Aromatic Hydrocarbons

Aromatic hydrocarbons, or arenes, are compounds known for their pleasant aroma. Most contain a benzene ring, which is highly unsaturated but retains its structure in most reactions. Aromatic hydrocarbons are classified as benzenoids (with a benzene ring) and non-benzenoids (without a benzene ring).

Aliphatic hydrocarbons lack delocalised electrons, whereas aromatic hydrocarbons have sigma bonds and delocalised π-electrons forming a ring. Their general formula is C_nH_n.

All six hydrogen atoms are equivalent in benzene, leading to one type of monosubstituted product. For disubstituted benzene derivatives, three positional isomers are possible:

Ortho (o-): 1,2 or 1,6 positions
Meta (m-): 1,3 or 1,5 positions
Para (p-): 1,4 positions

Examples include methylbenzene (toluene) and 1,2-dimethylbenzene (o-xylene).

1.0Structure of benzene

Benzene (C₆H₆), isolated by Michael Faraday in 1825, exhibits high unsaturation and unique stability. August Kekulé proposed its cyclic structure in 1865, with six carbon atoms arranged alternately with single and double bonds, each bonded to a hydrogen atom.

The structure suggested two isomeric 1,2-dibromo benzenes, but benzene forms only one ortho-disubstituted product. Kekulé resolved this by proposing oscillating double bonds.

Later, resonance theory explained benzene’s unusual stability and its preference for substitution over addition reactions. Benzene’s stability is also confirmed by its ability to form a triozonide, indicating three double bonds.

Resonance and stability of benzene

Benzene’s resonance, explained by Valence Bond Theory, shows it as a hybrid of Kekulé’s structures (A and B), represented by a hexagon with a circle denoting delocalised electrons. All six sp²-hybridized carbon atoms form sigma bonds in a hexagonal plane, with unhybridised p orbitals perpendicular to the ring, enabling electron delocalisation and stability.

X-ray diffraction reveals that benzene’s six C–C bonds are equal in length (139 pm) between a single and double bond. This indicates delocalised π electrons, making benzene more stable than cyclohexatriene. The delocalised electron cloud, formed above and below the plane of the ring, accounts for benzene’s resistance to addition reactions and its unusual stability.

2.0Aromaticity

Benzene is considered the parent aromatic compound, and this term is now applied to all ring systems with these properties.

Aromaticity refers to the unique stability of ring systems with specific characteristics:

Planarity
Complete delocalisation of π electrons
Presence of (4n + 2) π electrons (Hückel Rule, where n = 0, 1, 2, ...)

3.0Preparation of Benzene

Cyclic Polymerization of Ethyne: Under suitable conditions, benzene can be prepared by the cyclic polymerisation of ethyne (acetylene).

Decarboxylation of Aromatic Acids: When heated with soda lime (a mixture of sodium hydroxide and calcium oxide), the sodium salt of benzoic acid undergoes decarboxylation to yield benzene.

Reduction of Phenol: Benzene can be obtained by reducing phenol by passing its vapours over heated zinc dust.

4.0Physical Properties

Aromatic hydrocarbons are nonpolar molecules, typically colourless liquids or solids with a distinct aroma. Naphthalene, for example, is commonly used in toilets and for preserving clothes due to its unique scent and moth-repellent properties. These hydrocarbons are immiscible with water but easily dissolve in organic solvents. When burned, they produce a sooty flame.

5.0Chemical Properties

Arenes primarily undergo electrophilic substitution reactions, though they can also participate in addition and oxidation reactions under certain conditions. In electrophilic substitution, the attacking reagent is an electrophile (E+).

(i) Nitration: Benzene reacts with concentrated nitric acid and sulfuric acid (nitrating mixture) to introduce a nitro group into the benzene ring.

(ii) Halogenation: Arenes react with halogens in the presence of a Lewis acid like anhydrous FeCl₃, FeBr₃, or AlCl₃, resulting in haloarenes (e.g., chlorobenzene).

(iii) Sulphonation: A hydrogen atom in the benzene ring is replaced by a sulfonic acid group when benzene reacts with fuming sulfuric acid (oleum).

(iv) Friedel-Crafts Alkylation: Benzene reacts with an alkyl halide in the presence of anhydrous aluminium chloride to form alkylbenzenes, such as isopropylbenzene when treated with 1-chloropropane instead of n-propylbenzene.

(v) Friedel-Crafts Acylation: When benzene reacts with an acyl halide or acid anhydride in the presence of a Lewis acid (AlCl₃), acylbenzene is produced.

If excess electrophilic reagent is used, further substitution reactions can occur, replacing additional hydrogen atoms in the benzene ring with the electrophile. For instance, benzene treated with excess chlorine and anhydrous AlCl₃ may form hexachlorobenzene (C₆Cl₆).

6.0Carcinogenicity and Toxicity

Benzene and polynuclear hydrocarbons with more than two fused benzene rings are toxic and carcinogenic. These compounds are formed during the incomplete combustion of organic materials such as tobacco, coal, and petroleum. Once in the human body, they undergo biochemical reactions that damage DNA, leading to cancer.

7.0Uses of Aromatic Compounds

Aromatic compounds are essential in various industries:

Chemical Industry: Solvents (benzene, toluene) and synthesis of phenol and aniline.
Pharmaceuticals: Key ingredients in drugs like aspirin and paracetamol.
Plastics: Production of polystyrene and PET plastics.
Dyes: Used in manufacturing synthetic dyes and pigments.
Agrochemicals: Basis for herbicides, insecticides, and fungicides.
Explosives: Toluene for TNT production.
Perfumes & Flavors: Found in vanillin and coumarin.
Energy: Fuels and gasoline additives.
Research: Used in labs and advanced material synthesis.
Others: Moth repellents (naphthalene), anaesthetics (benzocaine), and lubricants.