Define the term electrophile.

An electrophile is a species that is attracted to a region of high electron density and that accepts a pair of electrons to form a new covalent bond.

Give two examples of electrophiles that attack alkenes.

The δ+ hydrogen atom of a hydrogen halide and the δ+ bromine atom of a polarised bromine molecule are both electrophiles that attack alkenes.

Must an electrophile carry a positive charge?

No, an electrophile need only be electron deficient, so a neutral molecule carrying a permanent or an induced dipole can act as one.

Describe what the first curly arrow represents in the addition of hydrogen bromide to ethene.

It runs from the carbon–carbon double bond to the hydrogen atom of the hydrogen bromide and represents the pair of pi electrons moving out to form a new bond to that δ+ hydrogen.

Describe what the second curly arrow represents in the first step of that addition.

It runs from the middle of the H–Br bond to the bromine atom and represents both of the bonding electrons moving onto bromine, which is heterolytic fission and leaves a bromide ion.

CCEA A-Level Chemistry 1110

Formulae, Equations and Amounts of Substance

Topic overview Ionic charges and the formulae of compound ions Writing and balancing full and ionic equations The mole, molar mass and Avogadro's constant Reacting masses and reactants in excess Hydrated salts and water of crystallisation

Atomic Structure

Topic overview Protons, neutrons and electrons Atomic number, mass number and the composition of ions Isotopes and relative isotopic mass Relative atomic, molecular and formula mass Mass spectra and relative atomic mass calculations Mass spectra of diatomic elements Electronic configuration and the building up principle Orbitals, electron spin and orbital shapes The s-, p-, d- and f-blocks of the Periodic Table Ionisation energies and their equations Trends in ionisation energy as evidence for shells and subshells

Bonding

Topic overview Ionic bonding and electron transfer Dot and cross diagrams for ionic compounds Covalent bonding and the shared pair Lone pairs and co-ordinate bonding Dot and cross diagrams for covalent molecules and ions The octet rule and its limitations Electronegativity and bond polarity Metallic bonding

Intermolecular Forces and Structure

Topic overview Van der Waals' forces and permanent dipole-dipole attractions Hydrogen bonding, boiling points and solubility Giant ionic, metallic and giant covalent structures Molecular covalent structures and melting points across Period 3

Shapes of Molecules and Ions

Topic overview Electron pair repulsion and VSEPR theory Shapes and bond angles up to four electron pairs Shapes with five and six electron pairs Polar bonds and polar molecules

Redox

Topic overview Rules for assigning oxidation states Oxidation states in peroxides, hydrides and Roman numeral names Oxidation, reduction and electron transfer Oxidising agents, reducing agents and disproportionation Half-equations and combining them into redox equations

The Halogens

Topic overview Colours, physical states and solubility of the halogens Chlorine with water and the treatment of drinking water Disproportionation of the halogens with sodium hydroxide Oxidising ability and displacement reactions Solid halides with concentrated sulfuric and phosphoric acid

Acid-Base Titrations

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Qualitative Tests

Topic overview Chemical tests for the gases Flame tests for metal cations Testing for the ammonium ion, sulfate and carbonate Distinguishing the halides, and the starch test for iodine

Formulae and Amounts of a Substance

Topic overview Empirical and molecular formulae Calculating formulae from composition data Molar gas volume and reacting gas volumes Percentage yield Atom economy

Nomenclature and Isomerism in Organic Chemistry

Topic overview Homologous series and functional groups IUPAC rules for naming organic compounds Structural, displayed and skeletal formulae Structural isomerism Geometrical isomerism and E–Z notation

Alkanes

Topic overview Alkanes as saturated hydrocarbons Boiling points of alkanes and van der Waals' forces Boiling points of branched isomers Complete and incomplete combustion of alkanes Pollutants from alkane fuels and the atmosphere Catalytic converters Radicals, homolytic fission and heterolytic fission Free radical substitution of alkanes

Alkenes

Topic overview Unsaturated hydrocarbons and the bromine water test Sigma and pi bonds in the carbon–carbon double bond Electron density at C=C, reactivity and catalytic hydrogenation Addition of chlorine and bromine to alkenes Addition of hydrogen chloride and hydrogen bromide Electrophiles, heterolytic fission and the addition mechanism Carbocation stability and Markovnikov addition Addition polymerisation of alkenes

Halogenoalkanes

Topic overview Classification and boiling points of halogenoalkanes Laboratory preparation of a halogenoalkane from an alcohol Nucleophiles and hydrolysis with aqueous alkali Reactions with ammonia and with potassium cyanide Relative rates of hydrolysis and carbon–halogen bond enthalpy Elimination using ethanolic potassium hydroxide CFCs and the ozone layer

Alcohols

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Equilibrium and Acid-Base Equilibria

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Isomerism

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Aldehydes and Ketones

Topic overview Structure and naming of aldehydes and ketones Boiling points and solubility of carbonyl compounds Preparation from primary and secondary alcohols Reaction with hydrogen cyanide and the nucleophilic addition mechanism Racemic mixtures from nucleophilic addition 2,4-dinitrophenylhydrazine and the identification of carbonyls Distinguishing aldehydes from ketones, and reduction with lithal

