Alkyl halide and amines (Sindh Board) 2nd year keynotes and solved exercise

6.1 Alkyl halides: primary, secondary, tertiary

Definition: Monohaloalkanes R–X, general formula CₙH₂ₙ₊₁X. Halogen (F, Cl, Br, I) bonded to sp³ carbon. 1°: C bonded to 1 C (e.g. 1-bromopropane) 2°: 2-bromopropane (CH₃–CHBr–CH₃) 3°: 2-bromo-2-methylpropane (CH₃–C(CH₃)₂–Br).

1°: CH₃–CH₂–CH₂–Br 2°: CH₃–CHBr–CH₃ 3°: (CH₃)₃C–Br 🔹 halogen-bearing carbon: 1,2,3 carbons attached

🧠 MEMORY PALACE: “Primary = one alkyl neighbor, secondary = two, tertiary = three. Methyl halide = no carbon attached to C–X (special).”

6.1.1 Structure: sp³ hybridized, methyl iodide

Methyl chloride: carbon sp³, four sp³ orbitals, bond angles 109°. Sigma bond C–Cl (sp³–p) and C–H (sp³–s). Methyl iodide: same hybridization, C–I bond longer due to larger iodine. Tetrahedral geometry.

Orbital picture: CH₃–I : I H
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H–C–I (sp³)
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H sp³ hybridized, 4 sigma bonds

✏️ QUICK HACK: “sp³ = 109.5°, imagine tripod + halogen. Methyl iodide: big I atom but still tetrahedral.”

6.1.2 Physical properties – elaborated

State: CH₃Cl, CH₃CH₂Br gases; C₅–C₁₈ liquids; >C₁₈ solids (colourless). Solubility: insoluble in water (no H-bond); soluble in organic solvents. BP/MP > alkanes due to dipole-dipole and van der Waals (larger halogen higher BP). Density: bromo, iodo alkanes denser than water.

🌊 TREND: RI > RBr > RCl > RF boiling point; iodides highest polarizability.

6.1.3 Preparation of alkyl halides

(A) Alcohol + HX: R–OH + HX → R–X + H₂O. Reactivity HI>HBr>HCl. For HCl: anhydrous ZnCl₂ catalyst.

C₂H₅OH + HCl —ZnCl₂—> C₂H₅Cl + H₂O ethyl chloride

(B) Phosphorus trihalide: 3 R–OH + PX₃ → 3 R–X + H₃PO₃ (X = Cl, Br, I).

3 C₂H₅OH + PCl₃ → 3 C₂H₅Cl + H₃PO₃

(C) Thionyl chloride (best): R–OH + SOCl₂ (pyridine) → R–Cl + SO₂↑ + HCl↑. Gaseous byproducts, pure alkyl halide.

CH₃CH₂OH + SOCl₂ → CH₃CH₂Cl + SO₂ + HCl

(D) Halogenation of alkane (free radical): CH₄ + Cl₂ —UV→ CH₃Cl + HCl (mixture, not efficient).

🧪 BEST METHOD: “Thionyl chloride + pyridine = no byproduct mess. PCl₃ works but produces H₃PO₃.”

6.1.4 Nucleophilic substitution (SN1 & SN2)

Polar C–X bond makes carbon electrophilic. SN1: 2-step, carbocation intermediate, 1st order kinetics. Rate = k[R–X]. Favoured by 3° alkyl halides, polar protic solvents. (CH₃)₃C–Br slow → (CH₃)₃C⁺ then +Nu⁻

SN1: (CH₃)₃C–Br → (CH₃)₃C⁺ + Br⁻ (slow, RDS) → (CH₃)₃C–OH (with H₂O)

SN2: concerted, backside attack, inversion. Rate = k[R–X][Nu⁻]. Favoured by 1° and methyl halides.

HO⁻ + CH₃–Br → [HO···CH₃···Br]‡ → HO–CH₃ + Br⁻

Carbocation stability: 3° > 2° > 1° > methyl (alkyl groups donate electron density).

methyl⁺(least) < CH₃⁺ < 1° < 2° < 3° (most stable)

📈 RATE PREDICTION: “SN1: 3° > 2° (carbocation stability). SN2: CH₃ > 1° > 2° > 3° (steric hindrance).”

