Terminology Used in Reaction Mechanisms (CIE A Level Chemistry)

• 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
• 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

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