Carboxylic Acids

Topic overview Structure, naming and physical properties of carboxylic acids Preparation from primary alcohols and aldehydes Formation by hydrolysis of esters and nitriles Salt formation with carbonates, hydroxides and ammonia Reactions with alcohols, phosphorus pentachloride and lithal

Derivatives of Carboxylic Acids

Topic overview Esters and acyl chlorides: structure and naming Boiling points and solubility of monoesters Making esters from alcohols with carboxylic acids or acyl chlorides The laboratory preparation of a liquid ester Fats and oils as esters of glycerol Transesterification, biodiesel and margarine Reactions of acyl chlorides with water and alcohols

Aromatic Chemistry

Topic overview The structure of benzene and delocalised π electrons The stability of benzene compared with alkenes Electrophilic substitution: bromination and nitration of benzene Friedel–Crafts reactions and the nitration of methyl benzoate

Spectroscopy and Chromatography

Topic overview The mass spectrum: molecular ion and base peak Fragmentation and the formulae of fragment ions High resolution mass spectrometry Nuclear magnetic resonance and the TMS standard Chemically equivalent hydrogens and chemical shift Integration curves Spin–spin splitting and the n+1 rule Deducing a structure from an nmr spectrum Paper and thin-layer chromatography Gas–liquid chromatography

Volumetric Analysis

Topic overview Iodine–thiosulfate titrations Manganate(VII) titrations Deducing titration equations from half-equations Back titration

Transition Metals

Topic overview The characteristic properties of the transition metals Electronic configurations of the metals and their ions Complexes, ligands and coordination number Oxidation number, redox and disproportionation Monodentate, bidentate and polydentate ligands Ligand strength and the entropy of ligand replacement Ligand replacement in copper(II) complexes The colours of the aqueous transition metal ions Hydroxide precipitates with sodium hydroxide Precipitates with aqueous ammonia and their dissolution Vanadium and the interconversion of its oxidation states

Electrode Potentials

Topic overview Standard electrode potential Types of half-cell Cell representation and calculating emf Non-rechargeable, rechargeable and fuel cells The hydrogen–oxygen fuel cell Electrode potentials and reactivity

Amines

Topic overview Structure and classification of amines Physical properties of amines Preparing aliphatic and aromatic amines Amines as bases Reaction with ethanoyl chloride and identifying amines Diazonium salts and coupling Colour and delocalisation

Amides and Polymers

Topic overview The amide group and its preparation Reactions of amides The basicity of amides Condensation polymerisation Polyesters and polyamides in use Comparing addition and condensation polymers

Amino Acids and Proteins

Topic overview The structure of amino acids Optical activity Zwitterions and physical properties Reactions of amino acids Peptides and protein structure Enzymes as catalysts Temperature, pH and enzyme activity Enzymes in analysis and industry

Chemistry in Medicine

Topic overview Indigestion remedies Analysing an indigestion remedy by back titration Treating the skin Silver nitrate and cisplatin Aspirin: synthesis and analysis Chromatography in identifying drugs Haemoglobin, carbon monoxide and edta Drugs, testing and judgement

Alkenes

Electrophiles, heterolytic fission and the addition mechanism

3 things to know
Unit AS 2

Overview

Term electrophile, electrophiles that attack alkenes and mechanism by which bromine adds to ethene.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Process

Compare the overall charge of the reactants with that of the species present at the intermediate stage of the electrophilic addition of hydrogen bromide to ethene

Key term

Why does hydrogen bromide add to an alkene by heterolytic rather than homolytic fission?

Key term

What intermediate is formed in the electrophilic addition of hydrogen bromide to an alkene?

Questions from this subtopic

Each one with the model answer underneath.

1
Define the term electrophile.
Model answer
An electrophile is a species that is attracted to a region of high electron density and that accepts a pair of electrons to form a new covalent bond.
2
Why does hydrogen bromide add to an alkene by heterolytic rather than homolytic fission?
Model answer
The hydrogen to bromine bond is already polar and the double bond is a region of high electron density, so the bond breaks unevenly with both electrons going to the bromine.
3
Give two examples of electrophiles that attack alkenes.
Model answer
The δ+ hydrogen atom of a hydrogen halide and the δ+ bromine atom of a polarised bromine molecule are both electrophiles that attack alkenes.
4
Must an electrophile carry a positive charge?
Model answer
No, an electrophile need only be electron deficient, so a neutral molecule carrying a permanent or an induced dipole can act as one.
5
Describe what the first curly arrow represents in the addition of hydrogen bromide to ethene.
Model answer
It runs from the carbon–carbon double bond to the hydrogen atom of the hydrogen bromide and represents the pair of pi electrons moving out to form a new bond to that δ+ hydrogen.
6
Describe what the second curly arrow represents in the first step of that addition.
Model answer
It runs from the middle of the H–Br bond to the bromine atom and represents both of the bonding electrons moving onto bromine, which is heterolytic fission and leaves a bromide ion.

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