Activity 01
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.
Compare homolytic and heterolytic bond fission.
Facilitation TipEncourage students to verbalise each step as they model it to reinforce their understanding.
What to look forUse 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.
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Activity 02
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.
Explain the mechanism for the free-radical substitution of methane.
Facilitation TipHave a clear, correct version of the mechanism ready to show immediately for instant feedback.
What to look forSet 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.
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Activity 03
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.
Analyse the electrophilic addition mechanism of bromine to ethene.
Facilitation TipInclude at least one correctly drawn mechanism to keep students on their toes.
What to look forProvide 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.
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Generate Complete Lesson→A few notes on teaching this unit
Start by firmly establishing the two types of bond breaking: homolytic for radicals and heterolytic for ions. Use analogies to make this stick. When introducing mechanisms, model the drawing process slowly and deliberately on the board, verbalising every step and the reasoning behind each curly arrow. Insist on neat, precise drawings from students from the very beginning.
Students will be able to draw and explain the step-by-step mechanisms for key organic reactions, using the formal language of curly arrows to accurately describe the movement of electrons.
Watch Out for These Misconceptions
Curly arrows show the movement of atoms or molecules.
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.
Free radicals are ions because they are reactive.
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.
The double bond in ethene attacks with both the sigma and pi bonds.
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.
Methods used in this brief