Colourful Solutions > The covalent model > Aromatic compounds

The Mad Science Lab

Higher-level only

Compounds with benzene rings are known as aromatic. In reality there are other compounds as well as benzene derivatives that are aromatic, however at this level benzene serves as a good example of many of the bonding concepts covered.

Syllabus ref: S2.2.12

Structure 2.2.12 - Benzene, C6H6, is an important example of a molecule that has resonance. (HL)

  • Discuss the structure of benzene from physical and chemical evidence.

Guidance

Tools and links

  • Reactivity 2.1, 2.2 - How does the resonance energy in benzene explain its relative unreactivity?
  • Reactivity 3.4 - What are the structural features of benzene that favour it undergoing electrophilic substitution reactions?
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Benzene

Benzene provides a good case study which examplified all of the preceding theories of bonding.

Benzene consists of six carbon atoms in a hexagonal ring, all of which are sp2 hybridised. This allows the perpendicularly oriented 'p' orbital of each carbon to overlap laterally all around the ring. The delocalised pi system produced leads to much greater stability than would be obtained by alternate double and single bonds.

Benzene has carbon atoms with perpendicular 'p' orbitals
'p' orbital overlap resulting in delocalisation

For many years there was doubt about the actual structure of benzene. It was possible to draw the structure with alternate double and single bonds, now known as a Kekule structure. Resonance ideas were evoked to suggest that there were two resonance Kekule forms that interchanged rapidly.

However, gradually evidence emerged that the actual structure of benzene involved delocalisation of electrons throughout the ring. Nowadays, the Kekule forms are used for mechanisms or simplicity when it is convenient to 'tie down' the electrons.

Benzene delocalisation

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Evidence for the ring structure of benzene

1. Energetics

Comparison of the addition reaction of cyclohexene and benzene with hydrogen. If benzene were simply three alternate double and single bonds (tri-ene) then hydrogenation of these three double bonds should release three times as much energy as hydrogenation of the one double bond in cyclohexene.

In fact, hydrogenation of benzene releases far less energy, suggesting that benzene's structure is much more stable than that of a tri-ene.

Benzene delocalisation energy

2. Bond characteristics

Other evidence for the delocalisation of electrons in benzene emerges from consideration of the bond lengths. If benzene were to consist of alternate double and single bonds, then these would be different lengths. However, studies show that all of the benzene C-C bonds are the same length, intermediate between the lengths of C-C single and double bonds.

This is evidence that all of the carbon-carbon bonds in benzene are identical in terms of electron density.

The bond order could be described as 1.5, i.e. one single + one half bond.

3. Isomerism

Absence of the existence of 1,2 disubstituted isomers in benzene compounds provides more evidence for a delocalised ring system.

For example, if benzene were to have an alternating double and single bond structure, 1,2 dichlorobenzene could exist with a double bond, or a single bond between the carbon atoms holding the chlorine atoms.

The fact that only one form of 1,2 dichlorobenzene can be prepared indicates that all of the C-C bonds are equivalent.

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Worked examples

Q228-01 The delocalisation of electrons is most likely to be significant in:
  1. CO2
  2. SO2
  3. HCOOH
  4. TiO2
Answer

The conditions necessary for delocalisation are parallel 'p' orbitals next to one another. In CO2 the two pi systems have to be at right angles to one another, preventing orbital overlap.

However, in SO2 the central sulfur atom is sp2 hybridised and two resonance forms can be drawn, indicating that the electrons are delocalised over the two oxygen atoms and the central sulfur atom.
Q228-02 What is the best description of the carbon-oxygen bond lengths in CO32-?
  1. One short and two long bonds
  2. One long and two short bonds
  3. Three bonds of the same length
  4. Three bonds of different lengths
Answer

There is an electron pair in a delocalised pi system that is spread over all four atoms. The system is stabilised by spreading out the charge in this way.

The result is that all three carbon - oxygen bonds have the same length and the same strength.

The ion is a perfect trigonal plane with all bond angles identical.
Q228-03 Compare the carbon-oxygen bond lengths in the HCOO- ion, giving your reasoning.
Answer

The negative charge on the carboxylate ion is spread out over the three atoms (carbon and two oxygen atoms). This stabilises the system and the carbon - oxygen bonds are identical, both in terms of length and strength.

The extra stability conferred by the delocalisation makes carboxylic acids more acidic than other organic compounds, such as alcohols. The equilibrium between molecular form and ionic form is influenced by the extra stability of the ion, producing free hydrogen ions in the process.

HCOOH(aq) ⇋ HCOO-(aq) + H+(aq)
Q228-04 Draw a graph representing the hydrogenation of cyclohexene and benzene and use it to explain the concept of delocalisation energy.
Answer

In the graph at the right hand side, the enthalpy of hydrogenation of cyclohexene (blue) is shown as -119 kJ mol-1.

As benzene adds three moles of hydrogen per mole of benzene, the expected enthalpy change on hydrogenation of benzene is three times the value for cyclohexene = 357 kJ mol-1.

However, experimentation reveals that the actual enthalpy change on hydrogenation of benzene is only 207 kJ mol-1 (red).

Thus benzene is 150 kJ mol-1 more stable than expected. This is due to the extra stability of the delocalised electrons in the ring system.

Q228-05 Describe the bonding in an ozone, O3 molecule.
Answer

Oxygen is in group 16 of the periodic table, it has six valence electrons. Three oxygen atoms have 18 valence electrons.

Although the Lewis structure has one double and one single bond between oxygen atoms, the double bond is actually a delocalised molecular orbital spread over all of the three oxygen atoms.

Both of the O-O bonds are identical, with a bond order of 1.5.

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