Relative Rates of Nucleophilic Substitution (HL) (HL IB Chemistry)

• Various factors affect the rate of nucleophilic substitution, regardless of S_N1 or S_N2, involving a halogenoalkane:
  1. The nature of the nucleophile
  2. The halogen involved (leaving group)
  3. The structure (class) of the halogenoalkane

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

• The table above shows that the C-I bond requires the least energy to break, i.e. it is the weakest carbon-halogen bond
  • During substitution reactions, the C-I bond will break 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 an 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

• 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

Diagram to show the relative reactivity of the halogenoalkanes

The trend in reactivity of halogenoalkanes

3. The structure (class) of the halogenoalkane

• Tertiary halogenoalkanes undergo S_N1 reactions, forming stable tertiary carbocations
• Secondary halogenoalkanes undergo a mixture of both S_N1 and S_N2 reactions depending on their structure
• Primary halogenoalkanes undergo S_N2 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

Primary, secondary and tertiary carbocations

The diagram shows the trend in the 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