• In organic reaction mechanisms, curly arrows represent the movement of electron pairs
• The arrow beings 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 displaced halogen atom is also called the leaving group)

Example of a nucleophilic substitution reaction to form methanol from chloromethane

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

Example of a nucleophilic addition reaction to form propane-2,2-diol from propanone