4.6.3 Mechanisms & Optical Activity

• Optical activity can be used to suggest the mechanism of a chemical reaction
• This is particularly the case for nucleophilic substitution
  • Nucleophilic substitution can occur via an S_N1 or S_N2 mechanism

S_N1 mechanism

• The S_N1 mechanism is a two-step reaction
  • In the first step, the C-X bond breaks heterolytically and the halogen leaves the halogenoalkane as an X^- ion
  • This leaves a trigonal planar, tertiary carbocation 
  • In the second step, the planar, tertiary carbocation is attacked by the nucleophile
  • The nucleophile is able to attack from either side of the planar carbocation, which results in the formation of a racemic mixture
• Therefore, a reaction with an S_N1 mechanism will produce a racemic mixture 

S_N1 Optical Isomers Mechanism

S_N2 mechanism

• The S_N2 mechanism is a one-step reaction
  • The nucleophile donates a pair of electrons to the δ+ carbon atom of the halogenoalkane to form a new bond
  • At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond 
  • The halogen leaves the halogenoalkane as an X^- ion

• For example, the nucleophilic substitution of bromoethane by hydroxide ions to form ethanol

The S_N2 mechanism of bromoethane with hydroxide causing an inversion of configuration

• The bromine atom of the bromoethane molecule causes steric hindrance
• This means that the hydroxide ion nucleophile can only attack from the opposite side of the C-Br bond
  • Attack from the same side as the bromine atom is sometimes called frontal attack
  • While attack from the opposite side is sometimes called backside or rear-side attack

• As the C-OH bond forms, the C-Br bond breaks causing the bromine atom to leave as a bromide ion
  • As a result of this, the molecule has undergone an inversion of configuration
  • The common comparison for this is an umbrella turning inside out in the wind

Inversion of configuration - umbrella analogy

• Therefore, if a reaction with an S_N2 mechanism starts with an enantiopure reactant then an enantiopure products will be formed