6.3.4 - Chemical Reactions of Halogenoalkanes

• reaction with water

  nucleophilic substitution (water=nucleophile)

  observations: cold water slowly hydrolyses halogenoalkanes replacing halogen atoms with an -OH group forming an alcohol

  RX + H2O -> ROH + H+ + X-
• reaction with aqueous silver nitrate in ethanol

  nucleophilic substition (water=nucleophile)

  observations: water hydrolyses halogenoalkane forming an alcohol and halide ions which react with silver ions producing a silver halide precipitate
• why don't halogen atoms don't react with silver atoms

  they are covalently bonded to carbon so give no precipiate
• why is ethanol needed in reaction with aqueous silver nitrate in ethanol

  halogenoalkanes don't mix with water so ethanol is added
• why is dilute nitric acid added when testing for halogens

  prevents unreacted hydroxide ions reacting with silver ions which would produce a precipitate
• uses of reaction of halogenoalkanes with aqueous silver nitrate in ethanol

  test for halogen in halogenoalkane

  investigate rate of reaction of halogenoalkane
• method to compare reactivities of halogenoalkanes

  react each halogenoalkane in a test tube with aqueous silver nitrate in ethanol

  the faster a precipitate forms the faster the rate of hydrolysis of the halogenoalkane
• rate of reaction of primary, secondary and tertiary halogenoalkanes

  tertiary halogenoalkane forms a precipitate almost instantly (faster rate of reaction)

  secondary halogenoalkane gives a slight precipitate that thickens with time

  primary halogenoalkane takes longer to produce a precipitate(slower rate of reaction)
• comparingn reactivity of halogenoalkanes with different halogens

  halogens further down group 7 makes halogenoalkane more reactive because halogen is bigger so increases bond length which decreases bond enthalpy of C-X bond so less energy to break them so react faster
• reaction with warm KOH (-OH ions)

  nucleophilic substitution (nucleophile=hydroxide ion)

  conditions: water as a solvent, lower temperatures, more dilute KOH

  general equation: RX + KOH -> ROH + KX

  product: alcohol

  (learn reaction mechanism)
• reaction with hot ethanolic KOH

  elimination - hydroxide acts as a base to remove H+ from halogenoalkane

  conditions: higher temperatures, concentrated KOH, pure ethanol, heating under reflux

  product: alkene

  usually with secondary and tertiary halogenoalkanes

  learn mechanism
• reaction with cyanide ions

  nucleophilic substitution reaction (nucleophile=cyanide ion)

  halogenoalkane heated under reflux with a solution of potassium cyanide in ethanol producing a nitrile

  increases length of carbon chain

  learn mechanism
• why is ethanol used in the reaction of halogenoalkanes with cyande ions

  if water was present substitution would be by -OH instead of -CN
• heating under reflux

  heating with a condenser placed vertically in the flask to prevent vapour escaping while heating so it condenses back into flask

  ensure that volatile substances condense and drip back into reaction mixture
• reaction with ammonia

  nucleophilic substitution (nucleophile=ammonia)

  warm halogenoalkane reacts with excess concentrated ethanolic ammonia in a sealed tube produces a primary amine

  learn mechanism
• problem with reaction of halogenoalkanes with ammonia

  primary amine can also react with halogenoalkane because there's a lone pair of electrons on the primary amine so a mixture of products may form

  limit this by having excess ammonia so there's a greater chance of ammonia being the nucleophile not the halogenoalkane