# Halogenoalkanes are used as intermediates in the synthesis of other organic compounds. A chemist reacts three different halogenoalkanes — 1-fluorobutane, 1-chlorobutane, and 1-iodobutane — with aqueous sodium hydroxide solution under the same conditions. The rate at which each halogenoalkane undergoes nucleophilic substitution differs. Explain why the rate of reaction differs across the three halogenoalkanes, and state which one reacts the fastest.

> Edexcel A-Level Chemistry (9CH0) — 6.4 Halogenoalkanes · Explain · 5 marks

> Halogenoalkanes undergo nucleophilic substitution reactions in which the halogen atom is replaced by another group. The strength of the carbon–halogen bond determines how readily this substitution occurs. Bond enthalpies: C–F = 485 kJ/mol, C–Cl = 339 kJ/mol, C–I = 218 kJ/mol.

## Mark scheme (5 marks)

1. 1-iodobutane reacts the fastest
2. The C–I bond has the lowest bond enthalpy / is the weakest bond
3. Less energy is required to break the C–I bond, so the substitution reaction occurs more readily / at a greater rate
4. 1-fluorobutane reacts the slowest because the C–F bond has the highest bond enthalpy / is the strongest bond
5. The rate increases going from fluorine to chlorine to iodine because bond enthalpy decreases in that order (C–F > C–Cl > C–I)

## Key terms

- [halogenoalkane](https://www.gradenine.co.uk/glossary/halogenoalkane)
- [nucleophilic substitution](https://www.gradenine.co.uk/glossary/nucleophilic-substitution)

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