6.4.1

Structure & Reactivity (A2 Only)

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Structure and Reactivity of Aromatic Compounds

Benzene has a cyclic and planar structure of formula C6H6. Its reactivity is different from other unsaturated compounds.

The structure of benzene

The structure of benzene

  • Benzene has delocalised p-electrons.
    • This makes benzene aromatic and more stable than its non-delocalised equivalent cyclohexa-1,3,5-triene.
  • Benzene has a hexagonal planar structure with its p-orbitals above and below the plane of the ring.
  • To be aromatic, the p-orbitals must overlap. This is why the structure is planar - so the p-orbitals can overlap.
Structural representation

Structural representation

  • To represent the mobility of double bonds in benzene, we draw benzene with a ring (as shown above).
  • The skeletal structure is an average structure to show the double bonds moving around the whole ring.
Substitution reactions

Substitution reactions

  • Benzene's stable structure affects its reactivity.
  • Benzene reactions are typically substitution, NOT addition.
    • This is different from alkenes which normally undergo addition reactions.
    • Substitution is favoured because it re-establishes the original structure - and so retains the very stable aromatic structure.

Evidence for the Structure of Benzene

Many pieces of evidence have been used to prove the structure of benzene.

Structure of benzene

Structure of benzene

  • For years, chemists argued over what the best structure was to represent benzene.
Bond lengths

Bond lengths

  • All carbon-carbon bonds have been shown to be the same length.
    • This length (0.139 nm) is somewhere between the C-C and C=C bond lengths.
Bond angles

Bond angles

  • The structure has been proved to have a bond angle of 120o at each carbon.
    • This means the structure must be planar.
Enthalpy of hydrogenation

Enthalpy of hydrogenation

  • The enthalpy of hydrogenation for benzene is more positive than for cyclohexa-1,3,5-triene.
    • Hydrogenation is a form of addition reaction.
    • So this reaction breaks the aromaticity in the benzene ring, which is why it is unfavourable.
  • This reflects how benzene is more stable due to aromaticity.
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1

Physical Chemistry

1.1

Atomic Structure

1.1.1Fundamental Particles1.1.2Isotopes & Mass Number1.1.3Mass Spectrometry1.1.4Electron Shells, Sub-Shells & Orbitals1.1.5Electron Configuration1.1.6Ionisation Energy1.1.7Factors Affecting Ionisation Energies1.1.8Trends of Ionisation1.1.9Specific Impacts on Ionisation Energies1.1.10End of Topic Test - Atomic Structure1.1.11A-A* (AO3/4) - Atomic Structure
1.2

Amount of Substance

1.2.1Relative Masses1.2.2The Mole1.2.3The Ideal Gas Equation1.2.4Empirical & Molecular Formulae1.2.5Balanced Equations1.2.6Percentage Yield1.2.7A-A* (AO3/4) - Percentage Yield1.2.8Atom Economy1.2.9End of Topic Test - Amount of Substance1.2.10A-A* (AO3/4) - Substances & Yield1.2.11Diagnostic Misconceptions - Moles
1.3

Bonding

1.3.1Ionic Bonding1.3.2Covalent & Dative Bonding1.3.3Carbon Structures1.3.4Metallic Bonding1.3.5Physical Properties1.3.6Shapes of Molecules1.3.7Polarity1.3.8Intermolecular Forces1.3.9Intermolecular Forces 21.3.10End of Topic Test - Bonding1.3.11Exam-Style Question - Shape of Molecules1.3.12A-A* (AO3/4) - Bonding1.3.13Diagnostic Misconceptions - Ions1.3.14Diagnostic Misconceptions - Ionic & Covalent1.3.15Diagnostic Misconceptions - Phase Change1.3.16Diagnostic Misconceptions - Boiling1.3.17Diagnostic Misconceptions - Polar Bonds
1.4

Energetics

1.4.1Enthalpy Changes1.4.2Bond Breaking and Making1.4.3Enthalpy Changes: Standard Conditions1.4.4Calorimetry1.4.5Hess' Law1.4.6Bond Enthalpies1.4.7End of Topic Test - Energetics1.4.8Diagnostic Misconceptions - Calorimetry
1.5

Kinetics

1.5.1Collision Theory1.5.2Maxwell-Boltzmann Distribution1.5.3Increasing Rates1.5.4End of Topic Test - Kinetics
1.6

