Topic 10 · page 8 of 27

Water as the nucleophile

Water is a weak nucleophile and hydrolyses a halogenoalkane slowly. Silver ions in aqueous silver nitrate catch the halide ion as a coloured precipitate, so the time it takes to appear measures how fast the C–X bond breaks.

11 min read10.810.8(iii)

Read first: Ammonia and cyanide

Water attacks the δ+ carbon 10.810.8(iii)

Water has two lone pairs on oxygen. It is a weaker nucleophile than OH⁻ because it carries no charge, but it will still attack a δ+ carbon. The halogenoalkane is hydrolysed: the halogen is replaced by –OH and the halide ion goes into solution.

CHX3CHX2Br+HX2O→CHX3CHX2OH+HX++BrX− \ce{CH3CH2Br + H2O -> CH3CH2OH + H+ + Br-}

Halogenoalkanes do not dissolve in water, and silver nitrate is in water. So the reaction is done in ethanol, which mixes with both. Ethanol is the solvent. Water is the nucleophile.

Silver nitrate is added to the mixture. Silver ions combine with the halide ions as soon as they form:

AgX+(aq)+BrX−(aq)→AgBr(s) \ce{Ag+(aq) + Br-(aq) -> AgBr(s)}

The overall change is:

CHX3CHX2Br+HX2O+AgX+→CHX3CHX2OH+HX++AgBr(s) \ce{CH3CH2Br + H2O + Ag+ -> CH3CH2OH + H+ + AgBr(s)}

Which precipitate? 10.810.8(iii)

Three test tubes with silver halide precipitates. Silver chloride, AgCl, is white. Silver bromide, AgBr, is cream. Silver iodide, AgI, is yellow. Below each is the halogenoalkane that gave it: a chloroalkane, a bromoalkane, an iodoalkane.
The colour of the precipitate shows which halogen was in the halogenoalkane. The colours are as in the data booklet.
Halogen in the halogenoalkane Halide ion Precipitate Colour
Cl Cl⁻ AgCl white
Br Br⁻ AgBr cream
I I⁻ AgI yellow

The colours are the ones you used for halide ions in Topic 8. Here they identify the halogen in an unknown halogenoalkane. Do not add nitric acid in this experiment: there is no carbonate to remove, and the acid would be an extra variable.

Worked exampleAn unknown halogenoalkane

A halogenoalkane is warmed with aqueous silver nitrate in ethanol. A cream precipitate forms slowly. Identify the type of halogenoalkane and write the equations.

  1. The cream precipitate is silver bromide, so the halogenoalkane contains bromine: it is a bromoalkane.
  2. The Br⁻ comes from hydrolysis by water: RBr+HX2O→ROH+HX++BrX−\ce{RBr + H2O -> ROH + H+ + Br-}.
  3. The silver ions precipitate it: AgX+(aq)+BrX−(aq)→AgBr(s)\ce{Ag+(aq) + Br-(aq) -> AgBr(s)}.

Five reagents on one page 10.8

Bromoethane in the centre with five arrows to its products. Aqueous KOH, reflux: ethanol. Ethanolic KOH, reflux: ethene. Excess NH3 in ethanol, sealed tube: ethylamine. KCN in ethanol, reflux: propanenitrile. AgNO3 in ethanol, water as nucleophile: ethanol, with a precipitate of AgBr.
The five reactions of a halogenoalkane with their reagents and conditions. Three use OH⁻, NH₃ or CN⁻ as a nucleophile, one uses water, and one uses OH⁻ as a base.
Reagent Condition Product from bromoethane Species and role
KOH(aq) heat under reflux ethanol OH⁻, nucleophile
KOH in ethanol heat under reflux ethene OH⁻, base
AgNO₃(aq) in ethanol warm ethanol, and an AgBr precipitate H₂O, nucleophile
NH₃ in ethanol, excess sealed tube, heat ethylamine NH₃, nucleophile
KCN in ethanol heat under reflux propanenitrile CN⁻, nucleophile

Check yourself

  1. Write the equation for the hydrolysis of 1-chlorobutane by water, and name the nucleophile.
  2. Write the ionic equation, with state symbols, for the formation of the precipitate in this reaction.
  3. Explain why ethanol is used as the solvent.
  4. A halogenoalkane gives a yellow precipitate. Name the precipitate and the halogen in the halogenoalkane.
  5. Explain why the time for the precipitate to appear can be used to compare the rates of hydrolysis.
  6. List the reagent, the condition and the role of the hydroxide ion for the conversion of a halogenoalkane into an alkene.

Answers cover these until you have tried the questions

  1. CHX3CHX2CHX2CHX2Cl+HX2O→CHX3CHX2CHX2CHX2OH+HX++ClX−\ce{CH3CH2CH2CH2Cl + H2O -> CH3CH2CH2CH2OH + H+ + Cl-}. The nucleophile is water.
  2. AgX+(aq)+ClX−(aq)→AgCl(s)\ce{Ag+(aq) + Cl-(aq) -> AgCl(s)}
  3. Ethanol is miscible with both the halogenoalkane (which does not dissolve in water) and the aqueous silver nitrate, so the two can mix and react.
  4. Silver iodide, AgI, so the halogen is iodine.
  5. The halide ion is released only when the C–X bond breaks, and the silver ions precipitate it at once. So the time for the precipitate to appear is the time taken for a fixed amount of bond breaking: the shorter the time, the faster the hydrolysis.
  6. Reagent: potassium hydroxide in ethanol (ethanolic KOH). Condition: heat under reflux. OH⁻ acts as a base.