1. Aromatic Hydrocarbons

Aromatic hydrocarbons, also known as arenes, are organic compounds that contain one or more benzene rings. These compounds exhibit special stability due to delocalized π-electrons in their ring structure, which follows Hückel's rule (4n+2 π electrons).

• Examples: Benzene (C₆H₆), Toluene (C₆H₅CH₃).
• Key Properties: High stability, resonance energy, undergo substitution reactions rather than addition reactions.

2. Classification of Organic Compounds

Organic compounds are broadly classified based on their structure and functional groups:

1. Acyclic or Open-Chain Compounds: Straight or branched chains (e.g., methane, ethane).
2. Cyclic Compounds: Carbon atoms form a ring.

• Aromatic: Contain benzene rings (e.g., naphthalene).
• Aliphatic: No benzene ring (e.g., cyclohexane).

1. Functional Groups: Specific atoms or groups determine properties (e.g., alcohol, aldehydes).

3. Carbonyl Compounds

Carbonyl compounds are organic molecules that contain a carbonyl group (C=O). They are classified into:

• Aldehydes (RCHO): Carbonyl group bonded to one hydrogen and one R group (e.g., formaldehyde).
• Ketones (RCOR'): Carbonyl group bonded to two R groups (e.g., acetone).
• These compounds are reactive due to the polarity of the carbonyl group, making them susceptible to nucleophilic addition reactions.

4. Homologous Series

A homologous series is a group of organic compounds with the same general formula and similar chemical properties, differing by a -CH₂- unit.

• Example: Alkanes (CnH2n+2), Alcohols (CnH2n+1OH).
• Importance:
• Predictable trends in physical properties (boiling point, melting point).
• Similar chemical reactivity.

5. Why Phenols Are More Acidic than Alcohols?

Phenols (C₆H₅OH) are more acidic than alcohols due to:

1. Resonance Stabilization of the Phenoxide Ion:

• When phenol loses a proton (H⁺), the resulting phenoxide ion (C₆H₅O⁻) is stabilized by resonance, spreading the negative charge over the aromatic ring.

1. No Such Stabilization in Alcohols:

• Alcohols like ethanol lose H⁺ to form an alkoxide ion (R-O⁻), which lacks resonance stabilization and remains unstable.

1. Inductive Effect:

• The electron-withdrawing effect of the aromatic ring further enhances the acidity of phenol.

Thus, phenols are acidic enough to react with bases like NaOH, while alcohols are not.