Reactivity of Halogenoalkanes

Halogenoalkanes react when a nucleophile attacks the δ+ carbon and the C-X bond breaks. Two factors could affect how fast this happens.1. Bond polarity: fluorine is the most electronegative halogen, so C-F is the most polar bond and its carbon is the most δ+. If polarity controlled the rate, fluoroalkanes would react fastest and iodoalkanes slowest.2. Bond enthalpy: the C-X bond must break. Going down the group the halogen atom gets bigger, the bond gets longer and weaker:C-F 467   C-Cl 346   C-Br 290   C-I 228 (kJ mol⁻¹)If bond enthalpy controlled the rate, iodoalkanes would react fastest.What actually happens: experiments show the rate order isiodoalkane > bromoalkane > chloroalkane > fluoroalkaneSo bond enthalpy is the dominant factor. The C-I bond has the lowest bond enthalpy, is broken most easily and reacts fastest. The C-F bond is so strong that fluoroalkanes are very unreactive. For example, 1-bromobutane reacts faster than 1-chlorobutane.Worked example: how much weaker is C-I than C-Br? 290 − 228 = 62 kJ mol⁻¹.The very strong C-F and C-Cl bonds also explain why CFCs are so stable in the lower atmosphere: they only break up high in the stratosphere, where UV radiation breaks the C-Cl bond.