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
An organic mechanism curly arrow
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
The free radical mechanism always contains initiation, propagation and termination steps
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
Curly arrows always move from an area of high electron density to an area of low electron density
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
In nucleophilic substitution reaction mechanisms, the nucleophile replaces an atom / group in the target molecule
- 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
In nucleophilic addition reaction mechanisms, the nucleophile is added to the target molecule
