What does unsaturated mean?

A hydrocarbon containing a C=C double bond, so it has fewer hydrogen atoms than the corresponding alkane.

Formula of a cycloalkene with one C=C

CₙH₂ₙ₋₂, e.g. cyclohexene C₆H₁₀.

What is a sigma bond?

A covalent bond formed by end-on overlap of orbitals along the bond axis.

A covalent bond formed by sideways overlap of p orbitals, with electron density above and below the plane.

Why is rotation about C=C restricted?

Rotating one end would break the sideways overlap of the pi bond.

Two conditions for geometric isomerism

A C=C bond (restricted rotation) and two different groups on each carbon of the C=C.

Why does but-1-ene have no geometric isomers?

C1 carries two identical hydrogen atoms, so there is only one arrangement.

What do cis and trans mean?

Cis: matching groups on the same side; trans: matching groups on opposite sides.

When does cis/trans naming break down?

When all four groups on the C=C are different, so there are no matching groups to compare.

How are priorities decided in E-Z?

By the atomic number of the atom attached to the double bond; higher atomic number has higher priority.

The higher-priority groups are on the same side of the double bond.

The higher-priority groups are on opposite sides of the double bond.

Alkenes and geometric isomerismEdexcel International A Level Chemistry: Flashcards

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General formula of alkenes

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General formula of alkenes: CₙH₂ₙ
What does unsaturated mean?: A hydrocarbon containing a C=C double bond, so it has fewer hydrogen atoms than the corresponding alkane.
Formula of a cycloalkene with one C=C: CₙH₂ₙ₋₂, e.g. cyclohexene C₆H₁₀.
What is a sigma bond?: A covalent bond formed by end-on overlap of orbitals along the bond axis.
What is a pi bond?: A covalent bond formed by sideways overlap of p orbitals, with electron density above and below the plane.
Why is rotation about C=C restricted?: Rotating one end would break the sideways overlap of the pi bond.
Two conditions for geometric isomerism: A C=C bond (restricted rotation) and two different groups on each carbon of the C=C.
Why does but-1-ene have no geometric isomers?: C1 carries two identical hydrogen atoms, so there is only one arrangement.
What do cis and trans mean?: Cis: matching groups on the same side; trans: matching groups on opposite sides.
When does cis/trans naming break down?: When all four groups on the C=C are different, so there are no matching groups to compare.
How are priorities decided in E-Z?: By the atomic number of the atom attached to the double bond; higher atomic number has higher priority.
What does Z mean?: The higher-priority groups are on the same side of the double bond.
What does E mean?: The higher-priority groups are on opposite sides of the double bond.
Priority order of Br, Cl, CH₃ and H: Br > Cl > CH₃ > H

Exam questions on Alkenes and geometric isomerism

A student studies hydrocarbons with a C=C double bond and builds models of several of them to compare their structures.
Describe how the carbon–carbon double bond in an alkene is made up of a sigma bond and a pi bond.2 marks
A chemist is explaining why some alkenes exist as pairs of isomers that cannot be interconverted at room temperature, and how these isomers are named.
Explain why pent-1-ene does not show geometric isomerism.2 marks
In an A Level class, but-2-ene and a halogenated alkene are used to illustrate geometric isomerism and the E-Z naming system.
But-2-ene, CH₃CH=CHCH₃, shows geometric isomerism. Explain why, and name the two isomers using the E-Z system.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).