Reactivity of Halogenoalkanes (Edexcel AS Chemistry): Revision Note

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 & Halogenoalkane Reactivity

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