Chemguide: Support for CIE A level Chemistry

Learning outcome 14.3(a)

These statements cover the bonding and shapes of ethane, ethene and benzene. Statement 14.3(a)(ii) also wants you to be able to predict the shapes of similar molecules. As long as you understand the bonding in ethane, ethene and benzene, that is quite easy to do.

Before you go on, you should find and read the statements in your copy of the syllabus.

Notice that benzene is in bold type in the syllabus statements. That means that it won't be examined until the final exam in a two year course.

Before you start, you should revise electronic structure and atomic orbitals by reading this page in the Basic Organic Chemistry part of Chemguide.

Bonding and shapes of methane and ethane

I know that methane isn't specifically mentioned by the syllabus, but you can't understand ethane without first understanding the bonding in methane.

You should read the page bonding in methane which includes both methane and ethane. Don't skip over the methane bit and go straight to ethane. It won't make any sense unless you read the whole page.

If you are asked to draw the shape of an ethane molecule, I suggest you use something like this:

Note:  If you have forgotten what the various ways of drawing bonds mean, re-read the page on how to draw organic molecules.

The bonding and shape of ethene

You will find everything you need on the page bonding in ethene.

By the end of this, you should understand that:

  • Ethene is a planar molecule.

  • The double bond in ethene consists of a sigma bond and a pi bond.

  • The pi bond lies above and below the plane of the molecule.

  • There is no rotation around a double bond. If you tried to twist the molecule, the pi bond would break, and that costs quite a lot of energy. (The pi bond would break because the unhybridised p orbitals would no longer be parallel to each other and touching sideways.)

The bonding and shape of benzene

You should read this only if you are in the last year of a two year course. It assumes that you have met quite a lot of chemistry already.

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.

  • 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.

Predicting shapes of similar molecules

It is hard to know exactly what CIE want for this, and at the time of writing I couldn't find a question asked about it.

As a general guide, remember that:

  • If you have a carbon atom with four single bonds attached to it, the shape around that carbon atom will be tetrahedral, with bond angles of about 109.5°.

  • If you have a carbon atom with two single bonds and a double bond attached to it, the shape around that carbon atom will be planar, with bond angles of about 120°. That is true whether the double bond is attached to another carbon atom or to, say, on oxygen atom.

  • A benzene ring is always planar. Other groups attached to it, of course, may not be.

Here are a few simple examples. The parts of the molecules in red are all planar. The two CH3 groups have a tetrahedral arrangement around the carbon atom.

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© Jim Clark 2010 (last modified June 2014)