For years smell (odour) used to be a chemist’s biggest tool. Now, this may sound a bit weird but some chemists even went on to taste the chemicals! The German-Swedish pharmaceutical chemist Carl Wilhelm Scheele had a habit of smelling and even tasting any new compound which he discovered. The word ‘aromatic compounds‘ began to be used to describe compounds with carbon and also having distinct odours. Since toluene and benzene have sweet smells, some compounds which are formed primarily of carbon and hydrogen are called aromatic hydrocarbons. As such, the aromatic compounds are quite stable and unsaturated.
Today, we are going to talk about these stable aromatic compounds.
The aromatic compounds are also known as arenes. Most of the aromatic compounds contain benzene. The benzene is an unsaturated structure. There are certain aromatic compounds as well which do not contain benzene. The compounds which contain a benzene ring are known aromatic compounds and the aromatic compounds which do not contain a benzene ring are called non-benzenoids.
Physical Properties of Aromatic compounds
- Aromatic compounds are non-polar.
- Most often they are colourless solids or liquids with a specific aroma.
- These aromatic compounds are non-soluble in water but dissolve readily in organic solvents.
- Since they have a high carbon content, therefore they burn with a sooty flame.
Chemical Properties of Aromatic Compounds
The arenes readily undergo electrophilic substitution reactions. They also undergo addition and oxidation reactions under special conditions.
Talking of electrophilic substitutions, arenes undergo nitration, halogenation, Friedel Craft’s alkylation, acylation, and sulphonation.
- Electrophilic Substitution Reactions
1. Nitration: When a benzene ring is heated with concentrated sulphuric acid and nitric acid, a nitro group is introduced into the benzene ring. The mixture of concentrated sulphuric acid and concentrated nitric acid is known as a nitrating mixture. 
2. Halogenation: Haloarenes are formed when arenes react with halogens in the presence of Lewis acids like anhydrous FeCl3, FeBr3 or AlCl3.
3. Sulphonation: Sulphonation is the reversible process which results in the production of benzene sulfonic acid on the addition of fuming sulfuric acid and sulfur trioxide.
4. Friedel-Crafts alkylation reaction: The arenes react with alkyl halides to yield alkylarenes. The Friedel-Crafts alkylation reaction is catalysed by Lewis acid. 
5. Friedel-Crafts acylation reaction: Monoacylated products are synthesized by the reaction between arenes and acyl halides or anhydrides. 
However, if the electrophilic reagent is used in excess, other hydrogens of the benzene ring may be replaced by the electrophile in a successive manner. For example: when benzene reacts with the excess of chlorine in the presence of anhydrous AlCl3, hexachlorobenzene is formed.
- Addition Reactions
1. Hydrogenation: Hydrogenation of benzene occurs at high temperature and/or in the presence of nickel catalyst. The product formed is cyclohexane.
2. Chlorination: Chlorination of benzene occurs, in the presence of ultra-violet light, which results in the production of benzene hexachloride. The end product benzene hexachloride is also known as gammaxane.
3. Combustion: Benzene often burns with sooty flame; when heated in air. The end results are carbon-dioxide and water.
Carcinogenicity and Toxicity of Aromatic Compounds
Most of the aromatic compounds contain benzene. Benzene is a colourless and flammable liquid which readily evaporates when exposed to air. Benzene can easily be formed by natural processes like volcanic eruptions and forest fires. The aromatic compound is a precursor for the synthesis of rubbers, dyes, plastics, lubricants, pesticides, drugs, and detergents. Benzene is also an industrial solvent and an additive to benzene. It is a natural component of crude oil as well as gasoline and is also present in cigarette smoke.
An exposure to benzene can occur in a variety of ways; like:
- at the workplace,
- environment (for example- exposure to cigarette smoke), &
- use of various commercial products.
Benzene is a carcinogenic aromatic compound (hydrocarbon). The laboratory research on human subjects, as well as model organisms, have demonstrated a link between benzene and cancer; particularly leukemia and other types of blood cancers.
The International Agency for Research on Cancer (IARC), which is a part of the World Health Organisation, has put forth evidence which links benzene to acute myeloid leukemia (AML), acute lymphocytic leukemia (ALL), chronic lymphocytic leukemia (CLL), multiple myeloma, and non-Hodgkin lymphoma.
The National Toxicology Program (NTP) classifies benzene as “known to be a human carcinogen.”
Quick Revision in 5 Points
- Benzene forms the primary substituent of the aromatic compounds.
- The aromatic compounds are non-polar, colourless, and have a particular odour.
- They can readily undergo electrophilic substitution reactions, like nitration, halogenation, Friedel Craft’s alkylation, acylation and sulphonation.
- The aromatic compounds can also undergo addition reactions like hydrogenation, chlorination, and combustion.
- The aromatic compounds containing benzene ring are known to be carcinogenic.
Results
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Oh, Snap! You need to read the article again and come back to this quiz.
#1. Choose the incorrect answer:
#2. Which of the following is not a property of the aromatic compounds:
#3. Arenes are:
#4. Friedel Craft’s alkylation reaction results in the production of
#5. Gammaxane is formed from: