Terminology Used in Reaction Mechanisms (Cambridge (CIE) AS Chemistry): Revision Note

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

Free-radical substitution

• A free-radical substitution reaction is a reaction in which halogen atoms substitute for hydrogen atoms in alkanes

• Free radical substitution involves three standard steps:

  • Initiation

  • Propagation

  • Termination

Initiation step

• The covalent Cl-Cl bond is broken by energy from the UV light

• Each atom takes one electron from the covalent bond

• This produces two radicals in a homolytic fission reaction

Cl–Cl → 2Cl^•

Propagation step

• The halogen free radicals are very reactive and will attack the unreactive alkanes

• One of the methane C-H bond breaks homolytically to produce an alkyl radical

CH₄ + Cl^• → ^•CH₃ + HCl

• The alkyl radical can attack another chlorine molecule to form a halogenoalkane

• This also regenerates the chlorine free radical

^•CH₃ + Cl₂ → CH₃Cl + Cl^• 

• The regenerated chlorine free radical can then repeat the cycle

Termination step

• Multiple products are possible, dependent on the radicals involved

  • They are commonly classed as desirable (wanted) and undesirable (unwanted)

• In the single substitution of methane with chlorine:

  • Chloromethane is the desirable (wanted) product:

^•CH₃ + Cl^• → CH₃Cl 

• Ethane and chlorine are undesirable (unwanted) products:

^•CH₃ + ^•CH₃ → CH₃CH₃ 

Cl^• + Cl^• → Cl₂

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