Nucleophilic Substitution in Halogenoalkanes (HL) (DP IB Chemistry)

Nucleophilic Substitution in Halogenoalkanes

• In nucleophilic substitution reactions involving halogenoalkanes, the halogen atom is replaced by a nucleophile

• The strength of any nucleophile depends on its ability to make its lone pair of electrons available for reaction

• The hydroxide ion, OH^-, is a stronger nucleophile than water because it has a full negative charge

  • This means that it has a readily available lone pair of electrons

• A water molecule only has partial charges, δ+ and δ-

  • This means that its lone pair of electrons is less available than the hydroxide ions

  • The lone pairs of electrons in a water molecule are still available to react

  Lewis diagram of OH^– and H₂O

Lewis formulae of the hydroxide ion and water molecule - illustrating the lone pairs of electrons and charges within their structures 

• 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

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

  Meaning of S_N1

'S' stands for substitution, N stands for nucleophilic and '1' shows that the rate of reaction depends upon the concentration of one reagent

• 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)

  • As the rate-determining step only depends on the concentration of the halogenoalkane, the rate equation for an S_N1 reaction is rate = k[halogenoalkane]

  • In terms of molecularity, an S_N1 reaction is unimolecular

  • This forms a tertiary carbocation intermediate (which is a tertiary carbon atom with a positive charge)

  • In the second step, the tertiary carbocation is attacked by the nucleophile

• This two-step process is evident in the energy profile diagram for an S_N1 reaction

Reaction profile for an S_N1 mechanism

The reaction profile for an S_N1 mechanism is a two-step mechanism so has two curves. The connection between the first two curves represents the carbocation intermediate

• For example, the nucleophilic substitution of 2-bromo-2-methylpropane by hydroxide ions to form 2-methyl-2-propanol

Example of S_N1 mechanism

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

Meaning of S_N2

'S' stands for substitution, N stands for nucleophilic and '2' shows that the rate of reaction depends upon 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

  • As this is a one-step reaction, the rate-determining step depends on the concentrations of the halogenoalkane and nucleophile which means that the rate equation for an S_N2 reaction is

rate = k[halogenoalkane][nucleophile]

• In terms of molecularity, an S_N2 reaction is bimolecular

• 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

• This one-step process is evident in the energy profile diagram for an S_N2 reaction

Reaction profile for an S_N2 mechanism

The reaction profile for an S_N2 mechanism is a one-step reaction so has one curve. The transition state always involves partial bonds

• For example, the nucleophilic substitution of bromoethane by hydroxide ions to form ethanol

Example of an S_N2 mechanism

The S_N2 mechanism of bromoethane with hydroxide causes an inversion of configuration

• The bromine atom of the bromoethane molecule causes steric hindrance

• This means that the hydroxide ion nucleophile can only attack from the opposite side of the C-Br bond

  • An attack from the same side as the bromine atom is sometimes called a frontal attack

  • While attack from the opposite side is sometimes called a backside or rear-side attack

• As a result of this, the molecule has undergone an inversion of configuration

• The common comparison for this is an umbrella turning inside out in the wind

• As the C-OH bond forms, the C-Br bond breaks causing the bromine atom to leave as a bromide ion

Diagram to demonstrate inversion of configuration

Inversion of configuration - umbrella analogy