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 SN1 or SN2 mechanism
SN1 mechanism
- The SN1 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 SN1 mechanism will produce a racemic mixture
SN1 Optical Isomers Mechanism
SN2 mechanism
- The SN2 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 SN2 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 SN2 mechanism starts with an enantiopure reactant then an enantiopure products will be formed