Chemistry (AS Level)·Halogen derivatives · NSSCAS 4.3.1 & 4.3.2
Halogenoalkanes: Reactions & Mechanisms
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Halogenoalkanes are alkanes carrying a halogen atom, and that one polar bond opens a whole family of reactions. Learn to read and name them, deduce their molecular formulae, and follow their nucleophilic substitution reactions of bromoethane — hydrolysis to an alcohol, reaction with cyanide to a nitrile, and reaction with ammonia to an amine — drawn out with curly arrows. Then meet elimination to an alkene, the reactivity trend set by carbon–halogen bond strength, the uses of inert fluoro compounds, and the CFC–ozone story.
What you'll learn in this lesson
By the end you should be able to (NSSCAS Chemistry (AS Level) 4.3.1 & 4.3.2):
- Interpret and use general, structural (condensed/semi-structural and displayed) and skeletal formulae of halogenoalkanes
- Name halogenoalkanes up to six carbons, naming halogeno groups in alphabetical order (e.g. 1,2-dibromoethane, 1-chlorobutane)
- Deduce the molecular formula of a halogenoalkane from a given structure
- Describe nucleophilic substitution of bromoethane: hydrolysis with aqueous OH⁻ to an alcohol
- Describe nucleophilic substitution of bromoethane: reaction with CN⁻ to form a nitrile (propanenitrile)
- Describe nucleophilic substitution of bromoethane: reaction with ammonia to form a primary amine
- Describe the elimination of hydrogen bromide from 2-bromopropane with hot ethanolic KOH to form propene
- Show the nucleophilic substitution mechanism with curly arrows, C–Br bond polarity and attack on the δ+ carbon
- Interpret the different reactivities of halogenoalkanes in hydrolysis using the relative strengths of the C–Hal bonds
- Explain the uses of fluoroalkanes and fluorohalogenoalkanes in terms of their chemical inertness
- Recall that CFCs deplete the ozone layer and the development of replacement compounds to allow the ozone layer to repair
- Explain bond energy, bond length and bond polarity and use them to compare the reactivities of covalent bonds
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