Properties

Physical Properties

Aromatic hydrocarbons are non-polar molecules. This non-polar nature governs nearly all their physical behaviour. They are usually colourless liquids or solids at room temperature, and they carry a characteristic aroma — the word "aromatic" itself comes from this property.

You are already familiar with naphthalene balls, which are used in toilets and for preserving clothes. Naphthalene has a strong, unique smell and acts as a moth repellent. This is a practical everyday example of an aromatic hydrocarbon in use.

Because they are non-polar, aromatic hydrocarbons are immiscible with water (like oil and water). However, they dissolve readily in organic solvents such as ether, benzene itself, or carbon tetrachloride. When burned, they produce a sooty flame due to their high carbon content — the carbon atoms do not get fully oxidised and appear as black soot.

Chemical Properties

The chemistry of arenes (another name for aromatic hydrocarbons) is dominated by one type of reaction: electrophilic substitution. The benzene ring is rich in $\pi$ electrons, making it attractive to electron-seeking species (electrophiles). Under normal conditions, this is the characteristic reaction.

However, under special, more vigorous conditions, arenes can also undergo two other types of reactions:

• Addition reactions — where the ring's double bonds are broken and atoms add across them.
• Oxidation reactions — where the ring is burned or otherwise oxidised.

Electrophilic Substitution Reactions

The common electrophilic substitution reactions of arenes are nitration, halogenation, sulphonation, Friedel-Crafts alkylation, and Friedel-Crafts acylation. In every case, the attacking reagent is an electrophile, denoted as $\text{E}^+$.

(i) Nitration

A nitro group ($-\text{NO}_2$) is introduced into the benzene ring. This is done by heating benzene with a mixture of concentrated nitric acid ($\text{HNO}_3$) and concentrated sulphuric acid ($\text{H}_2\text{SO}_4$). This mixture is called the nitrating mixture.

Nitrobenzene

(ii) Halogenation

Arenes react with halogens (like $\text{Cl}_2$ or $\text{Br}_2$) in the presence of a Lewis acid catalyst, such as anhydrous $\text{FeCl}_3$, $\text{FeBr}_3$, or $\text{AlCl}_3$, to yield haloarenes.

Chlorobenzene

(iii) Sulphonation

The replacement of a hydrogen atom by a sulphonic acid group ($-\text{SO}_3\text{H}$) is called sulphonation. It is carried out by heating benzene with fuming sulphuric acid (oleum), which is sulphuric acid containing dissolved sulphur trioxide ($\text{SO}_3$).

Benzenesulphonic acid

(iv) Friedel-Crafts Alkylation Reaction

When benzene is treated with an alkyl halide (like $\text{R-Cl}$) in the presence of anhydrous aluminium chloride ($\text{AlCl}_3$), an alkylbenzene is formed.

(v) Friedel-Crafts Acylation Reaction

The reaction of benzene with an acyl halide (like $\text{RCO-Cl}$) or an acid anhydride in the presence of a Lewis acid ($\text{AlCl}_3$) yields an acyl benzene (a ketone).

Further Substitution

If an excess of the electrophilic reagent is used, further substitution reactions can take place. Other hydrogen atoms on the benzene ring may also be successively replaced by the electrophile. For example, benzene treated with excess chlorine in the presence of anhydrous $\text{AlCl}_3$ can be chlorinated all the way to hexachlorobenzene ($\text{C}_6\text{Cl}_6$).

Mechanism of Electrophilic Substitution Reactions

According to experimental evidence, $\text{S}_\text{E}$ (Substitution, Electrophilic) reactions proceed via three distinct steps:

1. Generation of the electrophile ($\text{E}^+$)
2. Formation of a carbocation intermediate (arenium ion)
3. Removal of a proton from the carbocation intermediate

Step (a): Generation of the electrophile $\text{E}^+$

The Lewis acid catalyst ($\text{AlCl}_3$, $\text{FeCl}_3$) plays a crucial role here. It combines with the attacking reagent to generate the active electrophile.

