Course Topics

Classification of Alkyl Halides

Type Definition Example
Primary (1°) Halogen attached to a primary carbon (attached to one or no other carbon) CH₃–Cl, CH₃–CH₂–Cl
Secondary (2°) Halogen attached to a secondary carbon (attached to two other carbons) (CH₃)₂CH–Cl
Tertiary (3°) Halogen attached to a tertiary carbon (attached to three other carbons) (CH₃)₃C–Cl
Memory Tip

1° = one carbon neighbor, 2° = two carbon neighbors, 3° = three carbon neighbors!

Structure & Bonding

Alkyl halides consist of an sp³ hybridized carbon bonded to a halogen via a σ bond. The C–X bond is polar due to electronegativity difference.

C–X Bond Polarity: Cδ+–Xδ−
Halogen Electronegativity Bond Energy (kJ/mol)
F 4.0 467
Cl 3.0 346
Br 2.8 290
I 2.5 228
Memory Tip

Bond strength: C–F strongest, C–I weakest. Reactivity order: R–I > R–Br > R–Cl > R–F.

Reactivity Factors

Two main factors control reactivity:

  1. Bond Polarity: Greater electronegativity of halogen → more polar bond → more reactive.
  2. Bond Energy: Weaker bonds break more easily. C–I is weakest, so most reactive.
CRITICAL CONCEPT: Although C–F is most polar, it is least reactive due to very high bond energy. Reactivity is determined by bond energy, not polarity alone.

Overall Reactivity Order: R–I > R–Br > R–Cl > R–F

Memory Tip

Think: “I Break Easily” → Iodide most reactive, Fluoride least.

Nucleophilic Substitution Reactions

A nucleophile (electron-rich) attacks the electrophilic carbon, replacing the leaving group (halide).

R–X
Alkyl Halide
+
Nu⁻
Nucleophile
→
R–Nu
Product
+
X⁻
Leaving Group

Common Nucleophiles: HO⁻, CH₃O⁻, I⁻, CN⁻, NH₃, H₂O

Common Leaving Groups: I⁻, Br⁻, Cl⁻, H₂O (poor: OH⁻, NH₂⁻, RO⁻)

Memory Tip

Good leaving groups are weak bases (stable anions). Poor leaving groups are strong bases.

SN1 Mechanism

Two-step mechanism: First ionization to form carbocation, then nucleophile attack.

  1. Step 1 (Slow): R–X → R⁺ + X⁻ (formation of carbocation)
  2. Step 2 (Fast): R⁺ + Nu⁻ → R–Nu
CRITICAL CONCEPT: SN1 is favored for tertiary alkyl halides, in polar solvents, and gives racemic mixture (50% inversion, 50% retention).

Rate Law: Rate = k [R–X] (unimolecular)

Memory Tip

SN1 = “Substitution Nucleophilic Unimolecular” = One molecule in rate-determining step.

SN2 Mechanism

One-step concerted mechanism: Nucleophile attacks backside while leaving group departs.

Nu⁻
Approaching
→
R–X
Transition State
→
Nu–R
Inverted Product
+
X⁻
Leaving

Key Features:

  • 100% inversion of configuration (Walden inversion)
  • Favored for primary alkyl halides, strong nucleophiles, polar aprotic solvents
  • Steric hindrance slows SN2

Rate Law: Rate = k [R–X] [Nu⁻] (bimolecular)

Memory Tip

SN2 = “Substitution Nucleophilic Bimolecular” = Two molecules in rate-determining step.

SN1 vs SN2 Comparison

SN1 Reaction

  • Two-step mechanism
  • Carbocation intermediate
  • Unimolecular rate law
  • Racemic product mixture
  • Favored for 3° alkyl halides
  • Polar protic solvents favor
  • Weak nucleophiles okay

SN2 Reaction

  • One-step mechanism
  • Transition state, no intermediate
  • Bimolecular rate law
  • 100% inversion of configuration
  • Favored for 1° alkyl halides
  • Polar aprotic solvents favor
  • Strong nucleophiles required
Memory Tip

SN1 = 3° + polar solvent + weak nucleophile. SN2 = 1° + strong nucleophile + backside attack.

Elimination Reactions (E1 & E2)

Elimination reactions remove HX from alkyl halide to form alkene.

R–CH₂–CH₂–X + Base → R–CH=CH₂ + HX

E1 Mechanism: Two-step, carbocation intermediate, favored for 3° halides, polar solvents.

E2 Mechanism: One-step concerted, strong base required, favored for 1° and 2° halides.

CRITICAL CONCEPT: Elimination competes with substitution. Strong bases and high temperatures favor elimination. Bulky bases favor elimination over substitution.
Memory Tip

E1 = like SN1 (carbocation). E2 = like SN2 (concerted). Strong base → E2, weak base → E1/SN1.

Substitution vs Elimination

Factor Favors Substitution Favors Elimination
Substrate 1° and methyl 3° and 2°
Nucleophile/Base Weak base, good nucleophile Strong base, poor nucleophile
Solvent Polar aprotic (SN2), Polar protic (SN1) Polar protic (E1), Less polar (E2)
Temperature Lower temperature Higher temperature
Leaving Group Good leaving group Good leaving group

Example: Aqueous KOH → substitution (SN2). Alcoholic KOH → elimination (E2).

Memory Tip

“Aqueous KOH gives alcohol, Alcoholic KOH gives alkene.”

Carbocation Stability

Carbocations are positively charged carbon intermediates. Stability order:

3° > 2° > 1° > methyl

Reasons for stability:

  • Hyperconjugation: Donation of electron density from adjacent C–H bonds.
  • Inductive effect: Electron-donating alkyl groups stabilize positive charge.
  • Resonance: Delocalization of charge if possible (allylic, benzylic).
CRITICAL CONCEPT: More stable carbocations form faster in SN1/E1. Tertiary carbocations are most stable due to maximum hyperconjugation and inductive donation.
Memory Tip

More alkyl groups = more stable carbocation. 3° is king!

Applications & Importance

Alkyl halides are versatile intermediates in organic synthesis.

  • Functional group interconversion: Convert halides to alcohols, ethers, amines, nitriles, etc.
  • Grignard reagents: R–MgX from alkyl halides used in C–C bond formation.
  • Pharmaceuticals: Many drugs contain halogen atoms or are synthesized via alkyl halides.
  • Polymers: Vinyl chloride (CH₂=CHCl) for PVC production.
  • Agrochemicals: Pesticides and herbicides often contain halogen atoms.
  • Solvents: Dichloromethane, chloroform (though use declining due to toxicity).
CRITICAL CONCEPT: Understanding alkyl halide chemistry is essential for designing synthetic routes in medicinal chemistry, materials science, and industrial chemistry.
Memory Tip

Alkyl halides are the “Swiss Army knife” of organic synthesis—convertible to almost anything!