Reaction Mechanisms
Quick Notes
- Reaction mechanisms show how bonds break and form during a reaction.
- Free-radical mechanisms use dots (•) to represent unpaired electrons.
- Curly arrows are NOT required for radical mechanisms at this level.
- Other mechanisms (e.g., nucleophilic and electrophilic reactions) use curly arrows to show:
- Bond formation – curly arrow starts from a lone pair or a bond.
- Bond breaking – curly arrow starts from a bond.
Full Notes
Reactions of organic compounds can be explained using mechanisms.
Mechanisms show a possible pathway or sequence of steps that occur when molecules react – they are proposals for how reactions occur.
Specific mechanisms that are required for AQA A-level chemistry can be downloaded here.
Free-Radical Mechanisms
Free radical mechanisms and free radical substitution for AQA has been covered in much more detail here. This page is just an introduction to mechanisms.
Free radicals are species with an unpaired electron (•).
Free-radical substitution occurs in three steps:
- Initiation (Radicals Are Created)
A covalent bond breaks by homolytic fission (each atom gets one electron). - Propagation (Radicals React and Regenerate)
Radicals react with molecules, forming new radicals. - Termination (Radicals Are Removed)
Two radicals combine to form a stable molecule.
All three steps can be combined together to show the free-radical substitution of an alkane with a halogen.
For example: Methane and chlorine to form chloromethane.
Key points:
- Curly arrows are NOT used in radical mechanisms.
- Radicals are represented by dots (•).
Curly Arrow Mechanisms
How bonds are broken and formed in reactions is represented in mechanisms using curly arrows. Curly arrows show the movement of electron pairs.
The formation of a covalent bond is shown by a curly arrow that starts from a lone pair of electrons or from another covalent bond.
The breaking of a covalent bond is shown by a curly arrow starting from the middle of the bond.
Always be very clear when drawing curly arrows – start the arrows from either a lone pair of electrons or the middle of a bond with the arrow head pointing directly to where the electron pair are going.
Example Nucleophilic Substitution (Halogenoalkanes and OH−)
Curly arrow shows the C-Br bond breaking, with Br− leaving.
Nucleophile (OH−) attacks positively charged carbon.
Curly arrow shows the lone pair forming a new bond.