6.7 Chemical Reactions of Haloalkanes and Haloarenes 👩‍🔬

Haloalkanes and haloarenes love to show off three main tricks: nucleophilic substitution 🔁, elimination 🔥, and reactions with metals ⚙️. We’ll walk through each move step-by-step, then flag the hottest NEET take-aways at the end. :contentReference[oaicite:0]{index=0}

6.7.1 Reactions of Haloalkanes 🧲

1 Nucleophilic Substitution (🔁)

  • Big idea: A nucleophile (Nu⁻) dives onto the carbon bearing the halogen (X), kicks X⁻ out, and grabs the spot. The halogen is the “leaving group.”
  • Ambident nucleophiles: CN⁻ and NO₂⁻ can attack through two atoms (C or N; O or N) – so you can get nitriles (RCN) or isonitriles (RNC); alkyl nitrites (R–O–N=O) or nitroalkanes (R–NO₂). 🙌

🛣️ Two possible pathways

  1. SN2 (bimolecular)—one-step, backside attack, inversion of configuration.
    Rate law: \( \text{Rate}=k[\text{R–X}][\text{Nu}^-] \)
    Fastest for methyl > 1° > 2° ≫ 3° because bulky groups block the path. :contentReference[oaicite:1]{index=1}
  2. SN1 (unimolecular)—two-step: (i) the C–X bond breaks to give a carbocation \( (\text{R}^+) \); (ii) the nucleophile jumps on.
    Rate law: \( \text{Rate}=k[\text{R–X}] \)
    Needs a stable carbocation, so 3° > 2° ≫ 1°. Polar protic solvents (water, alcohol) help. :contentReference[oaicite:2]{index=2}

🎯 Stereochemistry snapshot

  • SN2 → Inversion 🌀: the “umbrella” flips inside-out.
  • SN1 → Racemisation 🔄: attack can come from either side of the planar carbocation, giving a 50 : 50 mix of enantiomers.
  • Retention happens when the spatial set-up stays the same (rare in these reactions).

📋 Quick reagent→product guide

  • R–X + OH⁻ → ROH (alcohol)
  • R–X + RO'⁻ → ROR' (ether)
  • R–X + CN⁻ → RCN (nitrile); with AgCN you get RNC (isonitrile)
  • R–X + I⁻ → RI (easy HI swap)

Remember: better leaving group order is I⁻ > Br⁻ > Cl⁻ ≫ F⁻. ⚡ :contentReference[oaicite:3]{index=3}

2 Elimination 🔥 (β-Elimination)

Heat a haloalkane with alcoholic KOH, and a β-H plus the halogen leave together to form an alkene.

\( \text{R–CH}_2\text{–CHX–R’} \xrightarrow[\text{alc.\ KOH}]{\Delta} \text{R–CH=CHR’} + \text{KX} + H_2O \)

Zaitsev’s rule: the favored alkene sports more alkyl groups on the double-bonded carbons. Think of it as “the more substituted, the merrier.” 💃 :contentReference[oaicite:4]{index=4}

3 Reactions with Metals ⚙️

  • Grignard formation: \( \text{R–X} + \text{Mg} \xrightarrow{\text{dry ether}} \text{R–Mg–X} \) → a super-reactive carbon-magnesium bond. Keep water away or you’ll just make RH again! 🚱
  • Wurtz reaction: 2 haloalkanes + Na (dry ether) → a new hydrocarbon with double the carbons (handy chain-doubler).

Substitution vs Elimination 🤔

The winner depends on:

  • Structure: 1° tends to SN2, 3° loves SN1 or E1/E2
  • Base/Nucleophile: bulky → elimination; small/strong → substitution
  • Conditions: higher heat & alcohol solvent push elimination

6.7.2 Reactions of Haloarenes 🪄

1 Nucleophilic Substitution (very slow) 🐢

Why so sluggish?

  1. Resonance: the C–X bond shares π-electrons with the ring → partial double-bond character, shorter (169 pm) and tougher to break.
  2. Hybridisation: the sp2 carbon holds electrons tighter than the sp3 carbon in haloalkanes.
  3. Unstable phenyl cation: no SN1 rescue.
  4. Nucleophile repulsion: electron-rich meets electron-rich → not friendly.

Still, you can swap Cl for OH at 623 K and 300 atm. Add an –NO₂ group at ortho/para and the swap speeds up, because the ring pulls electron density away and stabilises the intermediate. 🎯 :contentReference[oaicite:5]{index=5}

2 Electrophilic Substitution (🔁 on the ring)

The halogen is deactivating (slows the ring), yet it directs new groups to ortho & para spots via resonance.

  • Halogenation: add X₂ (+FeX₃)
  • Nitration: HNO₃ + H₂SO₄ → –NO₂
  • Sulphonation: SO₃ + H₂SO₄ → –SO₃H
  • Friedel-Crafts: R–Cl / RCOCl (+AlCl₃)
🧠 Tip: Chlorine withdraws electrons inductively (–I) but donates by resonance (+R). Resonance wins for direction, –I wins for rate, so the ring is less reactive overall but still o,p-directing.

3 Reactions with Metals ⚙️

  • Wurtz–Fittig: Alkyl Cl/Br/I + aryl Cl/Br/I + Na (dry ether) → alkylarene.
  • Fittig: Two aryl halides + Na (dry ether) → biaryl (Ar–Ar).

Important Concepts for NEET 🎯

  1. SN2 inversion vs SN1 racemisation: know the stereochemical outcome and the rate laws.
  2. Zaitsev’s rule: elimination favors the most substituted alkene.
  3. Order of reactivity: For SN2 → 1° > 2° ≫ 3°; for SN1 → 3° > 2° ≫ 1°; for leaving groups I > Br > Cl ≫ F.
  4. Haloarene resistance 🐢: resonance + sp2 carbon explain the low SN rates; –NO₂ at o/p positions turbo-charges the reaction.
  5. Grignard reagent basics: prepare in dry ether, reacts with any proton source → hydrocarbon. A must-know synthetic tool.

👍 Keep practicing mechanisms, and you’ll ace those reaction-pathway questions!