Chapter 6: Haloalkanes

Classification Based on Hybridization

• Alkyl Halides (R-X): Halogen bonded to sp³ hybridized carbon. (1°, 2°, or 3°).
• Allylic Halides: Halogen bonded to sp³ carbon next to a C=C double bond. Example: CH₂=CH-CH₂X.
• Benzylic Halides: Halogen bonded to sp³ carbon attached to an aromatic ring. Example: C₆H₅CH₂X.
• Vinylic Halides: Halogen bonded directly to sp² hybridized carbon of a C=C double bond. Example: CH₂=CH-X.
• Geminal & Vicinal Dihalides: Geminal (both halogens on same carbon), Vicinal (halogens on adjacent carbons).

Nature of C-X Bond & Dipole Moment

C-X bond is highly polarized (C^δ+ - X^δ-). Bond length increases from C-F to C-I. Bond enthalpy decreases from C-F to C-I.

Reason: Though F is more electronegative, the C-Cl bond distance is larger, making the product of charge and distance (dipole moment) highest for CH₃Cl.

From Alcohols (Best Methods)

• Darzen's Process (Best): R-OH + SOCl₂ → R-Cl + SO₂↑ + HCl↑. (Preferred because byproducts are escapable gases).
• Lucas Reagent: HCl + anhy. ZnCl₂ (Groves' Process). 3° alcohols react instantly.

From Hydrocarbons

• Markovnikov's Rule: Addition of HX to unsymmetrical alkenes. Negative part (X⁻) goes to carbon with fewer hydrogens.
• Kharasch Effect (Peroxide Effect): Anti-Markovnikov addition of HBr (ONLY HBr) in presence of peroxide. Br goes to carbon with MORE hydrogens.
• Allylic Bromination (NBS): N-Bromosuccinimide selectively brominates the allylic position without affecting the C=C double bond.

Halogen Exchange & Silver Salts

• Finkelstein Reaction (Alkyl Iodides): R-X + NaI (in dry acetone) → R-I + NaX.
• Swarts Reaction (Alkyl Fluorides): Heating R-Br with metallic fluorides (AgF, Hg₂F₂, CoF₃).
• Borodine-Hunsdiecker Reaction: Silver salt of carboxylic acid heated with Br₂ in CCl₄ gives alkyl bromide (Step-down reaction).
  R-COOAg + Br₂ → R-Br + CO₂ + AgBr

Boiling Points & Solubility

• Boiling Point Trend: For same alkyl group, BP increases with size of halogen (R-I > R-Br > R-Cl > R-F).
• Isomeric Halides: Boiling point decreases with branching. (Branching makes the molecule more spherical, decreasing surface area and van der Waals forces).
• Solubility: Insoluble in water (cannot form H-bonds) but soluble in organic solvents.

Stereochemistry (Optical Activity)

• Chiral Carbon: A carbon attached to four different groups. It lacks a plane of symmetry.
• Enantiomers: Non-superimposable mirror images. They rotate Plane Polarized Light equally but in opposite directions (dextro '+' and laevo '-').
• Racemic Mixture: Equimolar mixture of two enantiomers. Optically inactive due to external compensation.
• Retention & Inversion: Retention preserves spatial arrangement. Inversion flips it (like an umbrella in wind).

SN2 Mechanism (Bimolecular)

• Kinetics & Steps: Follows 2nd order kinetics. Single-step process without intermediate.
• Stereochemistry: Nucleophile attacks from the back, leading to 100% Inversion of Configuration (Walden Inversion).
• Reactivity Order: Primary (1°) > Secondary (2°) > Tertiary (3°). Driven by Steric Hindrance.

SN1 Mechanism (Unimolecular)

• Kinetics & Steps: Follows 1st order kinetics. Two-step process forming a Carbocation Intermediate.
• Stereochemistry: Carbocation is planar, nucleophile can attack from either side. Leads to Racemization.
• Reactivity Order: Tertiary (3°) > Secondary (2°) > Primary (1°). Driven by Stability of Carbocation.
• Note: Allylic and Benzylic halides show high SN1 reactivity due to resonance-stabilized carbocations.

Substitution vs Elimination

• Aqueous KOH (aq. KOH): Acts as a Nucleophile → Causes Substitution → Forms Alcohol.
• Alcoholic KOH (alc. KOH): Acts as a strong Base → Causes Elimination (Dehydrohalogenation) → Forms Alkene.
• Zaitsev (Saytzeff) Rule: In elimination, the more highly substituted alkene (more alkyl groups around double bond) is the major product.

Why are Haloarenes less reactive towards Nucleophilic Substitution?

• Resonance Effect: Lone pairs on halogen resonate with the benzene ring, giving the C-X bond a partial double bond character (hard to break).
• Hybridization: Carbon in haloarenes is sp² hybridized (more electronegative), holding electron pair tighter than sp³ carbon in haloalkanes.
• Instability of Phenyl Cation: Phenyl cation formed is highly unstable, ruling out SN1 mechanism.

Electrophilic Substitution Reactions

Halogens on benzene are Ortho/Para directing (due to +R effect) but overall Deactivating (due to strong -I inductive effect withdrawing electrons from the ring).

• Undergo Nitration (HNO₃/H₂SO₄), Sulfonation (H₂SO₄), and Friedel-Crafts Alkylation/Acylation (AlCl₃) at ortho and para positions.

Reaction with Metals

• Wurtz Reaction: 2R-X + 2Na (dry ether) → Alkane (R-R)
• Fittig Reaction: 2Ar-X + 2Na (dry ether) → Biphenyl (Ar-Ar)
• Wurtz-Fittig Reaction: Ar-X + R-X + 2Na (dry ether) → Alkylarene (Ar-R)
• Grignard Reagent: R-X + Mg (dry ether) → R-Mg-X. (Prepared in anhydrous conditions because water instantly converts it into an alkane).

Important Compounds & Impact

• Chloroform (CHCl₃): Slowly oxidized by air in presence of light to an extremely poisonous gas, Phosgene (COCl₂). Stored in dark bottles with 1% ethanol (ethanol converts phosgene to harmless diethyl carbonate).
• Iodoform (CHI₃): Used as an antiseptic. Its antiseptic action is due to the liberation of free iodine (I₂), not the compound itself.
• Freons (CFCs): Excellent refrigerants but cause depletion of the ozone layer via free radical chain reactions in the stratosphere.
• DDT: Highly effective insecticide but banned because it is non-biodegradable, causes biomagnification, and is toxic to wildlife.