3.3.4 SN1 & SN2

• In nucleophilic substitution reactions involving halogenoalkanes, the halogen atom is replaced by a nucleophile
• These reactions can occur in two different ways (known as S_N2 and S_N1 reactions) depending on the structure of the halogenoalkane involved

S_N2 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 S_N2 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

• The S_N2 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

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

S_N1 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 S_N1 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

• 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 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

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

Carbocations

• In the S_N1 mechanism, a tertiary carbocation is formed
• This is not the case for S_N2 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

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

• Secondary halogenoalkanes undergo a mixture of both S_N1 and S_N2 reactions depending on their structure