Why do haloalkanes react with nucleophiles, and how does the carbon-halogen bond control the rate?
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.
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What this topic is asking
OCR specification point 4.2.2 wants you to explain the polarity of the carbon-halogen bond, describe and give the mechanism for nucleophilic substitution (with hydroxide, cyanide and ammonia), explain the trend in hydrolysis rate down the halogens using bond enthalpy, describe elimination to alkenes, and outline how chlorofluorocarbons (CFCs) deplete the ozone layer. Haloalkanes are a versatile springboard for building larger molecules.
The carbon-halogen bond
Nucleophilic substitution
The three reactions OCR expects:
- With aqueous hydroxide (warm ): gives an alcohol (hydrolysis).
- With cyanide (ethanolic , reflux): gives a nitrile, adding one carbon to the chain (useful in synthesis).
- With ammonia (excess ethanolic , sealed tube): gives an amine.
The hydrolysis-rate trend
Elimination
CFCs and ozone
Examples in context
Example 1. The Montreal Protocol. Recognising that CFC-derived chlorine radicals were thinning the ozone layer led to a global ban on CFCs, a textbook case of radical chain chemistry driving environmental policy.
Example 2. Extending a carbon chain. Converting a haloalkane to a nitrile with cyanide, then hydrolysing the nitrile to a carboxylic acid, is a standard way to lengthen a carbon chain by one carbon, a key tactic in the synthesis questions of Modules 4 and 6.
Try this
Q1. Name the organic product when 1-bromopropane is warmed with aqueous potassium hydroxide. [1 mark]
- Cue. Propan-1-ol (nucleophilic substitution).
Q2. State and explain which of 1-chlorobutane and 1-iodobutane is hydrolysed faster. [2 marks]
- Cue. 1-iodobutane, because the bond is weaker (lower bond enthalpy) than and so breaks more easily.
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 20194 marks1-bromopropane reacts with warm aqueous sodium hydroxide. (a) Name the mechanism and the organic product. (b) Outline the mechanism, showing the curly arrows.Show worked answer →
(a) Nucleophilic substitution; the product is propan-1-ol (1).
(b) The bond is polar ( and ). A lone pair on the hydroxide ion attacks the carbon (curly arrow from to the carbon) (1), and at the same time the bond breaks heterolytically (curly arrow from the bond to Br) (1), releasing a bromide ion and forming propan-1-ol (1).
Markers reward the mechanism name and product, the curly arrow from the nucleophile to the polar carbon, and the curly arrow showing heterolytic loss of the halide.
OCR 20213 marksThree haloalkanes (1-chlorobutane, 1-bromobutane and 1-iodobutane) are warmed with aqueous silver nitrate in ethanol. State and explain the order in which precipitates appear.Show worked answer →
The iodoalkane reacts fastest (precipitate appears first), then the bromoalkane, then the chloroalkane (slowest) (1).
The rate of hydrolysis depends on the carbon-halogen bond enthalpy: is the weakest bond and the strongest (1), so breaks most easily, releasing iodide ions soonest to form the silver halide precipitate (1).
Markers reward the correct order (iodo fastest), the link to bond enthalpy ( weakest), and the connection between bond breaking and precipitate formation.
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.
- 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.
- 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.
- Group 2 reactivity and reducing power, reactions of Group 2 elements and their oxides and hydroxides, the halogens as oxidising agents, halide displacement, disproportionation of chlorine, and tests for halide ions.
An OCR H432 module 3 answer on Group 2 and the halogens: reactivity trends, reactions with water and oxygen, halogen displacement, disproportionation of chlorine, and halide ion tests.
Sources & how we know this
- OCR A-Level Chemistry A (H432) specification — OCR (2015)