Why is the carbon atom attached to the halogen in a halogenoalkane open to attack by a nucleophile?

The halogen is more electronegative than carbon, so it draws the bonding pair towards itself and leaves the carbon atom with a partial positive charge, making it electron-deficient and therefore attractive to a species carrying a lone pair.

What reagent and conditions convert a primary halogenoalkane into an alcohol?

The halogenoalkane is heated under reflux with aqueous sodium hydroxide or potassium hydroxide, usually with ethanol added so that the two layers mix.

Write the equation for the reaction of bromoethane with aqueous hydroxide ions, and check that it balances for charge.

The equation is CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻; the left-hand side carries a single negative charge and so does the right-hand side, so it balances for charge as well as for atoms.

Write the full equation for the reaction of 1-bromobutane with aqueous sodium hydroxide and name the organic product.

The equation is CH₃CH₂CH₂CH₂Br + NaOH → CH₃CH₂CH₂CH₂OH + NaBr, and the organic product is butan-1-ol.

Describe the mechanism for the reaction between a primary halogenoalkane and a hydroxide ion, saying where each curly arrow begins and ends.

The hydroxide ion attacks as a nucleophile: one curly arrow starts at a lone pair on its oxygen atom and ends at the δ+ carbon atom carrying the halogen, forming the new C–O bond. At the same time a second curly arrow starts at the C–Br bonding pair and ends on the bromine atom, so that bond breaks heterolytically and bromine leaves as a bromide ion carrying both electrons.

In the hydrolysis of a halogenoalkane, is the fission of the carbon–halogen bond homolytic or heterolytic?

It is heterolytic, because the halogen takes both of the shared electrons and departs as a negatively charged halide ion rather than as a neutral radical.

WJEC AS-Level Chemistry 1410QS

Formulae and Equations

Topic overview Charges on common ions Building formulae for ionic and molecular compounds Assigning oxidation numbers Using oxidation numbers to follow redox changes Balancing equations and using state symbols Ionic and half equations

Basic Ideas About Atoms

Topic overview Radioactive decay and nuclear equations Alpha, beta and gamma radiation Half-life, dating and the uses of radioisotopes: half-life of radioactive decay Half-life, dating and the uses of radioisotopes: adverse consequences for living cells Ionisation energies and electronic structure Orbitals and the filling of sub-shells Atomic spectra and the energy of light: atomic emission spectrum of the hydrogen atom Atomic spectra and the energy of light: relationship between energy and frequency Atomic spectra and the energy of light: significance of the frequency

Chemical Calculations

Topic overview Relative masses and the mass spectrometer: relative atomic Relative masses and the mass spectrometer: how a mass spectrometer works Relative masses and the mass spectrometer: molecular ion The mole, the Avogadro constant and molar mass: relative mass terms and simple mass spectra The mole, the Avogadro constant and molar mass: moles and percentage composition by mass Empirical and molecular formulae Concentration, solubility and titration calculations: concentration in mol dm-3 and in g dm-3 Concentration, solubility and titration calculations: stoichiometry and the limiting reagent Concentration, solubility and titration calculations: titration calculations and estimating a titre Gas volumes and the ideal gas equation Yield, atom economy and experimental uncertainty: concepts of atom economy and percentage yield Yield, atom economy and experimental uncertainty: how to estimate the percentage error

Bonding

Topic overview Ionic bonds and covalent bonds: ionic bonding in terms of ion formation Ionic bonds and covalent bonds: covalent bonding and forces Electronegativity, polarity and in-between bonds: concepts of electronegativity and bond polarity Electronegativity, polarity and in-between bonds: forces Forces between molecules Hydrogen bonding Shapes of molecules and ions: VSEPR principle and its use Shapes of molecules and ions: bond angles associated with linear

Solid Structures

Topic overview Ionic crystals: sodium chloride and caesium chloride Giant covalent structures: diamond and graphite Simple molecular solids: iodine and ice Metals and the electron sea Using structure to explain physical properties

