Name the two representations of the benzene molecule in common use.

The Kekulé structure and the delocalised ring structure.

Describe the Kekulé representation of benzene.

A flat hexagon of six carbon atoms, each carrying one hydrogen, with single and double bonds alternating round the ring.

Describe the delocalised representation of benzene.

A flat hexagon of six carbon atoms with a circle drawn inside it, the circle standing for the six delocalised electrons shared equally round the ring.

What do the two representations agree about concerning the shape of the molecule?

Both show a planar hexagon of carbon atoms with bond angles of 120°.

On what point about bonding do the two representations disagree?

The Kekulé structure predicts alternating short double bonds and longer single bonds, whereas the delocalised structure predicts six identical bonds.

OCR A-Level Chemistry B H433

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Representations of benzene and enthalpy of hydrogenation

3 things to know
Components 01, 02 and 03

Overview

Recall questions on the Kekulé and delocalised models, the equal bond lengths, and the hydrogenation enthalpy evidence for delocalisation.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

Two isolated double bonds would predict an enthalpy change of hydrogenation of what value for cyclohexa-1,3-diene, and what is found?

Key term

Why does the hydrogenation of benzene need more forcing conditions than that of cyclohexene?

Key term

What is formed when benzene is hydrogenated completely?

Questions from this subtopic

Each one with the model answer underneath.

1
Name the two representations of the benzene molecule in common use.
Model answer
The Kekulé structure and the delocalised ring structure.
2
Describe the Kekulé representation of benzene.
Model answer
A flat hexagon of six carbon atoms, each carrying one hydrogen, with single and double bonds alternating round the ring.
3
Describe the delocalised representation of benzene.
Model answer
A flat hexagon of six carbon atoms with a circle drawn inside it, the circle standing for the six delocalised electrons shared equally round the ring.
4
What systematic name corresponds to the Kekulé structure?
Model answer
Cyclohexa-1,3,5-triene.
5
What do the two representations agree about concerning the shape of the molecule?
Model answer
Both show a planar hexagon of carbon atoms with bond angles of 120°.
6
On what point about bonding do the two representations disagree?
Model answer
The Kekulé structure predicts alternating short double bonds and longer single bonds, whereas the delocalised structure predicts six identical bonds.

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A3 ⁴₂He → ¹²₆C.

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