The term aromatic is derived from the Greek word aroma which means fragrance or pleasant smell.
Aromatic hydrocarbons are benzene and compounds that resemble benzene in chemical behaviour. They are also called arenes. They have the general formula CnH2n-6y where n is the number of carbon and y is the number of rings. Some examples are

Characteristics of aromatic compounds
- They are cyclic compound having a planar ring.
- They have delocalization of π electrons (conjugated system having an alternate single and double bond.)
- They show slight unsaturation. They don’t give addition reaction easily.
- They easily give electrophilic substitution reactions.
- The percentage of carbon in the aromatic compound is higher than in the aliphatic compound. So, they burn with a sooty flame.
- They follow Huckel’s (4n+2)π rule.
Huckel’s Rule
For any organic compound to be aromatic, it must fulfil following conditions
- It must be cyclic and planar.
- It must contain the delocalization of π electrons.
- It must contain (4n+2)π electrons, where n=0,1,2,3….
when n=0, (4n+2)π =2π eg. cyclopropenyl cation
when n=1, (4n+2)π =6π eg. benzene,thiophene, furan, pyrrole, pyridine,etc
when n=2, (4n+2)π =10π eg.naphthalene
when n=3, (4n+2)π =14π eg. anthracene

Structure of benzene

Benzene is stabilized by resonance. The resonating structure of benzene is given by Kekule. Resonance is the phenomenon in which a compound can be represented by more than one structure.
In benzene C-C bond length is 1.39°A which is intermediate between C-C single bond length (1.50°A) and C-C double bond length (1.34°A).
Orientation of benzene
The arrangement of substituents in the benzene ring is called orientation. It is used to determine the position of the substituent in the ring.
The position to be taken up by the second substituents depend upon the nature of the substituent already present in the ring which directs the incoming substituent. This is called the directive influence of substituent. Based on directive influence, substituents are classified into two groups.
- Ortho para directing groups:
The group that directs the incoming substituent to ortho and para position is called ortho para directing group. eg. -Cl, -OH, -NH2, etc.
If S is an ortho para directing group and E is an electrophile (incoming substituent), then reaction takes place as.

Why -OH group is ortho para directing group?

From the above resonating structure, it is clear that the -OH group increases the electron density at ortho and para positions. So, incoming electrophile prefers to attack at ortho and para position. Hence, the -OH group is an ortho para directing group.
- Meta directing groups:
The group that directs the incoming substituent to meta position is called meta directing group. eg. -NO2, -CHO, -COOH, etc.
If S is an meta directing group and E is an electrophile (incoming substituent), then reaction takes place as.

Q. Why -NO2 group is meta directing group?

From the above resonating structure, it is clear that the -NO2 group decreases the electron density at ortho and para positions. Consequently, incoming electrophile prefers to attack at the meta position. So, -NO2 group is meta directing group.
Nomenclature of benzene derivatives

General methods of preparation of benzene
1. From acetylene: Acetylene is heated over a red hot copper tube to give benzene. This reaction is called polymerization or trimerization or aromatization of benzene.

2. From phenol: Phenol is heated with zinc dust to give benzene.

3. From sodium benzoate: Sodium salt of benzoic acid i.e. sodium benzoate is heated with soda lime (NaOH + CaO) to give benzene. This reaction is called soda-lime decarboxylation.

Physical properties
- It is a colourless liquid having sweet aromatic odour.
- It is insoluble in water but soluble in organic solvents.
- It boils at 80.1 °C and freezes at 5.5 °C.
- It is highly inflammable and poisonous.
Chemical properties
Addition reaction
1. Addition of hydrogen: Benzene is added with hydrogen in presence of Ni, Pt or Pd catalyst to give cyclohexane.

2. Addition of chlorine: Benzene is added with chlorine in presence of sunlight to give benzene hexachloride (BHC) which is used as insecticides.

Electrophilic substitution reaction
1. Halogenation: Benzene reacts with halogen in presence of Lewis acid like FeCl3, FeBr3 at cold and dark conditions to give halobenzene.
- Chlorination:

- Bromination:

- Iodination:

Note: Direct iodination of benzene cannot be carried out because HI formed is a strong reducing agent which reduces iodobenzene back to benzene. Hence the reaction is reversible. For the formation of iodobenzene, strong oxidizing agents like conc. HNO3 or Iodic acid (HIO3) is used.

