• 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

• 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

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

• 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

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

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