Benzene usually shows only the substitution electrophilic reactions. But benzene involves in a different and difficult reaction with chlorine. It is called additive reaction and it forms Benzene Hexachloride. Products, mechanism, uses of reaction and products are discussed in this tutorial.
Written by: Aloka Karunathilaka, (undergraduate), Department of Chemical and Process Engineering, Faculty of Engineering, University of Peradeniya, last modified: 01/06/2021
Benzene with chlorine can't react as a substitutional reaction without halogen carriers. But they are done another reaction with sun rays or UV rays it's called additive reaction. But this reaction is difficult to happen. The reason is the extra energy is required to break the displaced π electron cloud.
Benzene hexachloride is especially used in agriculture and the pharmaceutical industry. It used as insecticides, pesticides, seed treatments in forestry and for head lice and used for the treatment of the skull.
BHC (benzene hexachloride) is very acutely toxic. Therefore, a higher amount benzene hexachloride can damage the nervous system. Also, it's production process is very harmful to the environment. Benzene hexachloride can affect headache, dizziness, convulsions, brain development of the children's and change thyroid hormone levels. Sometimes benzene hexachloride is dangerous for life and health. benzene hexachloride pollutes the environment very badly.
Benzene with chlorine is not a spontaneous reaction without sunrays. This is because the non-polar molecule has no electron deficient areas and cannot function directly as an electrophile. But when anhydrous AlCl3, anhydrous FeCl3 or Fe powder is used as the catalyst, it reacts with benzene and chlorine to form chlorobenzene.
When the Cl2 molecule comes near to the benzene ring, the electrons in the Cl2 molecule are repelled by the electrons in the benzene ring. As a result of this, the Cl2 molecule is going to be slightly polarized. This polarized Cl2 reacts with AlCl3 and forms a positive chlorine ion which acts as an electrophile.
This electrophile attacks to the electrons of the benzene ring and forms a C-Cl bond which leads to a highly unstable intermediate carbocation. This unstable carbocation reacts with the earlier formed tetra chloroaluminate (III) ion and gives chlorobenzene at the end. Here, the mechanism used for the preparation of chlorobenzene is an electrophilic substitution.
Chlorobenzene is most used as an intermediate in the production of herbicides, Dyestuffs and rubber. Chlorobenzene is the high-boiling solvent for the production of paint removers, paints, drugs, adhesives, polishers and dyes. Also, chlorobenzene is used for making the polyurethane insulation via the production of diisocyanate.
This gets evaporates when exposing to air and dissolves slightly when mixing with water. When chlorobenzene is in the air it breaks into other chemical compounds. As chlorobenzene is a liquid, it doesn't bind with the soil. So, it flows into the underground water. But plants and animals do not like to store chlorobenzene.
The exposing to a high concentration of chlorobenzene will cause sleepiness, headaches, vomit, loss of sensation of parts of the body, feeling of disgust and etc. Also, this will affect the malfunction of parts of the nervous system. However, this is a common response when exposing to different kinds of chemicals for a long time.
When considering the effect on animals, it specially influences the brain, liver and kidneys. The chemical can cause severe injury to the liver and kidneys. Researchers had found that chlorobenzene doesn't affect the reproduction or cause birth defects.