Equilibria

1.6.1Le Chatelier1.6.2Equilibrium Constants1.6.3A-A* (AO3/4) - Energetics, Kinetics, Equilibria1.6.4End of Topic Test - Equilibria1.6.5Diagnostic Misconceptions - Solids
1.7

Redox

1.7.1Redox1.7.2End of Topic Test - Redox
2

Physical Chemistry 2 (A2 Only)

2.1

Thermodynamics (A2 Only)

2.1.1Born-Haber Cycles (A2 Only)2.1.2Born-Haber Cycle Calculations (A2 Only)2.1.3Entropy (A2 Only)2.1.4Free Energy (A2 Only)2.1.5End of Topic Test - Thermodynamics2.1.6A-A* (AO3/4) - Thermodynamics
2.2

Rate Equations (A2 Only)

2.2.1Rate Equations (A2 Only)2.2.2Rate Equations 2 (A2 Only)2.2.3The Arrhenius Equation (A2 Only)2.2.4Experimental Methods for Rates (A2 Only)2.2.5End of Topic Test - Rate Equations2.2.6Exam-Style Question - Rates2.2.7A-A* (AO3/4) - Rate Equations
2.3

The Equilibrium Constant Kp (A2 Only)

2.3.1Properties of Kp (A2 Only)
2.4

Electrochemical Cells (A2 Only)

2.4.1Electrochemical Cells (A2 Only)2.4.2Electrochemical Series (A2 Only)2.4.3Commercial Application (A2 Only)2.4.4End of Topic Test - Kp & Electrochemistry2.4.5A-A* (AO3/4) - Electrochemical Cells2.4.6Diagnostic Misconceptions - Salt Bridges2.4.7Diagnostic Misconceptions - Electrode, Electrolyte
2.5

Acids & Bases (A2 Only)

2.5.1Brønsted-Lowry Acids & Bases (A2 Only)2.5.2pH (A2 Only)2.5.3The Ionic Product of Water (A2 Only)2.5.4Weak Acids & Bases (A2 Only)2.5.5pH Curves & Titrations (A2 Only)2.5.6pH Curves & Titrations 2 (A2 Only)2.5.7Buffer Solutions (A2 Only)2.5.8End of Topic Test - Acids & Bases2.5.9Exam-Style Question - Weak Acids2.5.10A-A* (AO3/4) - Acids & Bases2.5.11Diagnostic Misconceptions - Ammonia is an Alkali2.5.12Diagnostic Misconceptions - Water's Neutrality2.5.13Diagnostic Misconceptions - Concentrate & Strength
3

Inorganic Chemistry

3.1

Periodicity & Trends

3.1.1The Periodic Table3.1.2Trends in the Periodic Table3.1.3End of Topic Test - Periodicity & Trends3.1.4Diagnostic Misconceptions - Atomic Radius
3.2

Group 2

3.2.1Group 2 Chemistry3.2.2End of Topic Test - Group 2
3.3

Group 7

3.3.1Introduction to Halogens3.3.2Reactions of Halogens3.3.3Chlorine & Chlorate(I)3.3.4End of Topic Test - Group 73.3.5Exam-Style Question - Group 73.3.6A-A* (AO3/4) - Periodicity, Group 2 & 7
4

Inorganic Chemistry 2 (A2 Only)

4.1

Period 3 (A2 Only)

4.1.1Period 3 Elements & Oxides (A2 Only)4.1.2Period 3 Oxides (A2 Only)4.1.3End of Topic Test - Period 3
4.2

Transition Metals (A2 Only)

4.2.1General Properties (A2 Only)4.2.2Substitution Reactions (A2 Only)4.2.3Shapes of Complex Ions (A2 Only)4.2.4Colours of Ions (A2 Only)4.2.5Variable Oxidation States (A2 Only)4.2.6Titrations (A2 Only)4.2.7Homogeneous Catalysts (A2 Only)4.2.8Heterogeneous Catalysts (A2 Only)4.2.9End of Topic Test - Transition Metals4.2.10A-A* (AO3/4) - Transition Metals
4.3

Reactions of Ions in Aqueous Solutions (A2 Only)

4.3.1Acidity (A2 Only)4.3.2Identification (A2 Only)4.3.3End of Topic Test - Reactions of Ions in Solution
5

