What are the characteristic reactions of the main families of organic compounds?
The characteristic reactions of alkanes (combustion and substitution), alkenes (addition), alcohols (oxidation, combustion and dehydration) and carboxylic acids, and the reaction types of combustion, substitution, addition and oxidation.
A CCEA Life and Health Sciences answer on organic reactions: the characteristic reactions of alkanes, alkenes, alcohols and carboxylic acids, and the reaction types of combustion, substitution, addition and oxidation.
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What this dot point is asking
CCEA wants you to describe the characteristic reactions of the main organic families, the alkanes, alkenes, alcohols and carboxylic acids, and to classify reactions as combustion, substitution, addition or oxidation. It builds directly on the functional-group naming from the previous dot point, because the functional group determines how a compound reacts.
Reactions of alkanes and alkenes
Alkanes have only strong, non-polar single bonds, so they are fairly unreactive. Their main reactions are complete combustion, burning in plenty of oxygen to give carbon dioxide and water and releasing energy (which is why they are fuels), and substitution with halogens such as chlorine in the presence of ultraviolet light, where a hydrogen atom is replaced by a halogen atom. Alkenes are far more reactive because of the carbon-to-carbon double bond, which is a region of high electron density. They undergo addition reactions: with bromine to form a dibromo compound (decolourising orange bromine water, the standard test for an alkene), with hydrogen (over a nickel catalyst) to form an alkane, and with steam to form an alcohol.
Reactions of alcohols
The oxidation of alcohols is a much-tested reaction. Distilling the product off as it forms stops oxidation at the aldehyde stage, while heating under reflux with excess oxidising agent drives it on to the carboxylic acid. The orange-to-green colour change of the dichromate is the visible sign that oxidation has happened, and it is used to detect alcohol (for example in older breathalyser tests). Dehydration is the reverse of the hydration of an alkene, linking the two families.
Reactions of carboxylic acids and summary of reaction types
Carboxylic acids (such as ethanoic acid) are weak acids: they dissociate only partially in water. They show typical acid reactions, neutralising bases and alkalis to form a salt and water, and reacting with carbonates to give a salt, water and carbon dioxide (which fizzes, a useful test for a carboxylic acid). They can also react with alcohols to form esters. Across the unit, the reaction types to recognise are: combustion (burning in oxygen, releasing energy), substitution (one atom or group replaced by another, as in alkanes with halogens), addition (atoms added across a double bond, as in alkenes), and oxidation (increase in oxygen or loss of hydrogen or electrons, as in alcohols to aldehydes and acids). Recognising the type lets you predict products.
Examples in context
Example 1. Margarine from vegetable oils. Vegetable oils contain carbon-to-carbon double bonds (they are unsaturated). Adding hydrogen across some of these double bonds over a nickel catalyst (an addition reaction) hardens the oil into a spread. This industrial hydrogenation is a direct application of alkene addition chemistry and links to the nutrition content of the qualification.
Example 2. Detecting alcohol by oxidation. Older breathalysers used acidified potassium dichromate, which turns from orange to green when it oxidises the ethanol in a person's breath. The extent of the colour change indicated the amount of alcohol, a practical use of the oxidation of a primary alcohol that connects organic chemistry to health and safety.
Try this
Q1. Name the reaction type and product when ethene reacts with bromine. [2 marks]
- Cue. Addition; the product is 1,2-dibromoethane (and bromine water is decolourised).
Q2. State the products of the complete oxidation of a primary alcohol and the colour change of the oxidising agent. [2 marks]
- Cue. A carboxylic acid (via an aldehyde); the acidified dichromate changes from orange to green.
Q3. State the test that distinguishes an alkene from an alkane. [1 mark]
- Cue. The alkene decolourises orange bromine water; the alkane does not.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA A2 26 marksDescribe the reaction of ethene with bromine, naming the type of reaction and the product, and explain how this reaction is used as a test to distinguish an alkene from an alkane.Show worked answer →
The answer needs the reaction, its type and product, and the practical test.
Reaction and type: ethene reacts with bromine in an addition reaction. The carbon-to-carbon double bond opens and a bromine atom adds to each of the two carbons, so ethene plus bromine gives 1,2-dibromoethane. No other product is formed (addition, not substitution).
The test: bromine water (orange or yellow-brown) is added to the compound. An alkene decolourises bromine water rapidly, turning it colourless, because the bromine adds across the double bond. An alkane does not react with bromine water under these conditions (no double bond), so the colour remains. Therefore decolourising bromine water distinguishes an alkene from an alkane.
Markers reward addition as the reaction type, 1,2-dibromoethane as the product, and the decolourising of bromine water by the alkene but not the alkane as the distinguishing test.
CCEA A2 25 marksEthanol can be oxidised. Describe what happens when ethanol is oxidised by an oxidising agent such as acidified potassium dichromate, naming the products of partial and complete oxidation and the colour change observed.Show worked answer →
Give the two oxidation products, name the oxidising agent change, and the colour change.
Partial oxidation: a primary alcohol such as ethanol is first oxidised to an aldehyde (ethanal). If the aldehyde is distilled off as it forms, oxidation stops there.
Complete oxidation: if the mixture is heated under reflux with excess oxidising agent, the aldehyde is oxidised further to a carboxylic acid (ethanoic acid).
Oxidising agent and colour change: acidified potassium dichromate is the oxidising agent. As it oxidises the alcohol it is itself reduced, and the colour changes from orange (dichromate) to green (chromium(III) ions). This colour change is the sign that oxidation has occurred.
Markers reward ethanal (aldehyde) for partial oxidation, ethanoic acid (carboxylic acid) for complete oxidation, and the orange-to-green colour change of the dichromate.
Related dot points
- The classification of organic compounds by functional group, homologous series and general formulae, IUPAC nomenclature, the different ways of representing organic molecules, and the meaning of structural isomerism.
A CCEA Life and Health Sciences answer on organic classification: functional groups, homologous series and general formulae, IUPAC nomenclature, the ways of representing organic molecules, and an introduction to structural isomerism.
- Structural isomerism (chain, position and functional group isomers), stereoisomerism including cis-trans (E-Z) isomerism in alkenes, the conditions needed for each, and why isomers can have different properties.
A CCEA Life and Health Sciences answer on isomerism: structural isomerism (chain, position and functional group), stereoisomerism including cis-trans (E-Z) isomerism in alkenes, the conditions for each, and why isomers differ in properties.
- The principles and uses of instrumental methods for identifying organic compounds, including mass spectrometry, infrared spectroscopy and chromatography, and how data from these methods are interpreted to determine structure.
A CCEA Life and Health Sciences answer on instrumental analysis: the principles and uses of mass spectrometry, infrared spectroscopy and chromatography, and how their data are interpreted to identify and determine the structure of organic compounds.
- Addition polymerisation of alkenes and condensation polymerisation, the structures of the polymers formed, the differences between the two types, and the uses and environmental impact of polymers including biodegradability and disposal.
A CCEA Life and Health Sciences answer on polymers: addition polymerisation of alkenes and condensation polymerisation, the structures formed, the differences between the two types, and the uses and environmental impact of polymers.
- The Bronsted-Lowry definitions of acids and bases, strong and weak acids, the pH scale and its relationship to hydrogen ion concentration, neutralisation reactions, and the use of titration to find an unknown concentration.
A CCEA Life and Health Sciences answer on acids, bases and pH: the Bronsted-Lowry definitions, strong and weak acids, the pH scale and hydrogen ion concentration, neutralisation reactions, and using titration to find an unknown concentration.
Sources & how we know this
- CCEA GCE Life and Health Sciences specification — CCEA (2016)