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Halogenoalkanes: Question 1

Syllabus 15.1

Multiple choice AS 1 mark

The table shows approximate average bond enthalpies for three carbon-halogen bonds:

Bond Bond enthalpy / kJ mol⁻¹
C–Cl 338
C–Br 276
C–I 238

Equal small volumes of 1-chlorohexane, 1-bromohexane and 1-iodohexane are each heated under reflux with an excess of hot aqueous sodium hydroxide, and the rate at which each is hydrolysed is compared.

Which statement correctly ranks the reaction rates and links this to the bond enthalpy data?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Why bond enthalpy, not polarity, controls the rate

Hydrolysis of a halogenoalkane begins with breaking of the carbon-halogen bond as the nucleophile (here OH−\text{OH}^-) attacks. The easier that bond is to break, the faster the reaction, and “easier to break” means a lower bond enthalpy, not a more polar bond.

From the table: C–Cl (338 kJ mol−1)>C–Br (276 kJ mol−1)>C–I (238 kJ mol−1)\text{C–Cl } (338\ \text{kJ mol}^{-1}) > \text{C–Br } (276\ \text{kJ mol}^{-1}) > \text{C–I } (238\ \text{kJ mol}^{-1})

So the C–I bond is the weakest of the three, even though iodine is the least electronegative halogen and so gives the least polar C–X bond. Reactivity towards nucleophilic substitution therefore increases as the bond enthalpy decreases: reactivity: C–Cl (slowest)<C–Br<C–I (fastest)\text{reactivity: C–Cl (slowest)} < \text{C–Br} < \text{C–I (fastest)}

Applying this to the three hexyl halides

All three compounds (1-chlorohexane, 1-bromohexane, 1-iodohexane) are primary halogenoalkanes with an identical hexyl (C6H13) alkyl group, so they all react with OH−(aq)\text{OH}^-(aq) by the same SN2 mechanism. The only thing that differs between them is the halogen, so the only thing that can change the rate is how strong the C–X bond is:

  • 1-Iodohexane has the weakest C–I bond (238 kJ mol⁻¹), so it is hydrolysed fastest.
  • 1-Chlorohexane has the strongest C–Cl bond (338 kJ mol⁻¹), so it is hydrolysed slowest.
  • 1-Bromohexane, with an intermediate bond enthalpy (276 kJ mol⁻¹), reacts at an intermediate rate.

Why the other options are wrong

  • B wrongly assumes that a more polar bond reacts faster; in fact chlorine’s high electronegativity makes the C–Cl bond the most polar of the three, yet it is hydrolysed the slowest, because that bond is also the strongest.
  • C ignores the bond enthalpy data entirely. The shared mechanism does not mean the halogen identity is irrelevant to rate.
  • D draws an unjustified conclusion from the C–Br value sitting “in the middle” of the table; a bond enthalpy of intermediate size gives an intermediate rate, not the fastest one.

Final answer

A, 1-iodohexane reacts fastest (weakest C–I bond) and 1-chlorohexane reacts slowest (strongest C–Cl bond); reactivity increases as C–X bond enthalpy decreases.