DP IB Chemistry: HL

Revision Notes

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First teaching 2014

Last exams 2024

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20.1.1 Nucleophilic Substitution Reactions

Nucleophilic Substitution Reactions

  • 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 structures of the hydroxide ion and water molecule, downloadable IB Chemistry revision notes

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

Exam Tip

In general:

  • A negatively charged ion will be a stronger nucleophile than a neutral molecule
  • A conjugate base will be a stronger nucleophile than its corresponding conjugate acid
    • e.g. the hydroxide ion is a stronger nucleophile than water

 

SN1 Mechanism

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

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)
    • As the rate-determining step only depends on the concentration of the halogenoalkane, the rate equation for an SN1 reaction is rate = k[halogenoalkane]
    • In terms of molecularity, an SN1 reaction is unimolecular
    • 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

Exam Tip

You are expected to know the difference between the heterolytic fission that features in SN1 reactions and homolytic fission in other reactions:

  • Heterolytic fission forms anions and cations and uses double headed arrows to show the movement of both electrons from the covalent bond
  • Homolytic fission forms free radicals and uses single headed arrows, sometimes called fish hooks, to show the movement of a single electron as the covalent bond breaks

SN2 Mechanism

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
      • As this is a one-step reaction, the rate-determining step depends on the concentrations of the halogenoalkane and nucleophile, the rate equation for an SN2 reaction is rate = k[halogenoalkane][nucleophile]
      • In terms of molecularity, an SN2 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

  • 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

  • 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
    • Attack from the same side as the bromine atom is sometimes called frontal attack
    • While attack from the opposite side is sometimes called backside or rear-side attack

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

Inversion of configuration – umbrella analogy, downloadable IB Chemistry revision notes

Inversion of configuration - umbrella analogy

Exam Tip

If you are asked to explain reaction mechanisms where there is an inversion of configuration, you will be expected to:

  • Use partial charges, δ+ and δ-, to help explain why the nucleophile attacks and the halogen leaves
  • Use dotted, wedge and tapered bonds to show the change in configuration of the atoms / functional groups around the carbon that is being attacked
  • Draw the transition state with the nucleophile attached to the carbon with a dotted bond and the halogen still attached to the carbon, also, with a dotted bond
  • Be aware that the compound you draw is a transition state and not an intermediate

Factors Affecting Nucleophilic Substitution

Factors affecting nucleophilic substitution

  • Various factors affect the rate of nucleophilic substitution, regardless of SN1 or SN2, involving a halogenoalkane:

    1. The nature of the nucleophile
    2. The halogen involved (leaving group)
    3. The structure (class) of the halogenoalkane
    4. Protic & aprotic solvents

1. The nature of the nucleophile

  • The most effective nucleophiles are neutral or negatively charged species that have a lone pair of electrons available to donate to the δ+ carbon in the halogenoalkane
  • The greater the electron density on the nucleophile ion or molecule; the stronger the nucleophile
    • Consequently, negative anions tend to be more reactive than their corresponding neutral species, e.g. hydroxide ions and water molecules (as previously discussed)

  • When nucleophiles have the same charge, the electronegativity of the atom carrying the lone pair becomes the deciding factor
    • The less electronegative the atom carrying the lone pair; the stronger the nucleophile
    • For example:
      • Ammonia is a stronger electrophile than water because the nitrogen atom in ammonia is less electronegative than the oxygen atom in water

    • This is because a less electronegative atom has a weaker grip on its lone pair of electrons, which means that they are more available for reaction

  •  The effectiveness of nucleophiles is as follows:

Strongest     CN- > OH- > NH3 > H2O     Weakest

2. The halogen involved (leaving group)

  • 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

Approximate Halogenoalkane Bond Energy Table

Approximate Halogenoalkane Bond Energy Table, downloadable IB 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 breaks heterolytically as follows:

R3C-I + OH-     →    R3C-OH + I-

  • 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

  • This idea can be confirmed by reacting the product formed by nucleophilic substitution of the halogenoalkane with aqueous silver nitrate solution
  • As a halide ion is released, this results in the formation of a precipitate
  • The rate of formation of these precipitates can also be used to determine the reactivity of the halogenoalkanes

Halogenoalkane Precipitates Table

Halogen Compounds Table 2_Reactivity of Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

  • The formation of the pale yellow silver iodide is the fastest (fastest nucleophilic substitution reaction) whereas the formation of the silver fluoride is the slowest (slowest nucleophilic substitution reaction)
  • This confirms that fluoroalkanes are the least reactive and iodoalkanes are the most reactive halogenoalkanes

 Halogen Compounds Reactivity of Halogenoalkanes, downloadable AS & A Level Chemistry revision notes

The trend in reactivity of halogenoalkanes

3. The structure (class) of the halogenoalkane

  • Tertiary halogenoalkanes undergo SN1 reactions, forming stable tertiary carbocations
  • Secondary halogenoalkanes undergo a mixture of both SN1 and SN2 reactions depending on their structure
  • Primary halogenoalkanes undergo SN2 reactions, forming the less stable primary 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
    • In primary carbocations, there is only one alkyl group
      • This is why tertiary carbocations are much more stable than primary ones

Halogen Compounds Stability of Carbocations, downloadable AS & A Level Chemistry revision notes

The diagram shows the trend in stability of primary, secondary and tertiary carbocations

  • Overall, the structure (class) has a direct effect on the formation of the carbocation and, therefore, the rate-determining step
  • Consequently, this affects the overall rate of the nucleophilic substitution reaction

Protic & Aprotic Solvents

4. Protic & Aprotic Solvents

    Hydrogen bonding

  • Protic, polar solvents contain a hydrogen atom bonded to a very electronegative nitrogen or oxygen atom
    • This means that they are capable of hydrogen bonding
    • Examples of protic solvents include ammonia, carboxylic acids, ethanol and water

  • Aprotic, polar solvents contain hydrogen atoms but they are not bonded to an electronegative atom
    • This means that they cannot participate in hydrogen bonding
    • Examples of aprotic solvents include ethanenitrile, ethyl ethanoate and propanone

    Solvation

  • Solvation is where solvent molecules surround a dissolved ion
    • In SN1 reactions, the rate-determining step is not the attack of the nucleophile
    • The rate-determining step is the formation of the carbocation intermediates and halide ion
    • Both ions could be stabilised by the use of a protic solvent, as shown in the following example:

Protic polar solvent stabilising carbocation intermediates and halide ions, downloadable IB Chemistry revision notes

Protic polar solvent stabilising carbocation intermediates and halide ions

    • In SN2 reactions, the rate-determining step is the attack of the nucleophile
    • The use of aprotic solvents does not solvate the nucleophile
    • This means that the nucleophile is more able to react and form the transition state

  • SN1 reactions are best conducted using protic, polar solvents
  • SN2 reactions are best conducted using aprotic, non-polar solvents

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