1.11Unit 1. Structure, Bonding and Introduction to Organic Chemistry

Alkenes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

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1.11 Alkenes — bonding, isomerism, addition reactions and polymers — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Describe bonding in alkenes, deduce E/Z isomers, explain electrophilic addition and the bromine test, and work with addition polymers and their disposal.Alkenes are unsatura…

Sigma (σ) bond
Pi (π) bond
Addition polymerisation
Repeat unit
E/Z isomerism
Electrophile

Part 1 of 10Alkenes and the C=C bond

Alkenes and the C=C bond

Alkenes are unsaturated hydrocarbons with general formula C_nH_2n and a C=C functional group, which lets them react in ways alkanes cannot.

Named alk + ene.

For chains of four or more carbons the position of C=C is given by numbering from the end nearest the double bond.
The lowest-numbered carbon of the double bond is written before -ene, e.g. but-1-ene.

Part 2 of 10Sigma and pi bonds in ethene

Sigma (σ) bond

Sigma (σ) bond end-on overlap of orbitals, with electron density symmetrical about the line joining the nuclei.
s orbitals and p orbitals can both overlap this way.

Pi (π) bond

Pi (π) bond sideways overlap of adjacent p orbitals, with lobes above and below the plane of the σ bond.
The two clouds together form one bond of two electrons.

In ethene each carbon uses three of its four outer electrons for σ bonds.

Two σ bonds go to hydrogens and one σ bond goes to the other carbon.
The fourth electron is in a p orbital that overlaps sideways to form the π bond.
The C=C is therefore one σ bond and one π bond.

Part 3 of 10E/Z and cis/trans isomerism

E/Z isomerism is stereoisomerism in which different atoms or groups are attached to each carbon of the C=C.

The π bond prevents rotation about C=C, so the groups stay fixed in position.

In saturated compounds there is free rotation about C-C.

cis/trans is a special case in which two groups are the same.

Cis

Cis the two groups are on the same side of the double bond.

Trans

Trans the two groups are on opposite sides of the double bond.

Each carbon of the C=C must carry two different groups.

2-methylpropene has no cis/trans isomers because both methyl groups are on one carbon.
But-2-ene and 1-chloroprop-1-ene do have them.

cis/trans fails when the groups on the double bond are all different, so E/Z must be used.

Part 4 of 10Naming E/Z isomers with CIP rules

Rank the two groups on each carbon by the atomic number of the first atom attached; the higher atomic number has priority.

If the first atoms are the same, compare the second atoms (ignore identical hydrogens) until a difference is found.

Z (zusammen = together) has the two higher-priority groups on the same side of C=C.

E (entgegen = opposite) has the two higher-priority groups on opposite sides.

Naming E/Z isomers with CIP rules

In 1-bromo-1-propen-2-ol, Br has priority over H and O has priority over C.

In CH₂Br versus CH₃CH₂ the second atom Br beats C, and in CH₂OH versus CH₃ the second atom O beats H.

Part 5 of 10Reactions of alkenes

The C=C has high electron density (four electrons), so it is attacked by electrophiles.

Electrophilic addition

Electrophilic addition an electrophile adds across C=C, the π bond breaks and a new single bond forms on each carbon.

Addition reactions of alkenes

Hydrogen (hydrogenation) with Ni catalyst gives the alkane.
Steam with phosphoric acid catalyst gives an alcohol.
Hydrogen halides, HX, give halogenoalkanes.
Halogens give dihalogenoalkanes.

Oxidation

Oxidation cold dilute acidified KMnO₄ turns from purple to colourless and forms a diol.
KMnO₄ supplies one oxygen atom and water supplies another oxygen and two hydrogens, so two OH groups add across C=C.
Alkanes are not oxidised, so this also distinguishes alkenes from alkanes.

Part 6 of 10Saturation test

Saturation test

Shake the unknown compound with bromine water, Br₂(aq), an orange or yellow solution.

