Unit 8: Haloalkanes

2. Preparation of Monohaloalkanes

From Alkanes – Free‑Radical Halogenation

Alkanes react with halogens (Cl₂, Br₂) under UV light or heat via a radical chain mechanism.

CH₄ + Cl₂  \(\xrightarrow{h\nu}\)  CH₃Cl + HCl

Selectivity follows the order: tertiary > secondary > primary C–H bonds.

From Alkenes – Hydrohalogenation (Markovnikov Addition)

Addition of HX across a double bond places the halogen on the more substituted carbon.

CH₂=CH₂ + HCl  \(\rightarrow\)  CH₃CH₂Cl   (chloroethane)
CH₃CH=CH₂ + HBr  \(\xrightarrow{\text{peroxide}}\)  CH₃CH₂CH₂Br   (anti‑Markovnikov with peroxide)

From Alcohols

Several reagents convert –OH to –X:

• HX (conc.): R‑OH + HX → R‑X + H₂O (SN1 for tertiary, SN2 for primary).
• Phosphorus trihalide (PX₃): 3 R‑OH + PX₃ → 3 R‑X + H₃PO₃.
• Phosphorus pentahalide (PX₅): R‑OH + PX₅ → R‑X + POX₃ + HX.
• Thionyl chloride (SOCl₂): R‑OH + SOCl₂ → R‑Cl + SO₂ + ↑ G (gas). Preferred for chlorides due to gaseous by‑products.

3. Physical Properties

4. Chemical Properties

Nucleophilic Substitution Reactions

Two mechanistic pathways dominate:

SN1 (Unimolecular)

• Rate‑determining step: formation of a carbocation (R⁺).
• Two‑step: ionization → nucleophilic attack.
• Favored by tertiary haloalkanes, polar protic solvents.
• Outcome: racemization (if chiral center).

R‑X  \(\xrightarrow{\text{slow}}\)  R⁺ + X⁻
R⁺ + Nu⁻  \(\rightarrow\)  R‑Nu

SN2 (Bimolecular)

• Concerted backside attack; inversion of configuration (Walden inversion).
• Favored by primary haloalkanes, strong nucleophiles, aprotic solvents.
• Rate = k[R‑X][Nu⁻].

Nu⁻ + R‑X  \(\rightarrow\)  [Nu…R…X]‡  \(\rightarrow\)  R‑Nu + X⁻

Typical Nucleophilic Substitutions

• Hydrolysis (to alcohol): R‑X + NaOH (aq) → R‑OH + NaX.
• Cyanide substitution (to nitrile): R‑X + KCN (alc.) → R‑CN + KX.
• Aminolysis (to amine): R‑X + excess NH₃ → R‑NH₂ + HX (primary amine); further alkylation possible.
• Williamson ether synthesis: R‑X + R′‑ONa → R‑O‑R′ + NaX (SN2).
• Thioether formation: R‑X + NaSR′ → R‑SR′ + NaX.
• Carbylamine test (for primary amines): R‑CH₂‑X + alcoholic KOH + chloroform → R‑CH₂‑NC + 3 KCl + 3 H₂O (isocyanide foul smell).
• Nitrite/nitroalkane formation: R‑X + AgNO₂ → R‑ONO (alkyl nitrite) (SN2); R‑X + AgNO₂ (heat) → R‑NO₂ (nitroalkane) via rearrangement.

5. Elimination Reactions (Dehydrohalogenation)

Treatment with alcoholic KOH (or NaOH) removes HX to give an alkene.

Saytzeff’s Rule

The more substituted (more stable) alkene is the major product.

CH₃CHBrCH₃ + alc. KOH  \(\rightarrow\)  CH₃CH=CH₂  (propene) + KBr + H₂O

For 2‑bromo‑butane, the major product is 2‑butene (more substituted) rather than 1‑butene.

6. Reduction Reactions

Haloalkanes can be reduced to alkanes:

• HI/Zn: R‑X + Zn + HI → R‑H + ZnX₂.
• Catalytic hydrogenation (H₂/Pd): R‑X + H₂ \(\xrightarrow{\text{Pd/C}}\) R‑H + HX.

7. Wurtz Reaction

Coupling of two alkyl halides with sodium metal yields a higher alkane.

2 R‑X + 2 Na  \(\rightarrow\)  R‑R + 2 NaX

Example: 2 CH₃CH₂Br + 2 Na → CH₃CH₂CH₂CH₃ (butane) + 2 NaBr.
Limitations: works best with primary halides; secondary/tertiary give alkenes due to competing elimination.

8. Trichloromethane (Chloroform, CHCl₃)
Preparation

From ethanol: CH₃CH₂OH + 4 Cl₂ \(\xrightarrow{\text{UV/heat}}\) CHCl₃ + 5 HCl.

From propanone (acetone): CH₃COCH₃ + 3 Cl₂ \(\xrightarrow{\text{UV/heat}}\) CHCl₃ + 3 HCl + HCOOH (formic acid) (via haloform reaction).


Chemical Properties

Oxidation (with O₂, light): 2 CHCl₃ + O₂ → 2 COCl₂ (phosgene) + 2 HCl.
Reduction (with Zn/HCl): CHCl₃ + 4[H] → CH₄ + 3 HCl.
Action on silver powder: 2 CHCl₃ + 6 Ag → C₂H₂ (acetylene) + 6 AgCl.
Reaction with conc. HNO₃: CHCl₃ + 3 HNO₃ → CCl₃NO₂ (chloropicrin) + 3 H₂O.
Reaction with propanone (acetone) in presence of base: CHCl₃ + CH₃COCH₃ → CH₃CCl₂COCH₃ (chloroacetone) + HCl (via haloform).
Aqueous alkali (haloform reaction): CHCl₃ + 4 NaOH → HCOONa (sodium formate) + 3 NaCl + 2 H₂O.


Summary
Monohaloalkanes serve as versatile intermediates in organic synthesis. Their reactivity is governed by the carbon‑halogen bond strength, the stability of possible carbocations or transition states, and the nature of the nucleophile/base employed. Mastery of nomenclature, preparation methods, physical trends, and the diverse substitution, elimination, reduction, and coupling reactions equips students to predict outcomes and design synthetic routes effectively.