3.4.5 Reactivity of Halogenoalkanes

• Nucleophilic substitution reactions can occur in two different ways (known as S_N2 and S_N1 reactions) depending on the structure of the halogenoalkane involved
  • Tertiary halogenoalkanes favour S_N1 reactions
  • Primary halogenoalkanes favour S_N2 reactions

S_N1 reactions

• In tertiary halogenoalkanes, the carbon that is attached to the halogen is also 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

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 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 S_N2 mechanism of bromoethane with hydroxide causing an inversion of configuration

Bond Enthalpy

• The halogenoalkanes have different rates of substitution reactions
• Since substitution reactions involve breaking the carbon-halogen bond the bond energies can be used to explain their different reactivities

Halogenoalkane Bond Energy

• The table above shows that the C-I bond requires the least energy to break, and is therefore the weakest carbon-halogen bond
• During substitution reactions, the C-I bond will, therefore, heterolytically break as follows:

R₃C-I + OH^-     →    R₃C-OH + I^-

                                                                   halogenoalkane          alcohol

• The C-F bond, on the other hand, requires the most energy to break and is, therefore, the strongest carbon-halogen bond
• Fluoroalkanes will, therefore, be less likely to undergo substitution reactions