oxidation numbers and the rules for assigning them, oxidation and reduction as loss and gain of electrons, oxidising and reducing agents, and the construction of half-equations and overall redox equations.
An OCR H432 module 2 answer covering oxidation number rules, oxidation and reduction as electron transfer, oxidising and reducing agents, and building half-equations and balanced redox equations.
Sources & how we know this
- OCR A-Level Chemistry A (H432) specification — OCR (2015)