How do halogenoalkanes react, and what mechanisms explain substitution and elimination?
Nucleophilic substitution of halogenoalkanes, the mechanisms and the effect of the C-X bond, elimination to form alkenes, the relative rates of hydrolysis, and the environmental impact of halogenoalkanes.
An Eduqas A-Level Chemistry C3.3 answer on nucleophilic substitution of halogenoalkanes, elimination to alkenes, the relative rates of hydrolysis, and the environmental impact of CFCs.
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What this topic is asking
Eduqas topic C3.3 covers the reactions of halogenoalkanes: nucleophilic substitution (and its mechanism), elimination to form alkenes, the trend in hydrolysis rate down the halogen group, and the environmental impact of halogenoalkanes such as CFCs. The mechanisms and the bond-enthalpy explanation of reactivity are the recurring exam focus.
The polar C-X bond and nucleophilic substitution
The three standard substitutions are: with warm aqueous hydroxide, giving an alcohol; with potassium cyanide in ethanol, giving a nitrile (which adds a carbon to the chain); and with excess ammonia, giving an amine.
Elimination
With hot, concentrated hydroxide dissolved in ethanol, a halogenoalkane undergoes elimination: a hydrogen and the halogen are removed from adjacent carbons to form a double bond and a hydrogen halide. The same reagent gives substitution in aqueous solution and elimination in ethanolic solution, so the solvent decides the outcome.
Relative rates of hydrolysis
The rate of hydrolysis depends on the carbon-halogen bond enthalpy, not on bond polarity. The C-I bond is the weakest and breaks most easily, so iodoalkanes hydrolyse fastest; C-Cl is the strongest, so chloroalkanes are slowest. Warming with aqueous silver nitrate in ethanol gives a silver halide precipitate (white AgCl, cream AgBr, yellow AgI), and the order in which they appear confirms the trend.
Environmental impact
Examples in context
Example 1. Building a longer chain with cyanide. Reacting a halogenoalkane with adds one carbon as a nitrile, which can be hydrolysed to a carboxylic acid; this is a key chain-lengthening step in synthesis routes.
Example 2. The Montreal Protocol. Recognising that CFC chlorine radicals catalytically destroy ozone led to a global ban; modern refrigerants are chosen to lack the persistent C-Cl bonds, a direct application of halogenoalkane chemistry to policy.
Try this
Q1. Name the organic product when 1-bromopropane is heated with potassium cyanide in ethanol. [1 mark]
- Cue. Butanenitrile, (the chain has gained one carbon).
Q2. State the colour of the precipitate formed when 1-iodobutane is warmed with aqueous silver nitrate. [1 mark]
- Cue. Yellow (silver iodide, AgI).
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 20184 marks1-bromobutane reacts with warm aqueous sodium hydroxide. (a) Name the type and mechanism of the reaction and the organic product. (b) Outline the mechanism using curly arrows.Show worked answer →
(a) Nucleophilic substitution; the product is butan-1-ol (2: 1 for mechanism name, 1 for product).
(b) The hydroxide ion (a nucleophile) is attracted to the carbon bonded to bromine; a curly arrow goes from a lone pair on to that carbon, and another from the C-Br bond to the bromine, which leaves as (2).
Eduqas 20214 marksThe rates of hydrolysis of 1-chlorobutane, 1-bromobutane and 1-iodobutane were compared by warming each with aqueous silver nitrate in ethanol. (a) State the order of reactivity. (b) Explain this order in terms of bond enthalpy.Show worked answer →
(a) 1-iodobutane reacts fastest, then 1-bromobutane, then 1-chlorobutane (slowest) (1).
(b) The rate depends on the strength of the carbon-halogen bond (1). The C-I bond has the lowest bond enthalpy and breaks most easily, so iodoalkanes hydrolyse fastest; the C-Cl bond is the strongest and breaks least easily, so chloroalkanes are slowest (2). A precipitate (yellow for iodide, cream for bromide, white for chloride) forms with silver nitrate.
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Sources & how we know this
- WJEC Eduqas GCE A Level Chemistry specification (from 2015) — WJEC Eduqas (2015)