The Periodic Table

Topic overview How the Periodic Table is organised Oxidation and reduction as electron transfer Trends across periods and down groups Group 2: reactions, trends and the reasons behind them — reactions with oxygen, water and dilute acid Group 2: reactions, trends and the reasons behind them — trends down the group Group 2: reactions, trends and the reasons behind them — thermal stability of the carbonates Group 2: reactions, trends and the reasons behind them — oxides and hydroxides Identifying ions, making salts and gravimetric analysis: identifying cations with sodium hydroxide Identifying ions, making salts and gravimetric analysis: preparing a soluble salt from an acid Group 7: the halogens — physical properties and the trend in volatility Group 7: the halogens — reactions of the halogens with metals Group 7: the halogens — displacement reactions and the trend Group 7: the halogens — halide ions as reducing agents, uses of chlorine and the chlorate equilibrium

Simple Equilibria and Acid-Base Reactions

Topic overview Reversible reactions and dynamic equilibrium Le Chatelier's principle and shifting the position of equilibrium The equilibrium constant Kc Acids, bases, salts and neutralisation pH, strong acids and weak acids Acid-base titrations and their calculations

Thermochemistry

Topic overview Enthalpy changes and their definitions Calculating enthalpy changes from experimental measurements Determining enthalpy changes in the laboratory Hess's law and energy cycles Average bond enthalpies

Rates of Reaction

Topic overview Collision theory and the conditions that change rate Measuring rate and relating it to concentration Energy profiles and activation energy Temperature and the Boltzmann distribution Catalysts and how they work Following reactions in the laboratory

The Wider Impact of Chemistry

Topic overview Social and economic consequences of chemical manufacture and energy supply Environmental consequences of chemical processes and energy production Green chemistry and sustainable practice

Organic Compounds

Topic overview Drawing and reading organic structures Naming organic compounds Chain length, functional groups and physical properties Structural isomerism Electrophiles, nucleophiles, radicals and bond fission

Hydrocarbons

Topic overview Alkane bonding, combustion and fossil fuels: combustion reaction of alkanes and benefits Alkane bonding, combustion and fossil fuels: combustion calculations and acid rain Radical substitution of alkanes The C=C bond and why alkenes are reactive E–Z isomerism in alkenes Electrophilic addition, Markovnikov's rule and testing for alkenes: mechanism of electrophilic addition Electrophilic addition, Markovnikov's rule and testing for alkenes: Markovnikov's rule and the bromine water test Electrophilic addition, Markovnikov's rule and testing for alkenes: testing for a C=C bond Hydrogenation and addition polymers: conditions required Hydrogenation and addition polymers: nature of addition polymerisation

Halogenoalkanes

Topic overview Nucleophilic substitution by hydroxide ions Elimination to form alkenes Reactivity of the carbon–halogen bond and the silver nitrate test Uses of halogenoalkanes, CFCs and the ozone layer: uses of halogenoalkanes and CFCs Uses of halogenoalkanes, CFCs and the ozone layer: radical mechanism Uses of halogenoalkanes, CFCs and the ozone layer: replacements for CFCs Heating under reflux in the laboratory

Alcohols and Carboxylic Acids

Topic overview Two routes to ethanol: hydration and fermentation — hydration of ethene Two routes to ethanol: hydration and fermentation — fermentation Two routes to ethanol: hydration and fermentation — comparing the two routes on rate Classifying alcohols and dehydrating them Oxidising alcohols to aldehydes, ketones and acids Chemical tests for alcohols and carboxylic acids Carboxylic acids, bases and salts Making an ester and separating it by distillation: esterification Making an ester and separating it by distillation: carrying out the preparation Making an ester and separating it by distillation: separating and purifying Making an ester and separating it by distillation: yield

Instrumental Analysis

Topic overview Reading a mass spectrum Infrared spectra and functional groups NMR spectra and atomic environments Putting the three techniques together

