Study Guide

AHL: Advanced organic reaction mechanisms

IB Chemistry HLΒ· R3: AHL MechanismsΒ· 15 min read

1. Electrophilic Addition to Conjugated Dienesβ˜…β˜…β˜…β˜†β˜†β± 6 min

πŸ“˜ Definition

Electrophilic addition to conjugated dienes

Addition of an electrophile to a conjugated diene forms a resonance-stabilized allylic carbocation intermediate. The nucleophile can attack two different positively charged carbons, producing two distinct addition products.

Example:

Addition of HBr to buta-1,3-diene

The delocalization of the positive charge across the allylic system is the key feature that distinguishes this reaction from electrophilic addition to simple alkenes. Product distribution depends on reaction temperature: kinetic control at low temperature favors the faster-forming 1,2-addition product, while thermodynamic control at high temperature favors the more stable 1,4-addition product.

πŸ“ Worked Example

Draw the complete mechanism for addition of HBr to buta-1,3-diene, and name both products.

  1. 1
    1. The terminal Ο€ bond of the diene attacks the electrophilic partially positive H from HBr. H adds to the terminal carbon to form a resonance-stabilized allylic carbocation and bromide ion.
  2. 2
    CH2=CHβˆ’CH=CH2+HBrβ†’[+CH2βˆ’CH2βˆ’CH=CH2↔CH2βˆ’CH2βˆ’CH+βˆ’CH3]+Brβˆ’CH_2=CH-CH=CH_2 + HBr \rightarrow [^+CH_2-CH_2-CH=CH_2 \leftrightarrow CH_2-CH_2-CH^+-CH_3] + Br^-
  3. 3
    1. Bromide ion attacks the positively charged C1, forming the 1,2-addition product.
  4. 4
    Brβˆ’++CH2βˆ’CH2βˆ’CH=CH2β†’BrCH2βˆ’CH2βˆ’CH=CH2Br^- + ^+CH_2-CH_2-CH=CH_2 \rightarrow BrCH_2-CH_2-CH=CH_2
  5. 5
    1. Bromide ion attacks the positively charged C4, forming the 1,4-addition product.
  6. 6
    Brβˆ’+CH2=CHβˆ’CH2βˆ’CH+CH3β†’CH2=CHβˆ’CH2βˆ’CH(Br)CH3Br^- + CH_2=CH-CH_2-CH^+CH_3 \rightarrow CH_2=CH-CH_2-CH(Br)CH_3

Exam tip:

Always state the effect of temperature on product distribution; this is a common 2-3 mark extended question.

2. Nucleophilic Addition-Elimination of Acyl Derivativesβ˜…β˜…β˜…β˜…β˜†β± 7 min

πŸ“˜ Definition

Nucleophilic addition-elimination

A two-step mechanism for acyl derivatives (acyl halides, esters, amides) where a nucleophile adds to the electrophilic carbonyl carbon, followed by elimination of a leaving group to reform the stable carbonyl double bond.

This mechanism is different from nucleophilic addition to aldehydes and ketones because acyl derivatives have a good leaving group attached to the carbonyl carbon. The reactivity of the acyl derivative correlates directly with the leaving group ability of the attached group: acyl halides are most reactive, amides are least reactive.

πŸ“ Worked Example

Draw the mechanism for base-promoted hydrolysis of methyl ethanoate.

  1. 1
    1. The hydroxide nucleophile attacks the electrophilic carbonyl carbon, breaking the Ο€ bond and forming a negatively charged tetrahedral intermediate.
  2. 2
    CH3COOCH3+OHβˆ’β†’CH3C(Oβˆ’)(OH)OCH3CH_3COOCH_3 + OH^- \rightarrow CH_3C(O^-)(OH)OCH_3
  3. 3
    1. The lone pair on the negatively charged oxygen reforms the carbonyl double bond, eliminating the methoxide leaving group.
  4. 4
    CH3C(Oβˆ’)(OH)OCH3β†’CH3COOH+βˆ’OCH3CH_3C(O^-)(OH)OCH_3 \rightarrow CH_3COOH + ^-OCH_3
  5. 5
    1. The strong base methoxide deprotonates the carboxylic acid to form a stable carboxylate ion and methanol, driving the reaction to completion.
  6. 6
    CH3COOH+βˆ’OCH3β†’CH3COOβˆ’+CH3OHCH_3COOH + ^-OCH_3 \rightarrow CH_3COO^- + CH_3OH

Exam tip:

Never forget the elimination step; examiners actively mark this as a key distinguishing feature from simple nucleophilic addition.