2. Nitration: Benzene reacts with conc. nitric acid and conc. sulphuric acid at 60 °C to give nitrobenzene. The role of the nitrating mixture (HNO3 + H2SO4) is to generate electrophile (nitronium ion: NO2+).

3. Sulphonation: Benzene is heated with conc. sulphuric acid to give benzene sulphonic acid.

4. Friedel-craft alkylation: Benzene reacts with an alkyl halide in presence of anhydrous AlCl3 to give alkylbenzene.

5. Friedel-craft acylation: Benzene reacts with an acyl halide or anhydride in presence of anhydrous AlCl3 to give aromatic ketone (acyl benzene).

Combustion/oxidation of benzene
C6H6 + 15O2 → 12CO2 + 6H2O + Energy
Uses of benzene
- As fuel.
- Dry cleaning of woollen clothes.
- Manufacture dyes, drugs, rubber, insecticides, etc.
Resonance effect (+R and -R effect)
Resonance is the phenomenon in which a molecule or ion cannot be represented by a single structure but can be represented by more than one structure to study its properties. More than one such structures of the same molecules or ions are called resonating structures and the real or actual structure is the hybrid of all resonating structures known as a resonance hybrid. The resonating structures are formed due to the delocalization of π-electrons or lone pair or odd electrons.
In resonance, there is an increase in electron density at one position and a corresponding decrease in electron density at another position. The distribution of electrons is affected by the presence of a group attached to the molecule. Atom or group of atoms which donates electrons shows positive resonance effect (+R): eg. -Cl, -OH, -NH2, etc. and those which withdraws electrons shows negative resonance effect (-R): eg. -NO2,-CHO, -COOH. Such resonance effect takes place in aromatic compounds known as a mesomeric effect.
For example, the OH group in phenol releases the electron towards the ring and increases the electron density at ortho and para positions. It is an example of the +R or +M effect. Due to this, phenol is more reactive than benzene.

The presence of a -NO2 group in nitrobenzene withdraws the electron from the ring and decreases the electron density at ortho and para positions. It is an example of the +R or +M effect. Due to this, nitrobenzene is less reactive than benzene.

In some aspect, inductive effect and resonance effect seems same but there are some differences between them. Let us take the example of chlorobenzene. The electronegativity of chlorine is greater than that of carbon. So, chlorine withdraws the electrons and shows the – I effect. On the other hand, the presence of lone pair of electrons in a chlorine atom releases electrons towards the ring and shows + R effect. Since the -I effect dominates over the +R effect in chlorobenzene, chlorine deactivates the ring. So, chlorobenzene is less reactive than benzene. Hence, -Cl is ortho para directing as well as a deactivating group.
Reactions Summary

Some Important Conversions

Some Important Questions
- State Huckel’s rule with example.
- Define the following with examples:
a. Chlorination of benzene
b. Nitration of benzene
c. Friedel craft alkylation
d. Friedel craft acylation
- What happens when:
a. Sodium benzoate is heated with soda lime.
b. Phenol is heated with zinc dust.
c. Benzene is reacted with H2/Ni.
d. Benzene is heated with acetic anhydride in presence of anhydrous AlCl3.
- Starting from benzene, how will you synthesize
a. BHC b. Cyclohexane c. Bromobenzene d. Toluene
- Write the structure of A and B.
a.\ Phenol\ \xrightarrow{Zn}\ A\ \xrightarrow{H_{2}/Ni}\ B\\ b.\ Sodium\ benzoate\ \xrightarrow{soda-lime}\ A\ \xrightarrow{CH_{3}Cl/anhy. AlCl_{3}}\\ c.\ Chloroform\ \xrightarrow{Ag}\ A\ \xrightarrow{red\ hot\ Fe\ tube}\ B
- The compound A (C7H6O2) reacts with NaOH solution gives B. B is heated with NaOH and CaO gives the compound C. C is subjected to Friedel craft methylation to give toluene. Identify A, B and C.