How do alkanes and alkenes react, and what mechanisms explain those reactions?
Alkanes from crude oil, combustion and free-radical substitution, alkenes and their electrophilic addition reactions, Markownikoff's rule, and addition polymerisation.
An Eduqas A-Level Chemistry C3.2 answer on alkanes (combustion and free-radical substitution), alkenes (electrophilic addition and Markownikoff's rule) and addition polymerisation.
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What this topic is asking
Eduqas topic C3.2 covers the two simplest hydrocarbon families: the saturated alkanes (their source in crude oil, combustion and free-radical substitution) and the unsaturated alkenes (their electrophilic addition reactions, Markownikoff's rule and addition polymerisation). The mechanisms are central, so curly-arrow accuracy earns the marks.
Alkanes: source and combustion
Alkanes are obtained from crude oil by fractional distillation and modified by cracking (breaking long chains into shorter, more useful ones, including alkenes). Complete combustion gives carbon dioxide and water and releases much energy; incomplete combustion in limited oxygen gives carbon monoxide or soot, which is why alkanes are major fuels but also a source of pollutants.
Free-radical substitution
Alkenes and electrophilic addition
Alkenes add hydrogen (with a nickel catalyst, to give an alkane), halogens (decolourising bromine water, the test for unsaturation), hydrogen halides and steam (with an acid catalyst, to give an alcohol).
Addition polymerisation
In addition polymerisation, many alkene monomers join through their double bonds to form a long saturated chain with no other product. Ethene gives poly(ethene); the repeat unit is drawn with continuation bonds and brackets. These polymers are chemically inert, which makes them useful but hard to dispose of.
Examples in context
Example 1. Bromine water test. Shaking an unknown with orange bromine water decolourises it if a is present (electrophilic addition), distinguishing an alkene from an alkane: a standard practical identification.
Example 2. Hydrogenation of vegetable oils. Adding hydrogen across the bonds of unsaturated oils (with a nickel catalyst) raises their melting point to make margarine, a direct industrial use of alkene addition.
Try this
Q1. Write the equation for the complete combustion of ethane. [1 mark]
- Cue. .
Q2. State the reagent and condition needed to convert ethene into ethanol, and name the type of reaction. [2 marks]
- Cue. Steam with a phosphoric acid catalyst (hydration); the reaction is electrophilic addition.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20195 marksMethane reacts with chlorine in the presence of ultraviolet light to form chloromethane. (a) Name the mechanism. (b) Write equations for the initiation step and the two propagation steps.Show worked answer β
(a) Free-radical substitution (1).
(b) Initiation: (1). Propagation: (1) and (1).
Markers reward the mechanism name, UV homolysis in initiation, and both propagation steps with radicals shown (1 for the overall correctness of radical notation).
Eduqas 20214 marksEthene reacts with hydrogen bromide to form bromoethane. (a) Name and outline the mechanism, showing curly arrows. (b) State the type of reagent that attacks the double bond and explain why.Show worked answer β
(a) Electrophilic addition (1). The electrons of the attack the hydrogen of HBr; a curly arrow goes from the double bond to H and another from the H-Br bond to Br, forming a carbocation and a bromide ion; the bromide then adds to the carbocation (2 for the mechanism with curly arrows).
(b) The double bond is attacked by an electrophile (1), because the bond is a region of high electron density that attracts electron-deficient (positively polarised) species (1).
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Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) β WJEC Eduqas (2015)