3. Radical Substitution Mechanismsβ˜…β˜…β˜…β˜†β˜†β± 5 min

πŸ“˜ Definition

Radical substitution

A chain reaction mechanism involving species with unpaired electrons (radicals), where a hydrogen atom on an alkane is substituted by a halogen atom.

Radical substitution proceeds in three distinct stages: initiation, propagation, and termination. Initiation requires UV light to break the weak halogen-halogen bond homolytically, forming two reactive halogen radicals. Propagation is the chain stage where radicals react to form new radicals, continuing the reaction. Termination ends the reaction when two radicals combine to form a stable neutral product.

πŸ“ Worked Example

Write the three stages of radical monochlorination of methane.

  1. 1
    1. Initiation: UV radiation breaks the Cl-Cl bond homolytically to form two chlorine radicals.
  2. 2
    Cl2β†’hΞ½2Clβˆ™Cl_2 \xrightarrow{h\nu} 2 Cl^\bullet
  3. 3
    1. Propagation: A chlorine radical abstracts a hydrogen from methane to form a methyl radical and HCl. The methyl radical then reacts with another Cl2 molecule to form chloromethane and a new chlorine radical that continues the chain.
  4. 4
    Clβˆ™+CH4β†’CH3βˆ™+HClCH3βˆ™+Cl2β†’CH3Cl+Clβˆ™Cl^\bullet + CH_4 \rightarrow CH_3^\bullet + HCl \\ CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet
  5. 5
    1. Termination: Two radicals combine to form a stable product, ending the chain reaction.
  6. 6
    CH3βˆ™+Clβˆ™β†’CH3ClCH3βˆ™+CH3βˆ™β†’CH3CH3CH_3^\bullet + Cl^\bullet \rightarrow CH_3Cl \\ CH_3^\bullet + CH_3^\bullet \rightarrow CH_3CH_3

Exam tip:

Always use single-headed fish-hook arrows for single electron movement in radical mechanisms; double-headed arrows are incorrect and will lose marks.

4. Common Pitfalls

Wrong move:

Using double-headed curly arrows for electron movement in radical mechanisms

Why:

Radical mechanisms involve movement of single unpaired electrons, not full electron pairs

Correct move:

Use single-headed (fish-hook) curly arrows to show single electron movement for all radical steps

Wrong move:

Forgetting to reform the carbonyl double bond in nucleophilic addition-elimination

Why:

Students confuse this mechanism with nucleophilic addition to aldehydes/ketones which lack a leaving group

Correct move:

After addition of the nucleophile, show the oxygen lone pair reforming C=O and eliminating the leaving group

Wrong move:

Claiming 1,2-addition is always the major product for conjugated diene reactions

Why:

Product distribution depends entirely on reaction temperature and control type

Correct move:

State 1,2-addition is major under kinetic control (low temperature) and 1,4-addition is major under thermodynamic control (high temperature)

Wrong move:

Drawing only one resonance form of the allylic carbocation in conjugated diene addition

Why:

Examiners require recognition that the intermediate is resonance-stabilized, which is why two products form

Correct move:

Draw both resonance forms connected by a resonance arrow to show delocalization of the positive charge

5. Quick Reference Cheatsheet

Mechanism Type

Key Steps

Exam Key Notes

Electrophilic addition (conjugated dienes)

Add electrophile β†’ resonance allylic carbocation β†’ nucleophile attack

1,2 = kinetic (low T), 1,4 = thermodynamic (high T)

Nucleophilic addition-elimination (acyl)

Add nucleophile β†’ tetrahedral intermediate β†’ eliminate leaving group β†’ reform C=O

Different from aldehyde/ketone addition; always show elimination

Radical substitution (halogenation)

Initiation (homolytic cleavage) β†’ propagation (chain) β†’ termination (radical combination)

Use single-headed arrows, UV light required for initiation

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2025 Β· Paper 2

    Ester hydrolysis mechanism question

  • 2024 Β· Paper 1

    1,2 vs 1,4 addition product prediction

  • 2023 Β· Paper 2

    Radical chlorination of propane

Going deeper

What's Next

This sub-topic builds on your foundational knowledge of organic reaction mechanisms and forms a core part of the AHL organic chemistry section of IB Chemistry HL. Mastery of these advanced mechanisms allows you to predict product outcomes for a wide range of synthetic organic reactions, which is critical for both multiple choice and extended response questions on the exam. Understanding how intermediate stability and reaction conditions control product distribution also underpins key concepts in multi-step synthesis and stereochemistry. Drawing curly arrows correctly is consistently tested, so practice this skill thoroughly before your exam.