Mechanisms & Optical Activity (Edexcel International A Level Chemistry)

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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

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, downloadable IB Chemistry revision notes

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, downloadable IB Chemistry revision notes

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

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Richard

Author: Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.