Advanced Chemical Principles and Molecular Dynamics · 6th Year · Organic Chemistry · Summer Term

Organic Reaction Mechanisms

Delve into the step-by-step processes of organic reactions, understanding how bonds are broken and formed in substitution and addition reactions.

TL;DR:Let's go beyond simply mixing chemicals and uncover the secret story of how reactions actually happen, one step at a time. We'll become detectives, following the trail of electrons to see how bonds are broken and new molecules are born.

• NCCA Curriculum Specifications
• Leaving Certificate Chemistry Syllabus: Organic Chemistry - Organic Reaction Mechanisms

About This Topic

This topic is a cornerstone of the Organic Chemistry section of the Leaving Certificate syllabus, moving students from simply knowing reactants and products to understanding the dynamic process of a reaction. It introduces the fundamental concepts of bond fission, distinguishing between the formation of radicals (homolytic) and ions (heterolytic). This lays the groundwork for explaining reaction pathways in a logical, step-by-step manner.

The two specific mechanisms mandated by the syllabus, free-radical substitution and electrophilic addition, are explored in detail. For free-radical substitution, the focus is on the reaction of methane and chlorine, emphasising the initiation, propagation, and termination steps and the role of UV light. For electrophilic addition, the reaction of ethene with bromine serves as the key example, introducing the concepts of electrophiles, pi-bond reactivity, and the formation of a carbocation intermediate. Mastery of these mechanisms requires a firm grasp of drawing conventions, particularly the use of 'curly arrows' to denote the movement of electron pairs, which is a vital skill for any further study in chemistry.

Key Questions

1. Compare homolytic and heterolytic bond fission.
2. Explain the mechanism for the free-radical substitution of methane.
3. Analyse the electrophilic addition mechanism of bromine to ethene.

Learning Objectives

• Distinguish between homolytic and heterolytic bond fission, identifying the products of each.
• Describe and illustrate the mechanism of free-radical substitution of methane using initiation, propagation, and termination steps.
• Explain and draw the mechanism for the electrophilic addition of bromine to ethene, correctly identifying the carbocation intermediate.
• Define the terms free radical, electrophile, and nucleophile.
• Correctly use curly arrows to represent the movement of electron pairs in a reaction mechanism.

Key Vocabulary

Mechanism	The step-by-step sequence of elementary reactions by which an overall chemical change occurs.
Homolytic Fission	The breaking of a covalent bond in such a way that each fragment gets one of the shared electrons, forming two free radicals.
Heterolytic Fission	The breaking of a covalent bond in such a way that one fragment gets both of the shared electrons, forming a cation and an anion.
Free Radical	A highly reactive atom or group of atoms with an unpaired electron.
Electrophile	A species that is electron-deficient and accepts a pair of electrons to form a covalent bond.
Carbocation	An organic ion that contains a positively charged carbon atom.

Watch Out for These Misconceptions

Common Misconception

Curly arrows show the movement of atoms or molecules.

What to Teach Instead

Curly arrows exclusively show the movement of a pair of electrons. They always start from a source of high electron density (like a covalent bond or a lone pair) and point towards an electron-deficient atom.

Common Misconception

Free radicals are ions because they are reactive.

What to Teach Instead

Free radicals are electrically neutral atoms or groups of atoms with a single, unpaired electron, formed by homolytic fission. Ions are charged species with a full positive or negative charge, formed by heterolytic fission where one atom takes both electrons.

Common Misconception

The double bond in ethene attacks with both the sigma and pi bonds.

What to Teach Instead

Only the electrons in the pi bond are involved in the electrophilic addition mechanism. The pi bond is weaker and its electrons are more exposed and available for attack compared to the electrons in the stronger, internal sigma bond.

Active Learning Ideas

Simulation Game

Mechanism Modelling with Molymods

Students use molecular model kits to build reactants like methane and ethene. They then physically break bonds and form new ones to represent each step of a mechanism, using paper arrows to show electron movement.

25 min·Pairs

Simulation Game

Whiteboard Relay Race

In small groups, students race to correctly draw a specified reaction mechanism on a large whiteboard. Each student is responsible for drawing one step of the mechanism before passing the marker to the next person.

20 min·Small Groups

Simulation Game

Spot the Mistake

Provide students with several pre-written mechanisms that contain common errors (e.g., incorrect curly arrows, wrong intermediate). In pairs, they must identify and correct the mistakes, explaining the reasoning.

15 min·Pairs

Real-World Connections

• The polymerisation of ethene to produce poly(ethene) is a commercially vital process that proceeds via a free-radical addition mechanism.
• The depletion of the ozone layer in the upper atmosphere is catalysed by chlorine free radicals generated from CFCs, highlighting the environmental impact of radical chain reactions.
• Understanding mechanisms allows pharmaceutical chemists to design synthetic routes to produce new medicines and to minimise the formation of unwanted by-products.
• The ageing process and some diseases in the human body are linked to damage caused by highly reactive free radicals, which is why antioxidants are important in our diet.

Assessment Ideas

Discussion Prompt

Use mini-whiteboards for students to draw the initiation step of a free-radical reaction or the formation of the carbocation in an addition reaction. This allows for a quick check of understanding across the whole class.

Quick Check

Set a past Leaving Certificate exam question on reaction mechanisms. This assesses their ability to recall and apply their knowledge in the format they will face in the state examination.

Peer Assessment

Provide students with a checklist of key features for each mechanism (e.g., 'curly arrow starts from pi bond', 'shows heterolytic fission of Br-Br'). They can use this to peer- or self-assess their own drawn mechanisms.

Frequently Asked Questions

Why is UV light needed for the reaction between methane and chlorine?

UV light provides the initial input of energy required to break the Cl-Cl bond homolytically. This first step, called initiation, produces the highly reactive chlorine free radicals that start the chain reaction.

What is the difference between an electrophile and a nucleophile?

An electrophile is an 'electron lover'; it is an electron-deficient species that accepts an electron pair. A nucleophile is a 'nucleus lover'; it is an electron-rich species with a lone pair of electrons that it can donate to form a new bond.

In the addition of bromine to ethene, why does the Br-Br bond break after the pi bond attacks it?

As the pi bond of ethene attacks one of the bromine atoms, it donates electron density to it. This induces a dipole in the Br-Br bond, weakening it and causing it to break heterolytically, with the other bromine atom taking both electrons to become a bromide ion (Br-).