ChemistryCHEM 10
Haloalkanes & Haloarenes
SN1, SN2, E1, E2 mechanisms, Grignard reagent, nucleophilic aromatic substitution
Concept Core
Essential theory — everything NCERT tests on Haloalkanes & Haloarenes
CLASSIFICATION & PREPARATION
Haloalkanes (alkyl halides): R-X. Haloarenes: Ar-X. C-X bond: polar (δ+C, δ-X). Reactivity: RI > RBr > RCl > RF.
Preparation: from alcohols (HX, SOCl₂), from hydrocarbons (halogenation), from alkenes (HX addition — Markovnikov).
SN1 vs SN2 MECHANISMS
SN1: 2 steps, carbocation intermediate, racemisation, polar protic solvent, favoured by 3° > 2° > 1°. Rate = k[RX].
SN2: 1 step, backside attack, inversion (Walden inversion), polar aprotic solvent, favoured by CH₃X > 1° > 2° > 3°. Rate = k[RX][Nu⁻].
ELIMINATION REACTIONS
E2: bimolecular, anti-periplanar arrangement, strong base, Zaitsev product (more substituted alkene).
E1: via carbocation, unimolecular, less common. Hofmann elimination: bulky base → less substituted product.
Competing reactions: SN vs E. High T and bulky base → elimination. Low T → substitution.
GRIGNARD & ORGANOMETALLIC
RMgX (Grignard reagent) formed in dry ether. Very reactive. Attacks C=O (aldehydes, ketones, esters) → alcohols. CO₂ → carboxylic acid. Cannot tolerate -OH, -NH, -COOH groups (react with Grignard).
HALOARENES & NAS
C-X bond in haloarenes: stronger due to partial double bond character (resonance). Difficult to hydrolyse.
Nucleophilic aromatic substitution: requires electron-withdrawing groups (NO₂) ortho/para to X. Addition-elimination (Meisenheimer complex) mechanism.
Reactions: Ullmann, Wurtz-Fittig. Diastereomers and enantiomers in haloalkane stereochemistry.
Fact & Formula Vault
High-yield facts, numbers, and formulas
SN1 vs SN2
SN1: 3°>2°>1°, racemisation
SN2: CH₃>1°>2°>3°, inversion
SN1: polar protic solvent
SN2: polar aprotic solvent
SN1: rate=k[RX], SN2: rate=k[RX][Nu]
Grignard Reactions
RMgX + HCHO → 1° alcohol
RMgX + RCHO → 2° alcohol
RMgX + R₂CO → 3° alcohol
RMgX + CO₂ → RCOOH
Nucleophilicity Order
I⁻ > Br⁻ > Cl⁻ > F⁻ (aprotic)
F⁻ > Cl⁻ > Br⁻ > I⁻ (protic)
RS⁻ > RO⁻ (soft base, better Nu)
Neutral: NH₃ > H₂O
Worked Examples
NEET-style questions solved step-by-step
EASYWhich alkyl halide undergoes SN1 more readily?▾
Which alkyl halide undergoes SN1 more readily?
Tertiary (3°) alkyl halide. SN1 proceeds through carbocation intermediate. Stability: 3° > 2° > 1° > CH₃. Polar protic solvents (water, alcohol) favour SN1.
MEDIUMGrignard reagent is prepared in dry ether. Why must the solvent be dry?▾
Grignard reagent is prepared in dry ether. Why must the solvent be dry?
Grignard reagent (RMgX) reacts readily with water/moisture: RMgX + H₂O → RH + Mg(OH)X. The reactive C-Mg bond is protonated. Dry conditions essential to prevent decomposition.
HARDWhy are haloarenes less reactive toward nucleophilic substitution than haloalkanes?▾
Why are haloarenes less reactive toward nucleophilic substitution than haloalkanes?
In haloarenes, C-X bond has partial double bond character due to resonance between ring and halogen lone pairs. Bond is stronger, shorter. Also, the benzene ring is electron-rich — repels nucleophiles.
Mistake DNA
Common NEET traps for this chapter
⚠ SN1 racemisation
SN1 proceeds via planar carbocation → nucleophile can attack from either face → racemic mixture (50% inversion + 50% retention). NOT pure inversion.
✓ Fix: SN1 = racemisation. SN2 = complete inversion (Walden).
⚠ Nucleophilicity in protic vs aprotic
In protic solvents: I⁻ > Cl⁻ (polarisability). In aprotic solvents: F⁻ > Cl⁻ (charge density). Students often confuse the two solvent conditions.
✓ Fix: Solvent type changes nucleophilicity ORDER.
⚠ Grignard limitations
Grignard fails in the presence of active H: -OH, -COOH, -NH groups, water. These protonate the Grignard before it can react with target.
✓ Fix: Keep Grignard anhydrous and free of protic functional groups.
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
2–3 Q/year. SN1 vs SN2 substrate preference, Grignard reactions (which alcohol type), haloarene NAS, Walden inversion.
High-Yield
SN2: inversion, CH₃ > 1° > 2° > 3°. SN1: racemisation, 3° > 2° > 1°. Grignard + HCHO → 1° alcohol. Haloarenes: resistant to nucleophilic substitution.
Strategy
Know SN1 vs SN2 table cold. Grignard alcohol type: HCHO→1°, RCHO→2°, R₂CO→3°. NAS: needs -NO₂ ortho/para activation.
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