Chemical Reactions of Benzene

• Benzene takes part in various reactions. flasks image by Wolszczak from Fotolia.com

  Benzene exhibits some unusual chemical features. Its six carbon atoms form a stable ring, even though each carbon atom shares an electron with only one hydrogen atom. This means that each carbon atom has an extra electron that does not join hands with hydrogen or anything else. Normally such lonely electrons make a substance highly reactive. But benzene achieves a semblance of stability through a phenomenon called resonance. This means that the negative charge of the unshared electrons does not strictly adhere to the carbon atom to which it belongs, but distributes itself throughout the benzene ring. Nevertheless, in spite of the stability thus achieved, benzene does take part in significant chemical reactions.

Substitution with Halogens

• Halogens, such as chlorine, bromine and iodine, can replace hydrogen atoms on a benzene ring, but only when heated in the presence of a catalyst. For example, the catalyst iron trichloride will induce a positive charge on one of the two atoms of a chlorine molecule. Consequently, the benzene ring, which is rich in unshared electrons, will attract this positively-charged chlorine, and one of the carbon atoms in the ring will unite with it. This will convert the benzene ring into an ion with a plus one charge. But the other chlorine atom, which has lost its partner, will remove the hydrogen atom from the carbon which the positively-charged chlorine has joined, thus restoring electrical neutrality to the benzene ring.

Other Electrophilic Reactions

• Other substances react with benzene in a similar fashion when a catalyst makes them electrophilic, or attracted to electrons. For example, when catalyzed by sulfuric acid, nitric acid and benzene react to form nitrobenzene, in which an NO2 group replaces one of the hydrogen atoms on the benzene ring. The Friedel-Craft Reaction takes place in a similar fashion. A chain of carbon atoms, rendered electrophilic by the catalyst aluminum chloride, unites with one of benzene's carbons, eventually causing the resident hydrogen atom to leave.

Multiple Substitutions

• More than one substituent can attach itself to a single benzene molecule. A single benzene ring may wind up with two or three chlorine atoms each attached to a different site. The attached substituents do not have to be the same. An NO2 group and a chlorine atom can unite with the same benzene ring. But electronic factors dictate the site where the substitutions take place. For example, if a chlorine atom is the first to unite with a benzene ring, the second substituent usually unites with either an adjacent carbon or else the carbon directly opposite. But if NO2 arrives first, the second substituent will prefer a site two carbons down.

Addition Reactions

• Benzene can take part in addition reactions---reactions in which something joins benzene without removing any of the original components. In hydrogenation, a second hydrogen atom unites with each carbon atom of the benzene ring. One or more halogen atoms can also unite with benzene without the removal of the resident hydrogen atoms. But these reactions can take place only at extreme pressures, 20 atmospheres for hydrogenation when rhodium is the catalyst.