Chemguide: Support for CIE A level Chemistry

Learning outcome 29: An introduction to A level organic chemistry

29.3: Shapes of aromatic organic molecules; σ and π bonds

Learning outcome 29.3.1

This statement is about the bonding and shape of benzene and other related molecules. Read the statement so that you know exactly what it wants.

The bonding and shape of benzene

Chemguide has two pages about the structure of benzene. The first discusses the older Kekulé structure for benzene, and you should read this because you will still find this structure in use, and you need to know what it means.

It is also important to understand that benzene is much more energetically stable than the Kekulé structure suggests. You need to know about delocalisation energy, which is introduced on this page.

You will find what you need in terms of modern bonding theory on the page about the bonding in benzene.

You need to understand that:

  • Benzene is a planar molecule.

  • There is a sigma bond joining each of the carbon atoms in the ring.

  • The most interesting and important part of the benzene structure is the delocalisation of the pi electrons.

  • Delocalisation makes benzene much more energetically stable than it would otherwise be.

  • Breaking the delocalisation costs energy, and benzene is reluctant to undergo reactions in which the delocalisation is permanently broken.

The syllabus also mentions "other aromatic molecules". These are compounds which contain planar ring systems with delocalised pi electrons instead of alternating single and double bonds.

At this level almost all of the compounds you meet will either be benzene itself or will contain a benzene ring with other groups attached.

But there don't have to be 6 carbons in a ring, and the ring might also involve other atoms like nitrogen. All that is important is that there should be delocalisation all around the ring for it to count as aromatic.

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© Jim Clark 2020