State the measured enthalpy of hydrogenation of benzene.

The measured value is −208 kJ mol⁻¹.

Calculate the delocalisation energy of benzene from those two values.

The difference between the expected −360 kJ mol⁻¹ and the actual −208 kJ mol⁻¹ is 152 kJ mol⁻¹, so benzene is 152 kJ mol⁻¹ more stable than the model with three separate double bonds.

Explain what the difference between the predicted and measured enthalpies of hydrogenation shows about benzene.

Less energy is released than predicted, so benzene starts from a lower energy than the model structure and is therefore more stable than three isolated double bonds would be.

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

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Intermolecular Forces and Structure

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Shapes of Molecules and Ions

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Redox

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The Halogens

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Acid-Base Titrations

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Qualitative Tests

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Formulae and Amounts of a Substance

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Nomenclature and Isomerism in Organic Chemistry

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Alkanes

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Alkenes

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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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Derivatives of Carboxylic Acids

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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

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Volumetric Analysis

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Transition Metals

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Electrode Potentials

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Amines

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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

Aromatic Chemistry

The stability of benzene compared with alkenes

3 things to know
Unit A2 1

Overview

Standard enthalpy of hydrogenation of cyclohexene, measured enthalpy of hydrogenation of benzene and why benzene undergoes substitution rather than addition.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

Compare the π electron density in benzene with that in an alkene.

Key term

State the standard enthalpy of hydrogenation of cyclohexene.

Numbers

Calculate the enthalpy of hydrogenation predicted for a benzene ring with three separate double bonds.

Questions from this subtopic

Each one with the model answer underneath.

1
State the standard enthalpy of hydrogenation of cyclohexene.
Model answer
The enthalpy of hydrogenation of cyclohexene is −120 kJ mol⁻¹.
2
Calculate the enthalpy of hydrogenation predicted for a benzene ring with three separate double bonds.
Model answer
Three times the value for one double bond gives 3 × −120 = −360 kJ mol⁻¹.
3
State the measured enthalpy of hydrogenation of benzene.
Model answer
The measured value is −208 kJ mol⁻¹.
4
Calculate the delocalisation energy of benzene from those two values.
Model answer
The difference between the expected −360 kJ mol⁻¹ and the actual −208 kJ mol⁻¹ is 152 kJ mol⁻¹, so benzene is 152 kJ mol⁻¹ more stable than the model with three separate double bonds.
5
Explain what the difference between the predicted and measured enthalpies of hydrogenation shows about benzene.
Model answer
Less energy is released than predicted, so benzene starts from a lower energy than the model structure and is therefore more stable than three isolated double bonds would be.
6
Explain why benzene undergoes substitution rather than addition.
Model answer
Addition would use up the delocalised π electrons and destroy the ring system, losing the extra stability, whereas substitution leaves the delocalised ring intact.

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The stability of benzene compared with alkenes

Overview

What you need to know

Compare the π electron density in benzene with that in an alkene.

State the standard enthalpy of hydrogenation of cyclohexene.

Calculate the enthalpy of hydrogenation predicted for a benzene ring with three separate double bonds.

Questions from this subtopic

State the standard enthalpy of hydrogenation of cyclohexene.

Calculate the enthalpy of hydrogenation predicted for a benzene ring with three separate double bonds.

State the measured enthalpy of hydrogenation of benzene.

Calculate the delocalisation energy of benzene from those two values.

Explain what the difference between the predicted and measured enthalpies of hydrogenation shows about benzene.

Explain why benzene undergoes substitution rather than addition.

Other ways to work this

What is inside Levo

The 3-step method, run for you

Cram a whole paper in one sitting

Blank Blitz makes facts automatic

Build It gets the exam shape right

Test It catches misconceptions

Brain Breaks between sessions

Every question in Aromatic Chemistry, on a schedule.