Practical Skills and Techniques

Topic overview Measuring mass, volume, time and temperature Standard solutions and titration technique Indicators and acid–base titration curves Heating, refluxing, distilling and purifying: Heating Heating, refluxing, distilling and purifying: purifying Melting temperature, chromatography and pH measurement Electrochemical cells, rate methods and working safely: Electrochemical cells Electrochemical cells, rate methods and working safely: rate methods and working safely

Planning, Analysis and Evaluation

Topic overview Planning an investigation and controlling variables Recording observations and presenting data Graphs, gradients and processing data Errors, uncertainty, accuracy and precision Evaluating a method and suggesting improvements Research and referencing

Core Experiments and Their Chemistry

Topic overview Gravimetric and acid–base analysis Enthalpy and rate experiments: Enthalpy Enthalpy and rate experiments: rate experiments Redox titrations and electrochemical cells Equilibrium constants and following a titration with a pH probe Organic synthesis, separation and identification: Organic synthesis Organic synthesis, separation and identification: identification tests and chromatography

Mathematical Skills in Chemistry

Topic overview Units, standard form and proportion in chemical calculation: volume, equilibrium, rate and energy units Units, standard form and proportion in chemical calculation: standard form Units, standard form and proportion in chemical calculation: proportion Units, standard form and proportion in chemical calculation: checking answers, buffers, rate constants and units Significant figures, means and uncertainty in experimental data: significant figures in multiplication Significant figures, means and uncertainty in experimental data: relative atomic mass as a weighted mean Significant figures, means and uncertainty in experimental data: absolute and percentage uncertainty Symbols, rearranging equations and logarithms: chemical symbols and what each one asserts Symbols, rearranging equations and logarithms: rearranging an equation Symbols, rearranging equations and logarithms: substituting into equations for Kc, enthalpy and pH Symbols, rearranging equations and logarithms: logarithms, the Arrhenius equation and rearranging formulae Graphs: plotting, gradients, intercepts and tangents — reading a spectrum Graphs: plotting, gradients, intercepts and tangents — reaction order from data and plotting good graphs Graphs: plotting, gradients, intercepts and tangents — Arrhenius plots, zero-order graphs and tangents Graphs: plotting, gradients, intercepts and tangents — initial rates from tangents and interpreting graphs Shape, symmetry and three-dimensional structure: Shape Shape, symmetry and three-dimensional structure: symmetry

Halogenoalkanes

7 subtopics
21 things to know
AS Unit 2

What Halogenoalkanes covers

Halogenoalkanes is examined on AS Unit 2 of WJEC Chemistry, Energy, Rate and Chemistry of Carbon Compounds. 103 recall questions across 7 subtopics.

Where it is assessed

AS Unit 2

Energy, Rate and Chemistry of Carbon Compounds.

Check the WJEC specification ↗

The 7 subtopics

Each one has its own page. WJEC AS Chemistry students sit this topic in the first year.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Which species is the nucleophile and which is the electrophilic centre when hydroxide ions react with bromoethane?

Key term

Why does the hydroxide ion attack the carbon atom rather than the bromine atom?

Key term

Water can also hydrolyse a halogenoalkane, but the reaction is much slower than with hydroxide ions. Why?

Key term

Why is ethanol rather than water used as the solvent when elimination is wanted?

Key term

Fluoroalkanes are in fact the least reactive halogenoalkanes towards hydroxide ions. Explain this apparent contradiction.

Key term

Why is ethanol added to the mixture in the silver nitrate test?

Key term

Why must sodium hydroxide not be used in place of water in the silver nitrate test?

Key term

How is ozone formed naturally in the stratosphere?

Key term

Why does a small quantity of CFC released at ground level have such a large effect?

Key term

Why does scientific evidence about ozone depletion require international rather than national action?

Key term

Why is a reaction such as the hydrolysis of a halogenoalkane heated under reflux rather than simply heated in an open flask?

