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10.2: Background

Difficulty Level: At Grade Created by: CK-12

At room temperature, benzene is a clear or pale yellow liquid with a sweet odor. It is highly flammable and highly toxic. Benzene forms as a result of forest fires, volcano eruptions, smoking, burning crude oil, and more.

Diagram of a benzene molecule

The molecular formula for benzene is C6H6. It consists of a carbon ring where each carbon is bonded to one hydrogen atom and two other carbon atoms. Carbon atoms have four valence electrons. So far, only three of these electrons are involved in bonds. Since each carbon has one more valence electron, each carbon has the ability to form one more bond. However, since all of the atoms are carbon atoms, every atom is equally attracted to the atoms on either side of it.

This causes benzene to exhibit a unique phenomenon called resonance. Each carbon's extra electron is shared equally between that carbon and the carbon adjacent to it. The Lewis structure depicting benzene would therefore look like this:

However, the electrons are not actually switching from carbon atom to carbon atom. A benzene molecule never exists in either of the states depicted in the Lewis structures above. This is indicated by the use of a single arrow as opposed to the double arrow used in equilibrium reactions. It is impossible to draw benzene as it is in the form of a Lewis structure, so instead it is depicted as an equal combination of the two structures. That is why benzene is called a resonant structure, or a structure that cannot be represented by one Lewis structure and is therefore depicted as a hybrid of two.

Ever since this compound was first discovered in 1825 by Michael Faraday, many structural representations have been used in an attempt to represent benzene:

The first four images have been used historically. The last two are both used today.

It was then used in the 19th and 20th centuries for a variety of uses such as aftershave and to decaffeinate coffee. The use of benzene to decaffeinate coffee is known as Roselius’s Process. In this process, the coffee beans are boiled in salt water. Benzene is then used as a solvent to attach to the caffeine leaving the coffee beans decaffeinated. However, this greatly alters the taste of the beans and is unsafe, as benzene is a dangerous substance.

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