This lesson provides a final overview of key organic reaction mechanisms, including substitution, addition, elimination, oxidation–reduction, polymerization, and a complete summary of organic chemistry learned so far.

1. Core Concepts (Short Notes)

Organic reaction mechanisms show how and why organic reactions occur. They explain the movement of electrons, which bonds break, and which new bonds form.

Most mechanisms involve:

• Movement of electron pairs (shown by curved arrows)

• Attack by nucleophiles/electrophiles

• Stability of intermediates

• Reaction conditions (heat, catalysts, acids, bases)

2. Types of Organic Reactions

28.1 Substitution Reactions

One atom/group replaces another.

(a) Nucleophilic Substitution (SN1 & SN2)

• Common in alkyl halides.

• SN1: Two-step, forms carbocation, tertiary > secondary > primary.

• SN2: One-step, inversion of configuration, primary > secondary > tertiary.

Examples:

• R–Br + OH⁻ → R–OH + Br⁻

(b) Electrophilic Substitution (Aromatic compounds)

• Benzene undergoes substitution due to delocalized ring.

• Examples: nitration, halogenation, sulfonation.

Example: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O

28.2 Addition Reactions

Typical of alkenes and alkynes.

Hydrogenation: C=C + H₂ (Ni catalyst) → alkane

Halogenation: C=C + Br₂ → dibromo compound

Hydrohalogenation: C=C + HCl → chloroalkane

Hydration: C=C + H₂O → alcohol (acid catalyst)

Markovnikov’s Rule: H attaches to carbon with more hydrogens already.

28.3 Elimination Reactions

Opposite of addition; removal of atoms/groups to form double bonds.

Example (dehydration of alcohols): CH₃CH₂OH → CH₂=CH₂ + H₂O (conc. H₂SO₄)

Example (dehydrohalogenation): R–CH₂–CH₂Br + KOH (alc.) → R–CH=CH₂ + KBr + H₂O

28.4 Oxidation & Reduction Reactions

Alcohol oxidation:

• 1° alcohol → aldehyde → carboxylic acid

• 2° alcohol → ketone

• 3° alcohol → no oxidation

Reduction reactions: Aldehydes/ketones → alcohols (NaBH₄)

28.5 Polymerization

Addition polymerization: Alkenes → polymers

• ethene → polyethene

• chloroethene → PVC

Condensation polymerization: Monomers join with loss of small molecules.

• diacids + diamines → nylon

• diacids + diols → polyester

3. Key Reaction Conditions

• Nickel → hydrogenation

• UV light → free radical substitution (alkanes + halogens)

• H₂SO₄ → dehydration & addition of water

• KMnO₄ → oxidation

• AlCl₃/FeBr₃ → halogenation of benzene

4. Reaction Mechanism Summary Table

5. Tips & Tricks for Exams

• Always identify functional groups first—the mechanism depends on them.

• Use Markovnikov’s rule for addition to asymmetrical alkenes.

• Memorize color changes of oxidizing agents (e.g., orange → green for dichromate).

• SN1 gives racemic mixtures; SN2 gives inversion (important exam point).

• For benzene reactions, always use substitution (not addition).

• In mechanism questions, draw curved arrows to show electron movement.

6. Final Organic Chemistry Summary

Key Families & Their Reactions

• Alkanes: substitution, combustion

• Alkenes/Alkynes: addition, polymerization

• Alcohols: oxidation, dehydration, substitution

• Aldehydes/Ketones: oxidation, reduction, nucleophilic addition

• Carboxylic acids: neutralization, esterification

• Esters: hydrolysis

• Amines: basic reactions, substitution

• Benzene: electrophilic substitution

Essential Skills

• Recognizing mechanisms

• Predicting products

• Writing structural formulas

• Understanding electron flow

• Applying reagent conditions correctly

7. Important Points to Remember

• Mechanisms explain how reactions occur.

• Reaction conditions dramatically influence products.

• Functional groups determine reactivity.

• Stability of intermediates affects pathway (SN1 vs SN2).

Organic chemistry builds progressively—each lesson connects to the next.