benzene

AROMATIC COMPOUNDS

1. Classification of Organic Compounds

• Organic compounds can be broadly classified into two categories:

  • Aromatic Compounds: Compounds that resemble benzene in chemical behavior.

  • Aliphatic Compounds: Compounds that include:

  • Alkanes

  • Alkenes

  • Cyclo compounds resembling open-chain compounds.

2. Distinguishing Features of Aromatic Compounds

• Aromatic compounds exhibit distinct properties that differentiate them from aliphatic compounds.

• Benzene Structure:

  • Molecular formula: $C<em>6H</em>6$

  • Degree of unsaturation: 4 (indicating hydrogen deficiency).

  • Degree of unsaturation formula: $ext{Degree of Unsaturation} = (2C + 2 + H - N - X)/2$

  • Where:

    • $C$ = Number of Carbon atoms

    • $H$ = Number of Hydrogen atoms

    • $N$ = Number of Nitrogen atoms

    • $X$ = Number of Halogens (e.g., F, Cl, Br, I)

3. Stability of Benzene Ring

• Reactivity: Benzene generally undergoes substitution reactions rather than addition reactions, which is contrary to expectations based on unsaturation degree.

• Example of Addition vs Substitution:

  • Bromination of Alkanes:

  • Alkanes react with potassium permanganate (KMnO4) losing color as a different compound is formed.

  • Bromination of Benzene:

  • No reaction with bromine in carbon tetrachloride; the red color of bromine persists.

3.1 Electrophilic Aromatic Substitution

• Mechanism involves the formation of a sigma complex where an electrophile attaches to the benzene, resulting in substitution of a proton (H+). This is suggested by:

  • $C<em>6H</em>6 + Br<em>2 ightarrow C</em>6H_5 Br + HBr$

4. Heat of Hydrogenation

• Benzene's heat of hydrogenation is lower than expected, signifying its stability. Comparison includes:

  • Heat evolved during hydrogenation of cyclohexane: $-28.6 ext{ kcal/mol}$

  • Heat evolved during hydrogenation of 1,4-cyclohexadiene: $-57.4 ext{ kcal/mol}$

• Resonance Energy: Explains stability as the energy released during hydrogenation is compared across various compounds:

  • Resonance energy of isolated double bonds is about $1.8 ext{ kcal/mol}$.

5. Benzene Bonding and Length

• All carbon-carbon bonds in benzene are equal in length at approximately $1.39 ext{ Å}$, intermediate between single ($1.54 ext{ Å}$) and double ($1.34 ext{ Å}$) bonds.

• Kekulé Structure: Friedrich Kekulé proposed a cyclic structure for benzene with alternating single and double bonds. Actual bonding shows uniformity in bond lengths due to resonance.

6. Resonance Structures of Benzene

• Benzene's resonance can be illustrated through several structural representations:

  • The delocalization of pi electrons allows for greater stability rather than localized bonds.

  • Benzene's true structure is often depicted as an average of the resonance contributors.

7. Nomenclature of Benzene Derivatives

• Positions on Benzene Ring:

  • Ortho (1,2 positioning)

  • Meta (1,3 positioning)

  • Para (1,4 positioning)

• Naming derivatives includes:

  • Basic nomenclature: e.g., Bromo-benzene, Iodo-benzene, Nitro-benzene.

  • Special names: Methyl-benzene is known as Toluene, Amine-benzene as Aniline, and Hydroxy-benzene as Phenol.

8. Reactions of Benzene

• Electrophilic Aromatic Substitution: A classic reaction in which benzene acts as a nucleophile due to its electron cloud.

• Halogenation (e.g., Bromination): Can be catalyzed by Lewis acids like Aluminium Chloride to promote substitution reactions.

• Nitration: Involves benzene reacting with nitric acid and sulfuric acid to produce Nitrobenzene.

8.1 Mechanism of Nitration

• The process involves generating the nitronium ion (NO2+) from nitric and sulfuric acids.

• Reaction: $C<em>6H</em>6 + HNO<em>3/ H</em>2SO<em>4 ightarrow C</em>6H<em>5NO</em>2 + H_2O$

9. Acylation and Alkylation Reactions of Benzene

9.1 Friedel-Crafts Alkylation

• Involves the attachment of alkyl groups to benzene using an alkyl halide and a strong Lewis Acid (e.g., Aluminum Chloride):

• General Reaction: $C<em>6H</em>6 + R-X <br>ightarrow C<em>6H</em>5-R + HX$

9.2 Friedel-Crafts Acylation

• Similar to alkylation, but results in acylated products (acyl-benzene). Reaction involves an acyl chloride:

• General Reaction: $C<em>6H</em>6 + RCOX <br>ightarrow C<em>6H</em>5C(O)R + HX$

10. Summary of Chemical Kinetics

• Chemical Reactions: Characterized by transformations from reactants to products involving bond breaking and formation.

• Factors effecting reaction rates include concentration, temperature, and physical state of reactants.

11. Kinetics: Types of Reactions

• Substitution: Removal or replacement of atoms or groups.

• Addition: Formation of new bonds by adding atoms/molecules.

• Elimination: Removal of atoms forming double bonds.

• Rearrangement: Rearranging structure while preserving the number of atoms.