Benzene and aromaticity

  • Characteristics of benzene & aromatic compounds
  • Modelling benzene
  • Physical and chemical characteristics of benzene
  • Derivatives of benzene

Learn.

Benzene is an aromatic compound, this is due to it having alternating single and double bonds.

Physics - Hot currents in benzene
aps.org

In order for a molecule to be aromatic, it must:

  1. Contain alternating single (sp3) and double bonds (sp2) meaning it is conjugated
  2. It must be cyclic
  3. Follow Hackle’s 4n + 2 rule, where n is an integer and the number of pi electrons (double bonds*2) in the system. (benzene contains 6)
  4. The system must also be planar

Due to this sharing effect of electrons amongst benzene molecules, known as the resonance structure. The alternating double and single bonds ‘jump’ between the two in valence bond theory.

Does a benzene ring continue to oscillate at absolute zero? - Chemistry  Stack Exchange
Chemistry stack exchange

Whilst this model may be useful, it is not accurate since the true molecule does not switch between phases. Rather the pi electrons are delocalized above and below the plane according to molecular orbital theory.

A pi bond forms when two p orbitals in the same phase overlap.

This results in the formation of bonding and anti bonding orbitals, there are 6 p electrons (one from each carbon) which are forming. Therefore, there all the bonding orbitals are filled resulting in benzene having lower energy i.e more stable then a cyclic diene (an unexpected result).

The Pi Molecular Orbitals of Benzene – Master Organic Chemistry
MasterOrganicChemistry.com

Unlike cyclohexene, benzene will not undergo electrophilic addition reactions. However, it does form a number of derivatives.

3.10: Benzene and its Derivatives - Chemistry LibreTexts
https://chem.libretexts.org

Sometimes o, m, p nomenclature is used to refer to the position of a functional group relative the highest priority functional group. On a benzene derivative a functional group attached to the carbon directly next to the highest priority functional group is an ‘o or ortho’, the next carbon along it is an ‘m or meta’ and three carbons down it is a ‘p or para’.

17.3: Nomenclature of Benzene Derivatives - Chemistry LibreTexts
https://chem.libretexts.org

Memorize.

Master.

Question 1. [2 marks]

Explain why benzene is more stable then expected, use both valence bond theory and molecular orbital theory in your answer.

Question 2. [2 marks]

Outline why benzene is considered an aromatic molecule.

Question 3. [1 mark each]

Name the following molecules:

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