• For chlorination, alkylation, and acylation: Anhydrous $\text{AlCl}_3$ (a Lewis acid) accepts a lone pair from the halogen in the reagent, forming a complex that then dissociates to give the electrophile.

  • Chlorination: $\text{Cl}_2 + \text{FeCl}_3 \rightarrow \text{Cl}^+ + [\text{FeCl}_4]^-$
  • Alkylation: $\text{R-Cl} + \text{AlCl}_3 \rightarrow \text{R}^+ + [\text{AlCl}_4]^-$
  • Acylation: $\text{RCO-Cl} + \text{AlCl}_3 \rightarrow \text{RCO}^+ + [\text{AlCl}_4]^-$ (the $\text{RCO}^+$ ion is called an acylium ion)

• For nitration: The electrophile is the nitronium ion ($\text{NO}_2^+$). It is produced by a transfer of a proton from sulphuric acid to nitric acid. This is a simple acid-base equilibrium.

  Step I:

  $\text{HNO}_3 + \text{H}_2\text{SO}_4 \rightleftharpoons \text{H}_2\text{NO}_3^+ + \text{HSO}_4^-$

  Protonated nitric acid

  Step II:

  $\text{H}_2\text{NO}_3^+ \longrightarrow \text{NO}_2^+ + \text{H}_2\text{O}$

  Nitronium ion

  In this process, sulphuric acid acts as an acid (proton donor) and nitric acid acts as a base (proton acceptor).

Step (b): Formation of Carbocation (Arenium Ion)

The electrophile ($\text{E}^+$) attacks the electron-rich benzene ring. This results in the formation of a $\sigma$-complex, also called an arenium ion. In this intermediate, one of the carbon atoms (the one attacked by the electrophile) becomes $\text{sp}^3$ hybridised.

The arenium ion is not stable, but it is stabilised by resonance. The positive charge is delocalised over three carbon atoms of the ring (the ortho and para positions relative to the point of attack).

Important

Loss of aromaticity

The sigma complex (arenium ion) loses its aromatic character because the delocalisation of $\pi$ electrons is interrupted at the $\text{sp}^3$ hybridised carbon. The ring is no longer a fully conjugated, planar system.

Step (c): Removal of Proton

To restore the stable aromatic character of the ring, the $\sigma$-complex releases a proton ($\text{H}^+$) from the $\text{sp}^3$ hybridised carbon. This proton is removed by the counter-ion present in the reaction mixture.

• In the case of halogenation, alkylation, and acylation, the proton is removed by $[\text{AlCl}_4]^-$.
• In the case of nitration, the proton is removed by $[\text{HSO}_4]^-$.

The removal of the proton regenerates the aromatic ring and gives the final substituted product.

Addition Reactions

Under normal conditions, benzene does not undergo addition reactions because it would lose its aromatic stability. However, under vigorous conditions, addition is possible.

Hydrogenation: At high temperature and/or pressure, in the presence of a nickel catalyst, benzene adds three molecules of hydrogen to give cyclohexane.

Cyclohexane

Halogenation (Addition): Under ultraviolet light, three molecules of chlorine add to benzene to produce benzene hexachloride ($\text{C}_6\text{H}_6\text{Cl}_6$), also known as gammaxane or BHC (benzene hexachloride). This is an insecticide.

Benzene hexachloride (BHC)

Combustion (Oxidation)

When heated in air (or oxygen), benzene burns with a sooty flame, producing carbon dioxide and water.

The general combustion reaction for any hydrocarbon ($\text{C}_x\text{H}_y$) can be written as:

Directive Influence of a Functional Group in Monosubstituted Benzene

When a monosubstituted benzene (benzene with one group already attached) is subjected to further substitution, the three possible disubstituted products (ortho, meta, and para) are not formed in equal amounts. Two distinct types of behaviour are observed:

1. The incoming group goes predominantly to the ortho and para positions.
2. The incoming group goes predominantly to the meta position.

This behaviour depends entirely on the nature of the substituent already present on the ring. It does not depend on the nature of the entering group. This is known as the directive influence of substituents.

Ortho and Para Directing Groups

Groups that direct the incoming group to the ortho and para positions are called ortho and para directing groups.