Reaction and mechanism types
Reaction and mechanism types
- Organic reactions fall into a few named types.
- Mechanisms show, step by step, how the electrons move.
- A curly arrow tracks each electron pair.
Types of reaction
| Reaction | What happens |
|---|---|
| addition | two molecules join into one |
| substitution | one atom/group is swapped for another |
| elimination | a small molecule is removed, making a double bond |
| hydrolysis | a molecule is split apart by water |
| condensation | two molecules join, losing a small molecule (e.g. water) |
- For organic redox, $[\text{O}]$ = one oxygen from an oxidising agent, $[\text{H}]$ = one hydrogen from a reducing agent.
Practice
Match each reaction type to what happens.
Addition joins; substitution swaps; elimination removes a small molecule to form a double bond.
Practice
In an addition reaction:
Addition combines two molecules into a single product (e.g. across a C=C bond).
Types of mechanism
- A free-radical reaction has three steps: initiation (radicals made), propagation (radicals react, make new radicals), termination (radicals join, chain stops).
- Main mechanisms: free-radical substitution (alkanes), electrophilic addition (alkenes), nucleophilic substitution (halogenoalkanes), nucleophilic addition (carbonyls).
- A curly arrow shows a pair of electrons moving (from a bond or a lone pair).
Practice
Put the steps of a free-radical mechanism in order.
Initiation makes radicals, propagation keeps the chain going, termination joins radicals to stop it.
Practice
In a mechanism, a curly arrow shows:
A curly arrow represents the movement of a pair of electrons, starting from a bond or a lone pair.
You've got it
Key idea
- reaction types: addition, substitution, elimination, hydrolysis, condensation; $[\text{O}]$ / $[\text{H}]$ for redox
- a free-radical mechanism runs initiation → propagation → termination
- main mechanisms: free-radical substitution, electrophilic addition, nucleophilic substitution/addition
- a curly arrow = a moving electron pair