Key term

When would you choose a water bath rather than a sand bath for heating a reaction mixture?

Key term

Why is ethanol added to a mixture of a halogenoalkane and aqueous sodium hydroxide before refluxing?

Key term

What reagent and conditions are needed to convert a halogenoalkane into an alkene?

Evaluation

When a CFC molecule absorbs ultraviolet radiation in the stratosphere, which bond breaks, and why that one?

Key term

Ozone and oxygen are both made only of oxygen atoms. Why does the overall conversion 2O₃ → 3O₂ matter so much?

Key term

Write an equation for the initial breakdown of CCl₂F₂ by ultraviolet radiation and state the type of bond fission.

Key term

What is an elimination reaction?

Key term

What is a CFC?

Evaluation

Why is the presence of C–H bonds in a halogenoalkane an advantage from an environmental point of view?

Key term

Suggest two non-halogenated substances that can be used as refrigerants in place of CFCs, and give a drawback of each.

Take Halogenoalkanes further

The same content, four other ways.

Questions from this topic

Each one with the model answer underneath.

1
Why is the carbon atom attached to the halogen in a halogenoalkane open to attack by a nucleophile?
Model answer
The halogen is more electronegative than carbon, so it draws the bonding pair towards itself and leaves the carbon atom with a partial positive charge, making it electron-deficient and therefore attractive to a species carrying a lone pair.
2
What reagent and conditions convert a primary halogenoalkane into an alcohol?
Model answer
The halogenoalkane is heated under reflux with aqueous sodium hydroxide or potassium hydroxide, usually with ethanol added so that the two layers mix.
3
Write the equation for the reaction of bromoethane with aqueous hydroxide ions, and check that it balances for charge.
Model answer
The equation is CH₃CH₂Br + OH⁻ → CH₃CH₂OH + Br⁻; the left-hand side carries a single negative charge and so does the right-hand side, so it balances for charge as well as for atoms.
4
Write the full equation for the reaction of 1-bromobutane with aqueous sodium hydroxide and name the organic product.
Model answer
The equation is CH₃CH₂CH₂CH₂Br + NaOH → CH₃CH₂CH₂CH₂OH + NaBr, and the organic product is butan-1-ol.
5
Describe the mechanism for the reaction between a primary halogenoalkane and a hydroxide ion, saying where each curly arrow begins and ends.
Model answer
The hydroxide ion attacks as a nucleophile: one curly arrow starts at a lone pair on its oxygen atom and ends at the δ+ carbon atom carrying the halogen, forming the new C–O bond. At the same time a second curly arrow starts at the C–Br bonding pair and ends on the bromine atom, so that bond breaks heterolytically and bromine leaves as a bromide ion carrying both electrons.
6
In the hydrolysis of a halogenoalkane, is the fission of the carbon–halogen bond homolytic or heterolytic?
Model answer
It is heterolytic, because the halogen takes both of the shared electrons and departs as a negatively charged halide ion rather than as a neutral radical.

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Halogenoalkanes revision questions

Is Halogenoalkanes examined at AS in WJEC Chemistry?: Yes. It sits in AS Unit 2, which you take in the first year, so it is on the AS paper as well as counting towards the full A level.
What does Halogenoalkanes cover in WJEC AS-Level Chemistry?: 7 subtopics: nucleophilic substitution by hydroxide ions, elimination to form alkenes, reactivity of the carbon–halogen bond and the silver nitrate test, uses of halogenoalkanes, cfcs and the ozone layer: uses of halogenoalkanes and cfcs, uses of halogenoalkanes, cfcs and the ozone layer: radical mechanism, uses of halogenoalkanes, cfcs and the ozone layer: replacements for cfcs and heating under reflux in the laboratory. Each one has its own page with questions and model answers.
How is Halogenoalkanes organised?: By the WJEC 1410QS specification, subtopic by subtopic, so it matches the order you are taught it in.

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