Organic Chemistry 1

5.1

Introduction

5.1.1Naming Conventions5.1.2Mechanism5.1.3Isomerism5.1.4End of Topic Test - Introduction to Organic Chem
5.2

Alkanes

5.2.1Fractional Distillation5.2.2Cracking5.2.3Combustion5.2.4Chlorination5.2.5End of Topic Test - Alkanes
5.3

Halogenoalkanes

5.3.1Substitution Reactions5.3.2Elimination Reactions5.3.3Ozone Depletion5.3.4End of Topic Test - Halogenoalkanes
5.4

Alkenes

5.4.1Introduction to Alkenes5.4.2Reactions of Alkenes5.4.3Polymerisation Reactions5.4.4Properties of Polymers5.4.5End of Topic Test - Alkenes
5.5

Alcohols

5.5.1Production of Alcohols5.5.2Oxidation of Alcohols5.5.3Oxidation of Alcohols - 25.5.4Elimination Reactions5.5.5End of Topic Test - Alcohols5.5.6Exam-Style Question - Alcohols
5.6

Organic Analysis

5.6.1Reactions5.6.2Reactions 25.6.3Mass Spectrometry5.6.4Infrared Spectroscopy5.6.5End of Topic Test - Organic Analysis
5.7

A-A* (AO3/4) - Organic 1

5.7.1A-A* (AO3/4) - Organic 1
6

Organic Chemistry 2 (A2 Only)

6.1

Optical Isomerism (A2 Only)

6.1.1Optical Isomerism (A2 Only)6.1.2End of Topic Test - Optical Isomerism
6.2

Aldehydes & Ketones (A2 Only)

6.2.1Aldehydes & Ketones (A2 Only)6.2.2End of Topic Test - Aldehydes & Ketones
6.3

Carboxylic Acids & Esters (A2 Only)

6.3.1Acids & Esters (A2 Only)6.3.2Acylation (A2 Only)6.3.3End of Topic Test - Carboxylic Acids & Esters
6.4

Aromatic Chemistry (A2 Only)

6.4.1Structure & Reactivity (A2 Only)6.4.2Reactions of Benzene (A2 Only)6.4.3End of Topic Test - Aromatic Chemistry
6.5

Amines (A2 Only)

6.5.1Nature & Preparation of Amines (A2 Only)6.5.2Basicity & Nucleophilicity (A2 Only)6.5.3End of Topic Test - Amines
6.6

Polymers (A2 Only)

6.6.1Condensation Polymers (A2 Only)6.6.2Biodegradability (A2 Only)
6.7

Biological Organic (A2 Only)

6.7.1Amino Acids (A2 Only)6.7.2Proteins (A2 Only)6.7.3DNA (A2 Only)6.7.4Anti-Cancer Drugs (A2 Only)6.7.5End of Topic Test - Biological Organic
6.8

Organic Synthesis (A2 Only)

6.8.1Organic Synthesis (A2 Only)6.8.2Examples of Synthetic Routes (A2 Only)6.8.3End of Topic Test - Polymers & Organic Synthesis
6.9

NMR Spectroscopy (A2 Only)

6.9.1Understanding NMR (A2 Only)6.9.2Carbon-13 NMR (A2 Only)6.9.3Hydrogen-1 NMR (A2 Only)6.9.4Hydrogen-1 NMR 2 (A2 Only)6.9.5End of Topic Test - NMR
6.10

Chromatography (A2 Only)

6.10.1Overview of Chromatography (A2 Only)6.10.2Thin-Layer Chromatography (A2 Only)6.10.3Column Chromatography (A2 Only)6.10.4Gas Chromatography (A2 Only)6.10.5End of Topic Test - Chromatography
6.11

A-A* (AO3/4) - Organic 2

6.11.1A-A* (AO3/4) - Organic 2

Practice questions on Structure & Reactivity (A2 Only)

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  1. 1
    Why is benzene planar?Multiple choice
  2. 2
    Where do the p-orbitals in benzene lie?Multiple choice
  3. 3
    The bond length in benzene is:Fill in the list
  4. 4
    The hydrogenation reaction on benzene:Fill in the list
  5. 5
    What are the three pieces of evidence for the structure of benzene?True / false
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End of Topic Test - Carboxylic Acids & Esters

Reactions of Benzene (A2 Only)