An unsaturated compound reacts by addition and decolourises the bromine water.

A saturated compound such as an alkane leaves it orange in the dark.

Part 7 of 10Electrophilic addition mechanism

Electrophilic addition mechanism

HBr is polar, with H δ+ and Br δ−.

The π electrons attack the H atom, which acts as an electrophile.
The H-Br bond breaks heterolytically, forming Br⁻ and a carbocation intermediate.
Br⁻, a nucleophile, attacks the carbocation to give the bromoalkane.

Br₂ is non-polar but is polarised when it approaches the double bond.

The electron density of C=C repels the Br-Br electrons, so the nearer Br is δ+ and the further Br is δ−.
The nearer Br acts as the electrophile, the bond breaks heterolytically and Br⁻ attacks the carbocation to give 1,2-dibromoethane.

Unsymmetrical alkenes give a major and a minor product.

Carbocation stability is tertiary > secondary > primary.
Propene and HBr give mainly 2-bromopropane (secondary carbocation) and a little 1-bromopropane.

Part 8 of 10Addition polymerisation

Addition polymerisation

Addition polymerisation many monomers containing at least one C=C join to form long chains, with the polymer as the only product.

The π bond of each monomer breaks and new C-C single bonds link the monomers.

Polymer

Polymer a long-chain molecule made of many repeating units.

Monomer

Monomer a small reactive molecule that joins with others to form a polymer.

Poly(ethene) and poly(chloroethene), PVC, are made from ethene and chloroethene.

Repeat unit

Repeat unit the smallest group of atoms that repeats along the chain, drawn in square brackets.
Its main chain is the monomer with C=C changed to C-C.
Ethenol, CH(OH)=CH₂, gives -CH(OH)-CH₂-.
To find the monomer, restore the C=C between the two main-chain carbons.

Part 9 of 10Polymer disposal

Addition polymers are non-biodegradable, so microorganisms cannot break them down.

Landfill

Takes up valuable land and plastics last for hundreds of years, so sites fill quickly.

Incineration

Releases a lot of heat energy but produces CO₂, a greenhouse gas.
PVC releases toxic HCl, which can be removed by scrubbing the waste gas with a base such as CaO: CaO + 2HCl → CaCl₂ + H₂O.

Recycling

Reduces landfill and incineration and conserves crude oil, since recycling needs less energy than making new plastic.

Biodegradable polymers

Polyesters and polyamides can be broken down by hydrolysis and microorganisms into carbon dioxide, water and other organic compounds.

Compostable polymers

Plant-based, e.g. starch bin liners and sugar-cane-fibre plates, and degrade leaving no harmful residues.

Part 10 of 10More from the specification

Spectroscopic structural identification
- analyzing infrared absorption frequencies provides an analytical means to detect carbonyl and hydroxyl functional groups.

Key terms

Term	Meaning
Alkene	An unsaturated hydrocarbon with a C=C bond, general formula CnH2n.
σ bond	A bond formed by end-on overlap of orbitals along the line between the nuclei.
π bond	A bond formed by sideways overlap of p orbitals above and below the plane of the nuclei.
E/Z isomerism	Stereoisomerism caused by restricted rotation about C=C with different groups on each carbon.
Electrophile	A species that accepts a pair of electrons to form a covalent bond.
Carbocation	An ion with a positively charged carbon, formed as an intermediate.
Monomer	A small reactive molecule that joins with others to form a polymer.
Repeat unit	The smallest group of atoms that repeats along a polymer chain.

Avoid these mistakes

Thinking C=C is two σ bonds — it is one σ bond and one π bond.

Using cis/trans when all four groups on the double bond are different — use E/Z.

Assuming Z means the same as cis in every case — Z means the higher-priority groups are on the same side.

Ignoring carbocation stability in unsymmetrical additions — the major product comes from the more stable carbocation.

Giving the monomer as the repeat unit — the monomer has C=C where the repeat unit has C-C.

Last updated: 18 October 2026