Relative Rates of Nucleophilic Substitution (HL) (DP IB Chemistry): Revision Note

Relative rates of nucelophilic substitution

• Various factors affect the rate of nucleophilic substitution of a halogenoalkane:

  1. The nature of the nucleophile

  2. The halogen involved (leaving group)

  3. The structure (class) of the halogenoalkane

• This is regardless of whether it is S_N1 or S_N2

1. The nature of the nucleophile

• The strength of a nucleophile depends on how readily it can donate a lone pair of electrons to the δ⁺ carbon atom in a halogenoalkane

• Two key factors influence this:

  • Charge: Negatively charged species are stronger nucleophiles than their neutral counterparts

    • For example, OH^- is stronger than H₂O

  • Electronegativity: When species have the same charge, the species with a lower electronegativity are stronger nucleophiles

    • For example, NH₃ is stronger than H₂O because nitrogen holds its lone pair less tightly than oxygen

•  The effectiveness of nucleophiles is:

Strongest     CN^- > OH^- > NH₃ > H₂O     Weakest

2. The halogen involved (leaving group)

• Substitution reactions involve breaking the carbon–halogen (C–X) bond

• The weaker the bond, the faster the reaction

  • This is because less energy is required for the halogen to leave

• This helps explain why different halogenoalkanes react at different rates

Relative bond energies of halogenoalkanes

• The approximate bond energies for different carbon–halogen bonds are:

  • C–F: 492 kJ mol^-1 (strongest bond)

  • C–Cl: 324 kJ mol^-1

  • C–Br: 285 kJ mol^-1

  • C–I: 228 kJ mol^-1 (weakest bond)

• The C–I bond is the weakest and requires the least energy to break

  • So, in substitution reactions this bond breaks heterolytically:

R₃C-I + OH^- → R₃C-OH + I^-

• The C-F bond is the strongest and requires the most energy to break

  • So, fluoroalkanes are less likely to undergo substitution reactions

Halogenoalkane precipitate colours

• When different halogenoalkanes undergo nucleophilic substitution, the halide ions released can be detected using aqueous silver nitrate

• Each halide forms a characteristic precipitate, except fluoride:

  • Fluoride (F^-) → no visible precipitate because silver fluoride is soluble in water

  • Chloride (Cl^-) → white precipitate of silver chloride (AgCl)

  • Bromide (Br^-) → cream precipitate of silver bromide (AgBr)

  • Iodide (I^-) → yellow precipitate of silver iodide (AgI)

• The pale yellow silver iodide forms most quickly, indicating that

  • Iodide ions are released fastest

  • Iodoalkanes are the most reactive

• No visible precipitate is formed with fluoroalkanes, indicating that:

  • Fluoride ions are not easily released

  • Fluoroalkanes are the least reactive

• This confirms the relative reactivity of the halogenoalkanes:

3. The class of halogenoalkane

• The classes of halogenoalkane are:

  • Primary

  • Secondary

  • Tertiary

• Primary halogenoalkanes undergo S_N2 reactions

  • They do not form carbocations

  • Nucleophiles attack directly, displacing the halide in a single step

• Secondary halogenoalkanes can undergo either S_N1 or S_N2 reactions

  • The preferred mechanism depends on:

    • The solvent

    • The temperature

    • The nucleophile involved

• Tertiary halogenoalkanes undergo S_N1 reactions

  • They form stable tertiary carbocations

Carbocation stability

• The stability of a carbocation increases with the number of alkyl groups attached to the positively charged carbon

  • This is due to the positive inductive effect

  • This is where alkyl groups push electron density towards the positively charged carbon, reducing its charge density

• Tertiary carbocations have three alkyl groups stabilising the charge, while primary ones have only one

• This difference in stability influences how easily the carbocation forms, and the rate of reaction

Primary, secondary and tertiary carbocations

• This trend is relevant for S_N1 reactions, which involve carbocation intermediates

  • Tertiary halogenoalkanes form the most stable carbocations and react fastest by the S_N1 mechanism

  • Primary halogenoalkanes react by the S_N2 mechanism and do not form carbocations, but are included here for comparison

• In S_N1 reactions, the formation of a carbocation depends on the class of halogenoalkane

  • Only tertiary halogenoalkanes form stable carbocations, making S_N1 the preferred pathway

• This directly influences the rate-determining step, which controls the overall rate of nucleophilic substitution