# AHL: Advanced organic reaction mechanisms

> IB Chemistry HL · R3: What are the mechanisms of chemical change?
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-advanced-organic-reaction-mechanisms/

This AHL module covers three core advanced organic mechanisms for IB Chemistry HL: electrophilic addition to conjugated dienes, nucleophilic addition-elimination of acyl derivatives, and radical substitution. You will learn to draw electron movement and predict products based on reaction conditions.

**Prerequisites:** [Basic organic reaction mechanisms](https://www.owlsprep.com/study/ib-chemistry-hl-basic-organic-reaction-mechanisms/); [Organic functional groups](https://www.owlsprep.com/study/ib-chemistry-hl-organic-functional-groups/)

## Learning objectives

- Distinguish between advanced organic mechanism classes for IB HL AHL
- Draw correct curly arrow mechanisms for the three core reaction types
- Explain the effect of intermediate stability on product distribution
- Predict major products based on reaction conditions and mechanism rules

## Electrophilic Addition to Conjugated Dienes

**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.

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

1. 1. The terminal π 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. $$CH_2=CH-CH=CH_2 + HBr \rightarrow [^+CH_2-CH_2-CH=CH_2 \leftrightarrow CH_2-CH_2-CH^+-CH_3] + Br^-$$
3. 2. Bromide ion attacks the positively charged C1, forming the 1,2-addition product.
4. $$Br^- + ^+CH_2-CH_2-CH=CH_2 \rightarrow BrCH_2-CH_2-CH=CH_2$$
5. 3. Bromide ion attacks the positively charged C4, forming the 1,4-addition product.
6. $$Br^- + CH_2=CH-CH_2-CH^+CH_3 \rightarrow CH_2=CH-CH_2-CH(Br)CH_3$$

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

## Nucleophilic Addition-Elimination of Acyl Derivatives

**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.

**Worked example:** Draw the mechanism for base-promoted hydrolysis of methyl ethanoate.

1. 1. The hydroxide nucleophile attacks the electrophilic carbonyl carbon, breaking the π bond and forming a negatively charged tetrahedral intermediate.
2. $$CH_3COOCH_3 + OH^- \rightarrow CH_3C(O^-)(OH)OCH_3$$
3. 2. The lone pair on the negatively charged oxygen reforms the carbonyl double bond, eliminating the methoxide leaving group.
4. $$CH_3C(O^-)(OH)OCH_3 \rightarrow CH_3COOH + ^-OCH_3$$
5. 3. The strong base methoxide deprotonates the carboxylic acid to form a stable carboxylate ion and methanol, driving the reaction to completion.
6. $$CH_3COOH + ^-OCH_3 \rightarrow CH_3COO^- + CH_3OH$$

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

## Radical Substitution Mechanisms

**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.

**Worked example:** Write the three stages of radical monochlorination of methane.

1. 1. Initiation: UV radiation breaks the Cl-Cl bond homolytically to form two chlorine radicals.
2. $$Cl_2 \xrightarrow{h\nu} 2 Cl^\bullet$$
3. 2. 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. $$Cl^\bullet + CH_4 \rightarrow CH_3^\bullet + HCl \\ CH_3^\bullet + Cl_2 \rightarrow CH_3Cl + Cl^\bullet$$
5. 3. Termination: Two radicals combine to form a stable product, ending the chain reaction.
6. $$CH_3^\bullet + Cl^\bullet \rightarrow CH_3Cl \\ CH_3^\bullet + CH_3^\bullet \rightarrow CH_3CH_3$$

> **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.

## Common pitfalls

- **Wrong:** Using double-headed curly arrows for electron movement in radical mechanisms
  - Why it fails: Radical mechanisms involve movement of single unpaired electrons, not full electron pairs
  - Correct: Use single-headed (fish-hook) curly arrows to show single electron movement for all radical steps
- **Wrong:** Forgetting to reform the carbonyl double bond in nucleophilic addition-elimination
  - Why it fails: Students confuse this mechanism with nucleophilic addition to aldehydes/ketones which lack a leaving group
  - Correct: After addition of the nucleophile, show the oxygen lone pair reforming C=O and eliminating the leaving group
- **Wrong:** Claiming 1,2-addition is always the major product for conjugated diene reactions
  - Why it fails: Product distribution depends entirely on reaction temperature and control type
  - Correct: State 1,2-addition is major under kinetic control (low temperature) and 1,4-addition is major under thermodynamic control (high temperature)
- **Wrong:** Drawing only one resonance form of the allylic carbocation in conjugated diene addition
  - Why it fails: Examiners require recognition that the intermediate is resonance-stabilized, which is why two products form
  - Correct: Draw both resonance forms connected by a resonance arrow to show delocalization of the positive charge

## Cheatsheet

| Mechanism Type | Key Steps | Exam Key Notes |
| --- | --- | --- |
| Electrophilic addition (conjugated dienes) | Add electrophile → resonance allylic carbocation → nucleophile attack | 1,2 = kinetic (low T), 1,4 = thermodynamic (high T) |
| Nucleophilic addition-elimination (acyl) | Add nucleophile → tetrahedral intermediate → eliminate leaving group → reform C=O | Different from aldehyde/ketone addition; always show elimination |
| Radical substitution (halogenation) | Initiation (homolytic cleavage) → propagation (chain) → termination (radical combination) | Use single-headed arrows, UV light required for initiation |

## 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.

- [AHL: Electrolytic cells and standard electrode potentials](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-electrolytic-cells-and-standard/)
- [AHL: Catalysis and reaction mechanisms](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-catalysis-and-reaction-mechanisms/)
- [AHL: Spectroscopic identification of organic compounds](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-spectroscopic-identification-of-organic/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-advanced-organic-reaction-mechanisms/