Reactivity of Halogenoalkanes (Cambridge (CIE) AS Chemistry): Revision Note

Reactivity of Halogenoalkanes

• 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

Halogenoalkane bond Eenergy table

• The table above shows that the C-I bond requires the least energy to break, and is therefore the weakest carbon-halogen bond

• During substitution reactions the C-I bond will therefore heterolytically break as follows:

R₃C-I + OH^–→R₃C-OH + I^– 

 halogenoalkane → alcohol

• 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

Aqueous silver nitrate

• Reacting halogenoalkanes with aqueous silver nitrate solution will result 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

• Chlorides

  • White (silver chloride) precipitate

• Bromides

  • Cream (silver bromide)

• Iodides

  • Yellow (silver iodide)

• The formation of the pale yellow silver iodide is the fastest (fastest nucleophilic substitution reaction)

• 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

 The trend in reactivity of halogenoalkanes