CIE AS Chemistry (9701) exams from 2022

Revision Notes

3.3.4 SN1 & SN2

Halogenoalkanes: SN1 & SN2 Mechanisms

  • In nucleophilic substitution reactions involving halogenoalkanes, the halogen atom is replaced by a nucleophile
  • These reactions can occur in two different ways (known as SN2 and SN1 reactions) depending on the structure of the halogenoalkane involved

SN2 reactions

  • In primary halogenoalkanes, the carbon that is attached to the halogen is bonded to one alkyl group
  • These halogenoalkanes undergo nucleophilic substitution by an SN2 mechanism
    • ‘S’ stands for ‘substitution’
    • ‘N’ stands for ‘nucleophilic’
    • ‘2’ means that the rate of the reaction (which is determined by the slowest step of the reaction) depends on the concentration of both the halogenoalkane and the nucleophile ions

 

Halogen Compounds SN2, downloadable AS & A Level Chemistry revision notes

  • The SN2 mechanism is a one-step reaction
    • The nucleophile donates a pair of electrons to the δ+ carbon atom 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 (heterolytic fission)
    • The halogen leaves the halogenoalkane as an X ion
  • For example, the nucleophilic substitution of bromoethane by hydroxide ions to form ethanol

 

Halogen Compounds SN2 of Bromoethane, downloadable AS & A Level Chemistry revision notes

The mechanism of nucleophilic substitution in bromoethane which is a primary halogenoalkane

SN1 reactions

  • In tertiary halogenoalkanes the carbon that is attached to the halogen is bonded to three alkyl groups
  • These halogenoalkanes undergo nucleophilic substitution by an SN1 mechanism
    • ‘S’ stands for ‘substitution’
    • ‘N’ stands for ‘nucleophilic’
    • ‘1’ means that the rate of the reaction (which is determined by the slowest step of the reaction) depends on the concentration of only one reagent, the halogenoalkane

 

Halogen Compounds SN1, downloadable AS & A Level Chemistry revision notes

  • 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 is the slow and rate-determining step)
    • This forms a tertiary carbocation (which is a tertiary carbon atom with a positive charge)
    • In the second step, the tertiary carbocation is attacked by the nucleophile
  • For example, the nucleophilic substitution of 2-bromo-2-methylpropane by hydroxide ions to form 2-methyl-2-propanol

 

Halogen Compounds SN1 of 2-bromo-2-Methylpropane, downloadable AS & A Level Chemistry revision notes

The mechanism of nucleophilic substitution in 2-bromo-2-methylpropane which is a tertiary halogenoalkane

Carbocations

  • In the SN1 mechanism, a tertiary carbocation is formed
  • This is not the case for SN2 mechanisms as a primary carbocation would have been formed which is much less stable than tertiary carbocations
  • This has to do with the positive inductive effect of the alkyl groups attached to the carbon which is bonded to the halogen atom
    • The alkyl groups push electron density towards the positively charged carbon, reducing the charge density
    • In tertiary carbocations, there are three alkyl groups stabilising the carbocation whereas in primary carbocations there is only one alkyl group
    • This is why tertiary carbocations are much more stable than primary ones

Halogen Compounds Stability of Carbocations, downloadable AS & A Level Chemistry revision notes

The diagram shows the trend in stability of primary, secondary and tertiary carbocations

 

  • Secondary halogenoalkanes undergo a mixture of both SN1 and SN2 reactions depending on their structure

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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