Terminology Used in Reaction Mechanisms
- In organic reaction mechanisms, curly arrows represent the movement of electron pairs
- The arrow begins at a bond or a lone pair of electrons and points to the species that accepts the lone pair of electrons
Curly arrows show electron pairs moving from the source (eg. a nucleophile) to its destination (eg. an electrophile)
Free-radical substitution
- A free-radical substitution reaction is a reaction in which halogen atoms substitute for hydrogen atoms in alkanes
- It involves the initiation, propagation and termination steps
Example of a free-radical substitution reaction to form chloromethane from methane
Electrophilic addition
- An electrophilic addition reaction is a reaction in which an electron rich region in a molecule is attacked by an electrophile (a species that likes electrons/negative charge) followed by the addition of a small molecule to give one product only
Example of an electrophilic addition reaction to form ethanol from ethene
Nucleophilic substitution
- A nucleophilic substitution reaction is a reaction in which an electron-rich nucleophile displaces a halogen atom
The general nucleophilic substitution reaction mechanism
- The C-X carbon of the halogenoalkane is electron deficient and has a δ+ charge
- The halogen atom, X, is more electronegative than the carbon atom which means that it pulls electrons towards itself and is δ–
- The nucleophile has a lone pair of electrons that it can donate to the δ+ carbon atom and form a covalent bond
- This causes the displacement of the halogen atom, X, which leaves as a halide ion, X–
- The displaced halide ion is known as a leaving group
Nucleophilic addition
- A nucleophilic addition reaction is a reaction in which a nucleophile (a species that likes a nucleus/positive charge) attacks an electron-deficient region in a molecule followed by the addition of a small molecule to give one product only
The general nucleophilic addition reaction mechanism
