Reactivity of Halogenoalkanes (Edexcel AS Chemistry)

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Reactivity of Halogenoalkanes

  • Nucleophilic substitution reactions can occur in two different ways (known as SN2 and SNreactions) depending on the structure of the halogenoalkane involved
    • Tertiary halogenoalkanes favour SN1 reactions
    • Primary halogenoalkanes favour SN2 reactions

SN1 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 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

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 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 SN2 mechanism of bromoethane with hydroxide causing an inversion of configuration, downloadable IB Chemistry revision notes

The SN2 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

Halogen Compounds Table 1_Reactivity of Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

  • 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

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.