Is it correct to say: Confusing SN1 (unimolecular, carbocation intermediate) with SN2 (bimolecular, concerted)?

SN1: 1st order, racemization, polar protic solvent, 3° > 2°. SN2: 2nd order, inversion, polar aprotic, 1° > 2°.

Is it correct to say: Writing Wurtz reaction using aqueous solvent?

Wurtz needs DRY ether; water would react with sodium.

Is it correct to say: Assuming aryl halides undergo SN2 easily?

Haloarenes are inert to normal SN. They need harsh conditions (Dow process, NaOH, 623 K, 300 atm).

Is it correct to say: Using alcoholic KOH for hydrolysis?

Alcoholic KOH → elimination (E2, gives alkene). Aqueous KOH → substitution (SN, gives alcohol).

Is it correct to say: Forgetting Saytzeff/Zaitsev's rule and predicting the terminal alkene?

Major product is the MORE substituted (thermodynamically more stable) alkene.

Is it correct to say: Ignoring solvent effects on SN1 vs SN2?

SN1 prefers polar protic (stabilizes carbocation); SN2 prefers polar aprotic (leaves nucleophile bare).

Halogen Derivatives

Haloalkanes, haloarenes, SN1, SN2, elimination, uses — Maharashtra HSC Chemistry Ch 10

📐 11 formulas✏️ 3 examples🎯 5 practice⚖️ 4 marks🏫 Maharashtra State Board📚 Class 12✓ 2025–26 syllabus

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Board Exam Tips

→SN1 vs SN2 mechanism comparison in tabular form (rate, stereochemistry, solvent effect, substrate order) is a Maharashtra HSC favourite (3 marks).
→Draw the transition state for SN2 clearly — pentavalent carbon with 5 partial bonds. Board is strict.
→For haloarenes explain the low reactivity: resonance stabilization + partial double-bond character of C–X bond + sp² hybridization + no back-side attack possible.
→Reactions with alcoholic vs aqueous KOH: alcoholic KOH gives elimination (alkene), aqueous KOH gives substitution (alcohol). Distinguish clearly.
→IUPAC naming: identify longest carbon chain including halogen; number to give lowest locant.

📐 Formulas(11)

SN2 Rate Law★ Board fav

Rate = k [R-X] [Nu^{-}]

SN1 Rate Law★ Board fav

Rate = k [R-X]

Reactivity Order (SN1)

3° > 2° > 1° > methyl

Reactivity Order (SN2)★ Board fav

methyl > 1° > 2° > 3°

Halide Leaving Group Order

R - I > R - B r > R - C l > R - F

Elimination E1 vs E2

E 2 : concerted, anti-periplanar; E 1 : stepwise via carbocation

Wurtz Reaction★ Board fav

2 R - X + 2 Na dry ether R - R + 2 NaX

Finkelstein Reaction

R - C l + NaI acetone R - I + NaCl

Swarts Reaction

R - C l + AgF ⟶ R - F + AgCl

Grignard Reagent Formation★ Board fav

R - X + Mg dry ether R - MgX

Rate — Haloalkane vs Haloarene

k_{alkyl halide} ≫ k_{aryl halide}

✏️ Solved Examples

1Solved Exampleeasy3 steps

Write IUPAC names of (i) (CH₃)₂CHCH₂Br, (ii) CH₃CHClCH₂CH₃.

(i) Longest chain has 4 C (butyl); methyl branch at C-3 is really iso-butyl.

(C H_{3})_{2} C H C H_{2} B r \to 1 -bromo-2-methylpropane

2Solved Exampleboard4 steps

Explain why 2-bromobutane on reaction with alc. KOH gives 2-butene as the major product (Saytzeff's rule).

The reaction is E2 elimination in alcoholic KOH.

C H_{3} C H B r C H_{2} C H_{3} alc. KOH alkene + HBr

3Solved ExampleHOTS4 steps

An optically active alkyl halide (S)-2-bromobutane undergoes SN2 reaction with hydroxide ion. Predict the stereochemistry of the product.

SN2 proceeds via back-side attack of OH⁻ on the electrophilic C.

OH^{-} + (S) -2-bromobutane \to TS \to product + Br^{-}

⚠️ Traps & Common Mistakes

⚠️Common Mistakes6

1
Confusing SN1 (unimolecular, carbocation intermediate) with SN2 (bimolecular, concerted)
✓SN1: 1st order, racemization, polar protic solvent, 3° > 2°. SN2: 2nd order, inversion, polar aprotic, 1° > 2°.
2
Writing Wurtz reaction using aqueous solvent
✓Wurtz needs DRY ether; water would react with sodium.
3
Assuming aryl halides undergo SN2 easily
✓Haloarenes are inert to normal SN. They need harsh conditions (Dow process, NaOH, 623 K, 300 atm).
4
Using alcoholic KOH for hydrolysis
✓Alcoholic KOH → elimination (E2, gives alkene). Aqueous KOH → substitution (SN, gives alcohol).
5
Forgetting Saytzeff/Zaitsev's rule and predicting the terminal alkene
✓Major product is the MORE substituted (thermodynamically more stable) alkene.
6
Ignoring solvent effects on SN1 vs SN2
✓SN1 prefers polar protic (stabilizes carbocation); SN2 prefers polar aprotic (leaves nucleophile bare).

🎯 Practice Yourself

🎯Practice Yourself5 questions

Q1
Complete: CH₃CH₂Br + KOH (aq) → ?
Q2
Identify the product: CH₃CH₂Br + Mg (dry ether) → ?
Q3
Which is faster in SN1: 2-chloro-2-methylpropane or 1-chlorobutane?
Q4
Give the IUPAC name of CH₃CH=CH−CH₂Cl.
Q5
Why is chlorobenzene less reactive than benzyl chloride toward nucleophiles?

📝 Notes

Halogen Derivatives — Maharashtra HSC Overview

Chapter 10 of the Balbharati Class 12 Chemistry textbook covers alkyl and aryl halides, their preparation, physical properties, mechanisms of nucleophilic substitution and elimination, and important named reactions.

Maharashtra syllabus specifics

SN1 vs SN2 tabular comparison with attention to rate law, stereochemistry, solvent, and substrate order is a signature Maharashtra Board question.
Named reactions — Wurtz, Wurtz–Fittig, Fittig, Finkelstein, Swarts — are listed explicitly in Balbharati and asked as 2-markers.
Aryl halide inertness explanation — resonance, sp², partial double bond in C–X — is a required 3-mark explanation.
Grignard reagent and its uses (with aldehyde, ketone, ester, CO₂) is a Balbharati-specific application-oriented topic.
Environmental effects of chlorofluorocarbons (CFCs), DDT, halon fire extinguishers — often asked as 2-mark descriptive questions.

Learning tips

Memorise SN1 vs SN2 tabular comparison — appears every year in some form.
For every named reaction, write: reactants → conditions → product with a curly-arrow mechanism if asked.
Draw curved arrows for mechanism — Balbharati marking scheme rewards clarity.
Learn IUPAC naming rules for polyhalogenated compounds — alphabetical order for prefixes.