Aromatic Hydrocarbons

Full Notes

Aromatic hydrocarbons, also called arenes, are a class of compounds that include one or more benzene rings. These compounds are notable for their relative stability due to resonance and their tendency to undergo electrophilic substitution reactions rather than addition.

Nomenclature and Isomerism

Aromatic compounds are named systematically using IUPAC rules.

Monosubstituted benzene

The substituent is simply prefixed to "benzene".

Disubstituted benzene

Ortho- (1,2-), Meta- (1,3-), Para- (1,4-) positions are used when only two substituents are involved and each is an isomer of the other.

For complex substitutions, the benzene ring is treated as a substituent (phenyl group).

Structure of Benzene

Benzene (C₆H₆) is a planar, hexagonal molecule in which each carbon atom is bonded to two other carbon atoms and one hydrogen atom. Chemists used to believe benzene had alternating single and double bonds, but experimental data shows all carbon–carbon bonds are of equal length, suggesting delocalisation.

• C–C bond lengths in benzene are equal (~0.140 nm). This is between the length of a single bond (0.154 nm) and a double bond (0.134 nm).
• Benzene’s structure cannot be represented accurately by alternating single and double bonds (as in the theoretical cyclohexa-1,3,5-triene which would also have a formula of C₆H₆).

Instead, it has a delocalised system of π electrons, formed by the sideways overlap of unbonded p orbitals from each carbon atom.

These six p electrons form a continuous cloud of electrons above and below the plane of the carbon atoms, leading to uniform bond lengths and a more stable structure.

Resonance Hybrid

Based on Valence Bond Theory, the idea of alternating double bonds in benzene is now better understood through the concept of resonance.

The actual structure is a resonance hybrid of the two possible Kekulé structures, commonly illustrated using a circle or dotted circle inside a hexagon (as shown in diagram), representing six delocalised electrons evenly shared across the six carbon atoms in the ring.

Aromaticity

Aromaticity is a concept used to explain the extra stability of benzene-like compounds. It is defined using structural and electronic criteria.

Hückel’s Rule:

• A molecule is aromatic if:
  • It is planar.
  • It is cyclic and conjugated.
  • It has (4n + 2) π electrons (n = 0, 1, 2...).

Preparation of Benzene

Benzene can be prepared from various methods using simple organic compounds. You should be aware of the following.

Cyclic Polymerisation of Ethyne

Decarboxylation of Aromatic Acid

Reduction of Phenol

Physical Properties

Benzene has some characteristics physical properties that give it a unique character.

• Colourless, highly flammable liquid.
• Pleasant aromatic odour.
• Insoluble in water but soluble in organic solvents.
• Boiling point: ~353 K. Melting point: ~279 K.

Chemical Properties – Electrophilic Substitution Reactions

Unlike alkenes, benzene resists addition reactions and undergoes electrophilic substitution to retain its aromaticity (ring of delocalised electrons).

Nitration

Halogenation

Sulphonation

Friedel–Crafts Alkylation

Friedel–Crafts Acylation

Further Substitution

If a reagent is excess, further substitution reactions may occur. This forms a poly substituted benzene. For example, benzene reacting with excess Cl₂ can form hexachlorobenezene (C₆Cl₆).

Other Reactions:

• Addition (under severe conditions): C₆H₆ + 3H₂ → C₆H₁₂ (Ni catalyst, pressure)
  C₆H₆ + 3Cl₂ → C₆H₆Cl₅ (uv light)
• Combustion: 2C₆H₆ + 15O₂ → 12CO₂ + 6H₂O
  (produces sooty flame due to high carbon content)

Electrophilic Substitution Mechanism

Benzene generally reacts with electrophiles by a general mechanism of electrophilic substitution.

• Step 1: Electrophilic attack
  Electrophile accepts a pair of electrons from π-bonding system in benzene ring.
• Step 2: Elimination of a proton (H⁺)
  The ring loses a hydrogen ion to restore ring of delocalised electrons and give aromatic stability.
• Step 3: Formation of substituted product
  Substitution rather than addition has occurred.

Forming electrophiles

As the general mechanism for benzene reactions is the same each time, it is how electrophiles are generated that is of most interest.

Example: Electrophile: Nitronium ion (NO₂⁺)
Generated in situ:

Example: Electrophile: Acyl or acylium ion
Generated by a halogen carrier catalyst with alkyl halide or acyl chloride

Directive Influence of a Functional Group in Monosubstituted Benzene

Groups already attached to the ring affect both reactivity and position of further substitutions. Understanding these effects helps predict major products in disubstituted arenes.

Ortho and Para Directing Groups:

Electron-donating groups (e.g. –OH, –NH₂, –CH₃). Increase electron density in the ring, directing new groups to positions 2 and 4 (ortho/para).

Increase electron density at ortho and para positions. New groups prefer to attach at these positions.

Meta Directing Groups:

Electron-withdrawing groups (e.g. –NO₂, –COOH, –CN). Withdraw electron density from the ring, reducing reactivity and directing to position 3 (meta).

Summary

• Aromatic hydrocarbons contain benzene rings with delocalised π electrons that obey Hückel’s rule.
• Benzene has equal C–C bond lengths due to resonance and undergoes electrophilic substitution.
• Common reactions include nitration, halogenation, sulphonation, and Friedel–Crafts reactions.
• Substituents direct further substitution to ortho/para or meta positions depending on their electronic effects.