6.1.5 Elimination: E1 & E2 (β-elimination)

Alcoholic KOH removes H–X from adjacent carbons → alkene. E1: unimolecular, carbocation, rate = k[R–X]. E2: bimolecular, concerted, anti-periplanar, rate = k[R–X][base]. Primary halides E2 example:

CH₃–CH₂–Br + OH⁻ (alc.) → CH₂=CH₂ + H₂O + Br⁻ (ethene)

E2 favoured by strong bulky base (e.g. t‑BuOK), heat. E1 similar conditions to SN1.

⚡ E2/Zaitsev: “More substituted alkene major. anti-periplanar H and leaving group.”

6.1.6 Substitution vs Elimination

Nucleophile/base competition. Substitution: Nu attacks α‑C. Elimination: base attacks β‑H. Solvent: polar protic favours Sub; Base strength: strong bulky base favours Elim; Temp: higher T favours Elim; Substrate: 3° favours E2, 1° favours SN2.

CH₃–CH₂–CH₂–Br + OH⁻ → SN2: CH₃–CH₂–CH₂–OH (major if low T, OH⁻) → E2: CH₃–CH=CH₂ (if heat, conc.)

♟️ MASTER KEY: “Strong bulky base + heat = elimination. Small strong nucleophile + low T = substitution.”

6.2 Grignard reagents (organometallic)

Preparation: R–X + Mg (anhydrous ether) → R–MgX. Ease: I > Br > Cl; smaller alkyl easier. Carbon–magnesium bond polar: C⁻–Mg⁺, carbon acts as strong nucleophile.

CH₃CH₂–Br + Mg (dry ether) → CH₃CH₂–MgBr (ethylmagnesium bromide)

Reactions: with H₂O → alkane (CH₃MgBr + H₂O → CH₄ + Mg(OH)Br). With CO₂ → carboxylic acid after hydrolysis. With ester → tertiary alcohol. With primary amine (CH₃NH₂) → alkane (CH₄).

CH₃MgBr + CO₂ → CH₃COOH (after H₃O⁺) CH₃MgBr + CH₃NH₂ → CH₄ + Mg(NHCH₃)Br

🚗 GRIGNARD GARAGE: “Anhydrous ether = indispensable. RMgX = carbanion equivalent. Never use protic solvents.”

6.3 Amines – all properties, preparation, basicity

Derivatives of NH₃: 1° (R–NH₂), 2° (R₂NH), 3° (R₃N). Physical: methylamine gas, trimethylamine fishy odor. Solubility: H-bond with water, decreases with larger R. N sp³ hybridized, pyramidal, lone pair. Basicity due to lone pair; alkyl groups increase basicity (inductive effect) but in water steric hindrance reduces solubility.

Preparation methods:

Alkylation: CH₃I + NH₃ → CH₃NH₂ + CH₃I → (CH₃)₂NH … mixture

Reduction of nitrile: CH₃–C≡N + 4[H] → CH₃–CH₂–NH₂ (ethylamine).

CH₃CN LiAlH₄ → CH₃CH₂NH₂

Reduction of amide: CH₃CONH₂ + 4[H] → CH₃CH₂NH₂ + H₂O (LiAlH₄).

🧪 PREPARATION TIP: “Alkylation gives mix; better to reduce nitriles/amides to get pure primary amine.”

6.3.6 Amides & Diazonium salt

Amide from amine + acyl chloride: CH₃NH₂ (methylamine) + CH₃COCl (acetyl chloride) —pyridine, heat→ CH₃CONHCH₃ (N‑methylacetamide) + HCl.

CH₃NH₂ + Cl–CO–CH₃ → CH₃–CO–NH–CH₃ (N-methylacetamide)

Diazonium salt: R–NH₂ + HNO₂ + HCl < 10°C → R–N₂⁺Cl⁻. Example: C₂H₅NH₂ → C₂H₅–N≡N Cl⁻ (ethyl diazonium chloride). Unstable above 10°C.

CH₃CH₂NH₂ + NaNO₂/HCl, <10°C → CH₃CH₂–N₂⁺Cl⁻

❄️ DIAZONIUM RULE: “Keep ice-cold! Otherwise N₂ escapes and phenol forms.”

Alkyl Halides & Amines

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