Lesson Plan: Haloalkanes & Haloarenes

Organic Chemistry (Class 12)

Haloalkanes & Haloarenes

This chapter introduces the C-X bond. The concepts of Nucleophilic Substitution ($S_N1/S_N2$) and Stereochemistry (Inversion vs Retention) introduced here are the foundation for the next three chapters (Alcohols, Aldehydes, Amines).

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⚠️ Prerequisites

Hydrocarbons: Markovnikov's Rule and Free Radical Halogenation.
GOC: Stability of Carbocations ($3^\circ > 2^\circ > 1^\circ$) and Inductive Effect.
Isomerism: Basic idea of Chirality/Optical Activity.

🧠 Study Approach

Mechanism First: Don't memorize reactions individually. Categorize them: Is it substitution? Elimination? Metal reaction? For substitution, ask: Is the substrate bulky ($S_N1$) or unhindered ($S_N2$)?

Study Sequence

1. Classification & Nature of C-X Bond

Primary ($1^\circ$), Secondary ($2^\circ$), Tertiary ($3^\circ$). Allylic vs Vinylic halides. Geminal vs Vicinal dihalides.
Nature of Bond: Polar ($C^{\delta+}-X^{\delta-}$). Bond strength decreases from C-F to C-I.

2. Methods of Preparation (Important)

From Alcohols: Best method uses $SOCl_2$ (gaseous byproducts). Lucas Reagent ($ZnCl_2/HCl$).
From Hydrocarbons: Free radical halogenation.
Halogen Exchange: Finkelstein ($NaI$/Acetone) and Swarts ($AgF, Hg_2F_2$).

3. Physical Properties

Boiling Point: Increases with molecular mass. For isomers, decreases with branching (surface area decreases).
Solubility: Slightly soluble in water (despite polarity, cannot form H-bonds).

4. Nucleophilic Substitution (CRITICAL)

$S_N2$: Bimolecular, Single Step, Back-attack, Inversion of config. Order: $1^\circ > 2^\circ > 3^\circ$.
$S_N1$: Unimolecular, Two Steps (Carbocation formation), Racemization. Order: $3^\circ > 2^\circ > 1^\circ$.
Stereochemistry: Chiral carbon, Enantiomers, Racemic mixture.

5. Elimination Reactions ($\beta$-Elimination)

Dehydrohalogenation using alcoholic KOH.
Saytzeff Rule: "Poor get poorer". The alkene with more alkyl groups attached to the double bond is the major product.

6. Reaction with Metals

Grignard Reagent ($R-Mg-X$): Organometallic compound. Extremely reactive towards moisture (forms alkane).
Wurtz Reaction: $2RX + 2Na \to R-R$ (Alkane coupling).

7. Reactions of Haloarenes

Low Reactivity: Why aryl halides don't undergo Nu- substitution easily (Resonance, $sp^2$ carbon).
Dow's Process: Drastic conditions ($623K, 300atm$) to convert Chlorobenzene to Phenol.
EAS: Halogen is ortho/para directing but deactivating.

🎯 How to Practice

$S_N1$ vs $S_N2$ Table: Create a comparison table (Order, Steps, Stereochem, Solvent). Apply this to 10 random halides to predict the mechanism.

Conversion Maps: Start with Ethanol. Convert it to Ethyl Chloride, then to Butane, then to Ethene. Linking these reactions is key for board exams.

Name Reactions: Make flashcards for Finkelstein, Swarts, Wurtz, Wurtz-Fittig, Fittig, and Friedel-Crafts reactions.

📝 Quick Revision Notes

Reactivity Order

$S_N1$: $3^\circ > 2^\circ > 1^\circ$

$S_N2$: $1^\circ > 2^\circ > 3^\circ$

Ambident Nucleophiles

$CN^-$: $KCN \to R-CN$ (Cyanide), $AgCN \to R-NC$ (Isocyanide)
$NO_2^-$: $KNO_2 \to R-ONO$, $AgNO_2 \to R-NO_2$

Grignard Reagent $$R-X + Mg \xrightarrow{\text{Dry Ether}} R-Mg-X$$ $$R-Mg-X + H_2O \to R-H + Mg(OH)X$$

Stereochemistry

Inversion (Walden) in $S_N2$.
Racemization (50% d + 50% l) in $S_N1$.

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