15. Aromatic Hydrocarbons

• Aromatic hydrocarbons are compounds containing one or more hexagonal benzene rings.
• The term aromatic comes from the Greek word ‘aroma’ meaning sweet fragrance.
• Hence, the term was originally used to designate the compounds having a sweet smell. In contrast to aliphatic compounds, which were fatty or waxy (Greek: Aliphos = fat)
• Later, it was found that they contain one or more benzene rings. Aromatic compounds may be defined as compounds containing benzenoid rings or benzene. e.g.,

Characteristic of aromatic compounds

Later on, further study revealed that many other aromatic compounds do not resemble benzene. Now, the compounds having all the following characteristics are called aromatic compounds.

1. Aromatic compounds are cyclic (closed chain) or ring compounds with flat structures.
2. All the atoms in the ring are sp² hybridised, hence making the ring planar.
3. They have a conjugated system, i.e. have alternate single and double bonds in the ring.
4. They contain a delocalized system of (4n+2) π electrons. Where, n is an integer, n = 0, 1, 2, 3, 4…. This is known as Hückel’s rule.

Kekulé’s structure

Resonance theory

Electron Delocalisation in Benzene

The extra stability of benzene and the fact that its C-C bonds are all of equal length can be explained using the following model, which is currently accepted by most chemists.

The carbon atoms in the ring are bonded to one another and their hydrogen atoms by σ-bonds. This leaves one unused p orbital on each carbon, each containing a single electron. The p orbitals are perpendicular to the plane of the ring, with one lobe above and one below this plane (Figure -1). Each p orbital overlaps sideways with the two neighbouring orbitals to form a single π-bond that extends as a ring of charge above and below the plane of the molecule (Fig. 2)

a: σ-bonded skeleton (C & H b: σ- skeleton with π- bonds
Atoms omitted for clarity)
Fig 1: Bonding in benzene (Plane of the molecules is Perpendicular to the paper)

Fig 2: Computer-generated model of the benzene molecule (Planar symmetrical shape)

The electrons in the π-bond cannot be said to ‘belong to’ any particular carbon atom. Each electron is free to move throughout the entire π-system, so the electrons are said to be delocalised. It is this delocalisation that gives benzene its extra stability; any system in which electron delocalisation can occur is stabilised. The reason for this is not hard to see. Electrons tend to repel one another, so a system in which they are as far apart as possible will involve minimum repulsion and will therefore be stabilised.

To conform with this model, the structural formula of benzene is nowadays often written as rather than . The delocalisation of π-electrons has a profound effect on the chemical properties of benzene.

The word aromatic is used to describe any system that is stabilised by a ring of delocalized π- electrons. Non-aromatic compounds (such as alkanes and alkenes) are called aliphatic.

The structure described above means that the benzene molecule is planar, symmetrical, and nonpolar. Its lack of polarity results in benzene being a liquid at room temperature and makes it immiscible in water. Its boiling point is 80 °C and its melting point 6⁰C. The surprisingly high melting point is due to the ease with which highly symmetrical benzene rings can pack into a crystal lattice. Compare it with that of the structurally similar but less symmetrical compound methylbenzene, C₆H₅CH₃, which melts at -95⁰C.

(From the book- Chemistry in Context- by Graham Hill and John Holman)

Resonance in Chemistry Explained in Simple Words with Examples

1. From Ethyne (by polymerisation)

Benzene can be prepared by polymerisation of Ethyne (acetylene) in a red-hot copper tube. This is addition polymerisation. It can also be called trimerisation, as three molecules form a benzene molecule.

2. From Phenol (by heating with zinc dust)

Phenol gives benzene when heated with Zn dust.

3. From Sodium benzoate (by soda lime decarboxylation)

Another general method of preparation of benzene is soda lime decarboxylation of sodium benzoate.

1. Electrophilic Aromatic Substitution reactions

Though benzene and aromatic compounds are highly unsaturated compounds, these do not commonly undergo addition reactions; instead undergo electrophilic substitution reactions. This is because of resonance stabilisation in aromatic compounds; they do not try to lose aromaticity. Hence undergo electrophilic aromatic substitution. One more factor that supports electrophilic substitution is that the aromatic ring is electron-rich, so prone to attack by an electrophile.

a. Halogenation

Chlorination:

b. Nitration

c. Sulphonation

d. Friedel-Crafts alkylation

e. Friedel-Crafts acylation

2. Addition Reactions

a. Addition of H₂

b. Addition of Cl₂

(An important insecticide)

3. Combustion reaction

1. To produce polystyrene via Ethylbenzene and styrene.
2. To produce cumene, which is converted into phenols for the production of resins & adhesives.
3. To prepare cyclohexane used for the manufacture of nylon.
4. To prepare aniline (for dyes) and dodecylbenzene (for detergents).

Naming the di-substituted Benzenes

Effect on reactivity

• Activating groups:
• Deactivating groups:

Effect on orientation

-o,-p directing groups

m- directing groups