Halogenoalkanes

• Halogenoalkanes


  Can be classified as primary, secondary or tertiary
• Prefix of halogen atom

  • Fluoro for F
  • Chloro for Cl
  • Bromo for Br
  • Iodo for I
• Primary Halogenoalkanes


  • One carbon attached to the carbon atom with the halogen
• Secondary Halogenoalkanes


  • Two carbons attached to the carbon atom with the halogen
• Tertiary Halogenoalkanes


  • Three carbons attached to the carbon atom with the halogen
• Nucleophile


  Electron pair donor e.g. :OH-, :NH3, :CN-
• Nucleophilic Substitution with aqueous hydroxide ions


  1. Change in functional group: halogenoalkane ⟶ alcohol

  2. Reagent: potassium (or sodium) hydroxide

  3. Conditions: In Aqueous solution; Heat under reflux

  4. Mechanism: Nucleophilic substitution

  5. Role of reagent: Nucleophile, OH
• Halogenoalkanes are insoluble in water so ethanol is added to solubilise the halogenoalkane so it can react with the hydroxide ions
• Nucleophilic Substitution with aqueous silver nitrate


  1. Change in functional group: halogenoalkane ⟶ alcohol

  2. Reagent:Water,H₂O

  3. Conditions: In Aqueous solution; in ethanol

  4. Mechanism: Nucleophilic substitution,Hydrolysis(splitting a molecule by a reaction with water)

  5. Role of reagent: Nucleophile, H₂O
• Water is a poor nucleophile but it can react slowly with halogenoalkanes in a substitution reaction
• Nucleophilic substitution with cyanide ions


  1. Change in functional group: halogenoalkane ⟶ nitrile

  2. Reagent: Potassium cyanide

  3. Conditions: In ethanol, reflux

  4. Mechanism: Nucleophilic substitution

  5. Role of reagent: Nucleophile, CN⁻
• Forming a nitrile from a halogenoalkane results in the length of the carbon chain increasing by one
• Nucleophilic substitution with ammonia


  1. Change in functional group: halogenoalkane ⟶ primary amine

  2. Reagent: NH₃ dissolved in ethanol

  3. Conditions: Heating under pressure in a sealed tube

  4. Mechanism: Nucleophilic substitution

  5. Role of reagent: Nucleophile, NH₃
• Amine


  Based on ammonia but one or more of the hydrogen atoms are replaced by alkyl group
• Elimination with alcoholic hydroxide ions


  1. Change in functional group:halogenoalkane ⟶ alkene

  2. Reagents: Potassium (or sodium) hydroxide

  3. Conditions: In ethanol; Heat

  4. Mechanism: Elimination

  5. Role Of reagent: Base, OH
• Elimination:removal of small molecule (often water) from the organic molecule
• Tertiary halogenoalkanes


  Most reactive, since it reacts the fastest with water
• Primary halogenoalkane


  Least reactive
• Weaker C-X bonds


  Break more easily-so they react faster
• Bond enthalpy depends on the size of the halogen


  The larger the halogen,the longer the C-X bond,and the lower bond enthalpy
• The size of the halogen increases down group 7


  Iodoalkanes have the weakest bonds,and are hydrolysed the fastest .Fluoroalkanes have the strongest bonds,so hydrolysed the slowest
• SN2(primary)


  Primary halogenoalkanes don't do SN1 mechanism because they would only form an unstable primary carbocation
• SN1(tertiary)


  Carbocation is stabilised by the electron releasing methyl groups around it. Also the bulky methyl groups prevent the hydroxide ion from attacking the halogenoalkane
• CORE PRACTICAL 4: Investigation of the rates of hydrolysis of some halogenoalkanes