Haloalkanes are identified by hydrolysis with aqueous silver nitrate in ethanol. The ethanol is a solvent in which both the haloalkane and the aqueous silver nitrate mix.Method: add the haloalkane to silver nitrate solution in ethanol and warm in a water bath at about 50 °C.Water acts as a nucleophile and hydrolyses the C−X bond, releasing halide ions:R−X + H2O → R−OH + H+ + X− The halide ions then react with silver ions to form a precipitate of silver halide:Ag+(aq) + X−(aq) → AgX(s) The colour of the precipitate identifies the halogen:• chloroalkane: white precipitate of AgCl
• bromoalkane: cream precipitate of AgBr
• iodoalkane: yellow precipitate of AgIThe rate at which the precipitate appears also differs: iodoalkanes react fastest and chloroalkanes slowest. This is because the C−I bond has the lowest bond enthalpy and is broken most easily; bond enthalpy, not polarity, controls the rate.Worked example: 1-chlorobutane, 1-bromobutane and 1-iodobutane are tested at the same time. A yellow precipitate appears first (1-iodobutane), then cream (1-bromobutane), and white (1-chlorobutane) forms last.Fluoroalkanes give no precipitate in this time, because the C−F bond is so strong.
• bromoalkane: cream precipitate of AgBr
• iodoalkane: yellow precipitate of AgIThe rate at which the precipitate appears also differs: iodoalkanes react fastest and chloroalkanes slowest. This is because the C−I bond has the lowest bond enthalpy and is broken most easily; bond enthalpy, not polarity, controls the rate.Worked example: 1-chlorobutane, 1-bromobutane and 1-iodobutane are tested at the same time. A yellow precipitate appears first (1-iodobutane), then cream (1-bromobutane), and white (1-chlorobutane) forms last.Fluoroalkanes give no precipitate in this time, because the C−F bond is so strong.