Haloalkanes — Elimination Reactions

• E1 and E2 elimination mechanisms and how they differ from substitution.
• Zaitsev's rule: the more substituted alkene is the major product.
• Hofmann's rule: when bulky base gives the less substituted alkene.
• Conditions that favour elimination (E) over substitution (SN).

Level 1 — Dehydrohalogenation

Elimination: Removal of HX from a haloalkane to form an alkene. Also called dehydrohalogenation.

CH₃CH₂Br + KOH (alc.) → CH₂=CH₂ + KBr + H₂O

E1 (Unimolecular):

1. Ionisation: R−X → R⁺ + X⁻ (same as SN1 step 1).
2. Base removes β-H from carbocation → alkene. Favoured by: 3° substrates, polar protic, weak base, high temperature.

E2 (Bimolecular): One concerted step: base removes β-H simultaneously as X⁻ leaves → anti-periplanar requirement (H and X must be 180° apart — anti conformation). Favoured by: strong base (KOH/EtOH, t-BuOK), 2°/3° substrates.

Level 2 — Regioselectivity rules

Zaitsev's rule: Elimination preferentially gives the more substituted (more stable, thermodynamically more stable) alkene — major product.

Example: 2-bromobutane + KOH → but-2-ene (major, more substituted) + but-1-ene (minor).

Hofmann's rule: When the base is bulky (e.g., t-BuO⁻/K), steric hindrance prevents attack at less accessible β-H → less substituted alkene formed (Hofmann product).

Competition SN vs E:

JEE tip: Alcoholic KOH (KOH in ethanol) = elimination. Aqueous KOH = substitution. Temperature is the deciding factor.

NCERT spotlight — Anti-periplanar geometry in E2

E2 requires the H and X being eliminated to be anti-periplanar (dihedral angle 180°). This has stereochemical consequences — specific diastereomers react faster, and specific alkene geometry (E/Z) results. In cyclohexane rings: trans-diaxial arrangement required for E2.

Saytzeff vs Hofmann in quaternary ammonium: When quaternary ammonium salt undergoes elimination (Hofmann elimination), the least substituted alkene (Hofmann product) is always obtained regardless of base — different mechanism applies.

Predict major and minor products when 2-bromobutane reacts with KOH in ethanol.

Step 1 — Identify: 2-bromobutane = CH₃CHBrCH₂CH₃; KOH/EtOH → elimination.
Step 2 — β-carbons to C2 (where Br is): C1 (has 3H) and C3 (has 2H).
Step 3 — Possible alkenes:
   Removing H from C1 → but-1-ene (CH₂=CHCH₂CH₃) — less substituted (1 alkyl group on double bond).
   Removing H from C3 → but-2-ene (CH₃CH=CHCH₃) — more substituted (2 alkyl groups on each C of double bond).
Step 4 — Zaitsev: but-2-ene is major product.
Step 5 — but-2-ene can be cis or trans; trans (E) more stable → E-but-2-ene predominates.
Answer: Major = (E)-but-2-ene; Minor = but-1-ene.

Applications — petroleum refining and polymer industry

Elimination reactions are used in petroleum refining to create alkenes (cracking). Alkenes are feedstock for polymerisation (polyethylene, polypropylene). Industrial dehydration of alcohols (H₂SO₄/heat) follows Zaitsev's rule. Pharmaceutical synthesis uses selective elimination to introduce double bonds.

• Write the major product when 2-methylpropan-2-ol undergoes elimination with H₂SO₄/heat.
• How does t-BuOK (potassium tert-butoxide) change the regiochemistry of elimination?
• Give one condition to promote E over SN for a secondary alkyl halide.

Open the Practice tab for graded questions on Elimination Reactions.

• Use the platform-native live simulation or PhET-style tool for this topic (number line, Venn, physics playground, molecule builder, sensor dashboard, etc.).
• Mirror / body / home activity: physically do the concept (count objects, measure, role-play) and photograph or describe for portfolio.
• Voice or text reflection with AI Mentor: explain the concept to a younger student or family member.

• "Explain this concept to a Class 6 student using one real example from an Indian home, school, market, or festival."
• "What is one common mistake students make here, and how would you catch yourself making it?"
• Stretch: "How does this connect to coding, robotics, money, health, environment, or a future career?"

• Complete the core practice + one extension activity (photo, table, short reflection, or mini-project) for base XP + topic badge.
• 5-7 day streak or family discussion note = multiplier + visible artifact in parent/principal dashboard.
• Best real-world application stories (anonymised) featured on class or national leaderboard.

• One hands-on project or measurement using the Drishti kit or household items that makes the concept physical.
• Direct link to at least one Future Skill track (Money Management, Green Tech, Cyber Defenders, Micro-Entrepreneurship, AI Mastery, Sustainable Living, Personality Development).
• Coding extension where relevant (simple script, simulation, or data logging).

This material emphasises experiential "learning by doing", competency (apply/create/analyse), vocational exposure, critical thinking, and multidisciplinary connections. Designed to feed live worlds, AI Mentor (with memory), gamification, robotics, parent analytics, and future skills — not just exam prep.

Portfolio Evidence Idea: Your photo/table/reflection/project + one sentence on "How this helps me in real life or a possible future path."

Open the Practice tab for aligned questions (easy/medium/hard + case-based) with full AI scaffolding.

See curriculum for cross-links and the full future-skills/robotics chapters.