IB Chemistry How to Solve: IB Chemistry HL – Organic Synthesis & Mechanisms (SN1/SN2, E1/E2, Resonance, FMO Theory)

How to Solve: IB Chemistry HL – Organic Synthesis & Mechanisms (SN1/SN2, E1/E2, Resonance, FMO Theory)

Complete Guide

Introduction

"Mastering SN1/SN2, E1/E2, resonance, and FMO theory doesn’t just get you 10+ marks on your IB Chemistry HL exam—it lets you predict how drugs, plastics, and even DNA are built in real labs. One wrong mechanism choice? Zero marks. One correct arrow-push? Full credit."

WHAT YOU NEED TO KNOW FIRST

1. Basic organic structures – You must recognize primary, secondary, tertiary carbons and halides.
2. Nucleophiles vs. bases – Know the difference (nucleophiles attack carbons; bases attack hydrogens).
3. Arrow-pushing rules – Curly arrows show electron movement; they start at lone pairs or bonds and end at atoms or bonds.

KEY TERMS & FORMULAS

1. Substitution (SN1 vs. SN2)

2. Elimination (E1 vs. E2)

3. Resonance

4. FMO Theory (Frontier Molecular Orbital)

STEP-BY-STEP METHOD

Step 1: Identify the Reaction Type

• Substitution? Look for a nucleophile (e.g., OH⁻, CN⁻, NH₃).
• Elimination? Look for a strong base (e.g., OH⁻, t-BuO⁻, NaNH₂).
• Both possible? Check substrate (1° = SN2/E2; 3° = SN1/E1).

Step 2: Determine the Mechanism (SN1/SN2/E1/E2)

Step 3: Draw the Mechanism

• SN2: One step. Nucleophile attacks from backside. LG leaves simultaneously.
• SN1: Two steps. (1) LG leaves → carbocation. (2) Nucleophile attacks.
• E2: One step. Base removes H anti-periplanar to LG. Double bond forms.
• E1: Two steps. (1) LG leaves → carbocation. (2) Base removes H⁺.

Step 4: Check for Resonance (If Applicable)

• If a carbocation or anion forms, draw resonance structures.
• Identify the major contributor (most stable structure).

Step 5: Apply FMO Theory (If Required)

• HOMO = Nucleophile’s orbital (e.g., lone pair on OH⁻).
• LUMO = Electrophile’s orbital (e.g., C-X σ in alkyl halide).
• Strongest reaction when HOMO and LUMO energies are close.

Step 6: Predict the Product

• Substitution: Replace LG with nucleophile.
• Elimination: Remove H and LG, form double bond.
• Resonance: Draw all possible structures, circle the major one.

WORKED EXAMPLES

Example 1 – Basic (SN2 vs. E2)

Question: What is the major product when 1-bromopropane reacts with NaOH in DMSO? Steps:
1. Identify reaction type: NaOH is a strong nucleophile and base → SN2 or E2.
2. Check substrate: 1° alkyl halide → SN2 favored (E2 requires strong base + heat).
3. Draw mechanism: - OH⁻ attacks C-Br from backside. - Br⁻ leaves.
4. Product: Propan-1-ol (SN2 product).

What we did and why: - 1° substrate + strong nucleophile → SN2. - No heat → elimination unlikely.

Example 2 – Medium (SN1 vs. E1 with Resonance)

Question: What is the major product when 2-bromo-2-methylpropane reacts with H₂O? Steps:
1. Identify reaction type: H₂O is a weak nucleophile/base → SN1 or E1.
2. Check substrate: 3° alkyl halide → SN1 or E1.
3. Draw mechanism (SN1): - (1) Br⁻ leaves → tertiary carbocation. - (2) H₂O attacks carbocation → oxonium ion. - (3) Deprotonation → 2-methylpropan-2-ol.
4. Check for elimination (E1): - (1) Br⁻ leaves → tertiary carbocation. - (2) H₂O removes H⁺ → 2-methylpropene.
5. Determine major product: - SN1 favored at low temps (no heat given). - Product: 2-methylpropan-2-ol.

What we did and why: - 3° substrate + weak nucleophile → SN1. - No heat → substitution dominates.

Example 3 – Exam-Style (Disguised FMO + Resonance)

Question: Explain why the reaction of CH₃Br with NH₃ is faster than with H₂O, using FMO theory. Steps:
1. Identify nucleophiles: NH₃ (stronger) vs. H₂O (weaker).
2. Compare HOMO energies: - NH₃ has a higher-energy HOMO (less electronegative N vs. O). - Higher HOMO = better overlap with LUMO (C-Br σ).
3. FMO interaction: - Stronger overlap → faster reaction.
4. Conclusion: NH₃ reacts faster because its HOMO is closer in energy to the C-Br LUMO.

What we did and why: - FMO theory explains reactivity differences. - Higher HOMO = better nucleophile.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP

"Listen up—this is your 60-second survival guide. For substitution: 1° = SN2, 3° = SN1. For elimination: strong base + heat = E2, weak base = E1. Resonance? Draw all structures, pick the one with full octets and minimal charge. FMO? HOMO attacks LUMO—higher HOMO = faster reaction. Solvents matter: polar aprotic = SN2/E2, polar protic = SN1/E1. And if the question says ‘explain using FMO,’ don’t just say ‘stronger nucleophile’—compare orbital energies. Now go crush that exam."