Halogenoalkanes Booklet

• Halogenoalkanes

  Much more reactive than alkanes, have many uses including as refrigerants, solvents and in pharmaceuticals, use of some has been restricted due to effect of chlorofluorocarbons (CFCs) on the atmosphere
• Nucleophilic substitution reactions

  Halogenoalkanes are susceptible to attack by nucleophiles due to electron deficient carbon atom created by electron withdrawing electronegative halogen bonded to carbon atom
• Reaction with potassium cyanide, KCN

  • Name : Nucleophilic Substitution with KCN acting as a nucelophile
  • Reagents: Halogenoalkane and KCN
  • Conditions : Reflux, Reagents dissolved in aqueous ethanolic solution
  • Product : Nitrile with a C chain longer by 1 than original haloalkane
  • Use: Chain lengthening, forming amines by reduction of the nitrile
• Reaction with sodium hydroxide, NaOH

  • Name: Nucleophilic Substitution with NaOH acting as nucleophile
  • Reagents: Halogenoalkane and NaOH(aq)
  • Conditions: Refluxed, warm aqueous solution
  • Product: Alcohol, alkene; mixture of products forms due to NaOH acting as base as well as nucleophile
  • Use: Can introduce -OH group into molecule before further reaction
  • Elimination reaction forming alkene also takes place
• Reaction with Ammonia, NH3

  • Name: Nucleophilic Substitution with NH3 acting as nucleophile
  • Reagents: Excess conc.NH3(aka 880-ammonia)
  • Conditions: Excess concentrated ammonia to minimise further substitution
  • Product(s): Amine(s)
  • Use: Formation of amines (limited by further substitution to secondary amines, tertiary amines and quaternary ammonium salts)
• Nucleophile

  A species that donates a pair of electrons to form a new bond
• Substitution (nucleophilic substitution)

  Replacement of the halogen atom by another group
• Bond enthalpy, as applied to a carbon–halogen bond

  The energy required to break the bond
• Elimination

  • Name: Elimination with OH- acting as a base
  • Reagents: Hot conc.NaOH/KOH
  • Conditions: Reagents in ethanolic solution, reflux
  • Products: Major product = alkene (may be mixture of isomeric alkenes and some alcohol from nucleophilic substitution)
  • Use: Producing an alkene but non-terminal longer chain halogenoalkanes produce a mixture of isomers
• Why can haloalkanes undergo nucleophilic substitution?

  Electrodeficient carbon atom created by electron withdrawing electronegative halogen
• Why is nucleophilic substitution not used in industry for preparation of ethanol?

  • Cost of halogenoalkane starting material is very high
  • Mixture of products : low atom economy
• Rates of hydrolysis

  • Hydrolysis is splitting apart C-X bonds using water
  • Order of reactivity is based off of bond enthalpy
  • I, Br, Cl, (F)
• Why is using the same number of drops of each halogenalkane not a fair test?

  • They have different densities
  • In the same volume they will have different masses
  • And due to their different masses and Mrs, they will have different moles
  • To make it fair you must calculate the number of moles you would like to use in the test for all haloalkanes and measure appropriate volumes
• Why must excess Ag2CO3 or NaOH be neutralized before adding AgNO3?


  • Will make the white ppt. of AgOH which looks like the Cl ppt.
  • Misleading result
• How might you be able to block the attacking groups in some directions?

  Using a protecting group
• Elimination must happen on H atoms which are 1 away from the carbon with the halogen
• It is the electrons of the carbon, not the halogen which are moved around