Benzene

Benzene is the parent compound of all aromatic compounds. It is usually illustrated as a hexagon with its points representing each of six carbon atoms. If no other atoms are shown as being attached to each point of the hexagon, then a hydrogen atom is presumed to be attached. Each of the carbon atoms is illustrated as being joined to its two neighbors by either a single or a double line. These lines represent single and double bonds respectively. Two alternate arrangements of these bonds are possible.

The concept of the structure of benzene as described above, was proposed by Kekulé in 1872. He suggested that these alternating single and double bonds continuously changed position with each other extremely rapidly, causing the molecule to vibrate. This vibration was so fast that it could not be detected.

Kekulé's theory is now considered to be obsolete and was replaced with another which emphasised that neither one of the possible structures with alternating single and double bonds was correct, and that neither structure actually existed. Rapid vibration between them did not take place at all, and the real structure was somewhere between the two alternate structures drawn in diagrams. These two alternate structures were merely conveniences for explanation and were called resonance hybrids.

In the same way, it must be remembered that structural formulae of chemicals are for convenience only. The different configurations shown for benzene (and other compounds) are convenient representations only and are not the reality. They do not necessarily describe the real structure.

The current explanation for benzene's structure is that the bonds between the carbon atoms of benzene are all the same. Each carbon atom is bonded to its neighbor with one electron from each atom. Since each atom has two neighbors, this uses two electrons from each atom. Another electron from each carbon is used to bond the hydrogen attached to it. The remaining six electrons orbit the atomic nuclei at right angles to the plane of the ring and also overlap each other, consequently blurring their orbits both above and below the ring. As a result, the electrons are shared equally between the carbon atoms and exist as two clouds, one above and one below the plane of the carbon ring. Since these latter six electrons are not confined to specific carbon atoms, they are said to be delocalised. This is usually represented in structural formulae as a hexagon with a circle in the centre to represent the shared nature of the electrons.

The delocalised electron structure of benzene

In general the explanation above is adequate. Modern opinion is that the electrons involved in the bonds forming benzene (and other compounds) are not restricted to single atoms. Rather, they are associated with particular atoms for a percentage of the time. In the case of the carbon to carbon bonds, the percentage is extremely high, as it is with the electrons forming the bonds between the carbon and hydrogen atoms. All of the electrons, however, will spend some time associated with other parts of the molecule, including the delocalised electron cloud. Similarly, the electrons in the delocalised state will spend a small fraction of their time localised with individual atoms.

As energy is required to keep electrons close to protons in atomic nuclei, this electron cloud must contain the energy necessary to keep it where it is. It is this energy which is fundamentally responsible for absorption of radiation and ultimately the phenomenon we know as colour. This is due to a process called resonance. Do not confuse this process of resonance with the term resonance hybrid that is used to describe the structure of benzene. Remember that resonance hybrids do not exist.

Resonance is the induction of a response in one energy system from another energy system in close proximity, which is operating at the same energy level. In this case the two systems are the energy in the electron cloud and electromagnetic radiation.

By definition, dyes are aromatic compounds. This means that they must have at least one aryl ring (often called a phenyl or benzene ring) in their structure. The delocalised electrons in this aryl ring is the fundamental cause of the absorbance of light in these organic compounds, and consequently the appearance of colour in dyes. Benzene itself absorbs at about 200 nm. This is outside the range that humans can perceive, which is about 400-700 nm. To us, therefore, benzene appears to be colourless.

 

Reference
Much of the data for this page was extracted, with permission, from TheMeter Periodic Table
Alessandri, Stefano
Periodic Table of the Elements
TheMeter Internet web site.

Fessenden R J, and Fessenden J S, (1990)
Organic Chemistry,, 4th ed.,
Brooks/Cole Publishing Company, Pacific Grove, California, U.S.A.

Almost any modern organic chemistry reference text used for 1st or 2nd year university organic chemistry courses would contain much the same information.

 


 

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