3.3 Halogenoalkanes

• what are halogenoalkanes?

  alkanes with 1 or more halogens attached to it
• how does boiling point change down group 7?

  Boiling point increases down the group

  - as you descend group 7 the number of electrons in the halogen atoms increases

  - more electrons = stronger Van der Waals between molecules and so more energy is needed to overcome these forces

  - this leads to the boiling points increasing as we go down the group
• what do halogenoalkanes have and therefore make them susceptible to attack from?

  polar bond that make them susceptible to attack by nucleophiles
• why is the polar bond formed?

  halogens are more electronegative than carbon so they pull electrons towards themselves in a covalent bond
• what is a nucleophile?

  electron pair donor
• give examples of nucleophiles

  ammonia :NH3

  hydroxide ions :OH-

  cyanide ions :CN-
• what do curly arrows show in mechanisms?

  movement of electron pairs
• name the mechanism for the reaction of halogenoalkanes with a nucleophile

  nucleophilic substitution
• what are the conditions needed for a hydroxide ion to react with halogenoalkanes?

  1. warm aqueous NaOH

  2. under reflux
• what is formed by the reaction of a hydroxide ion and halogenoalkane?

  alcohol
• what are the conditions needed for cyanide ions to react with halogenoalkane?

  1. warm ethanolic KCN

  2. under reflux
• what is formed by the reaction of a cyanide ion and halogenoalkane?

  nitrile
• what are the conditions needed for ammonia to react with halogenoalkanes?

  1. heat with ethanolic ammonia

  2. excess ammonia
• why do you need excess ammonia?

  in the second phase of this reaction another molecule of NH3 acts as a base by reacting with hydrogen on the intermediate
• what is produced from the reaction between ammonia and nucleophilic substitution?

  amine and an ammonium ion
• explain the trend in reactivity in halogenoalkanes

  halogenoalkanes become more reactive as you go down the group
• what determines reactivity?


  the bond strength / bond enthalpy determines the reactivity of haloalkanes not bond polarity
• how do halogenoalkanes go under elimination reactions?

  with hydroxide ions using ethanol solvent
• what are the conditions for elimination of halogenoalkanes?

  1. warm ethanolic NaOH

  2. under reflux
• describe the elimination mechanism

  1. the OH- will attack the hydrogen on a carbon adjacent to the carbon with the halogen on. OH- acts as a base (proton acceptor) forming water

  2. the electrons in the bond move to form a double bond between two carbons

  3. the C-X breaks, both electrons move from the bond to the halogen
• what is formed from elimination of halogenoalkanes?

  an alkene
• how does the solvent used play a role in deciding if a reaction is elimination or substitution?

  when reacting NaOH with a halogenoalkane we can make:

  - an alkene - when using ethanol as a solvent (OH- acts as a base)

  - an alcohol - when using water as a solvent (OH- acts as a nucleophile)
• what are CFCs?


  molecules that have had all their hydrogens replaced by chlorine and fluorine
• why are chlorofluorocarbons (CFCs) bad?

  break down ozone in the atmosphere
• what happens to form the radical that breaks down ozone and why?

  - C-Cl bonds are broken down by UV radiation in the atmosphere

  - C-Cl bonds are broken easiest by UV as they have the lowest bond enthalpy
• give equations to show the break down of ozone by CFCs?

  Initiation:

  CCl3F -> CCl2F + Cl* (under UV light)

  Propagation:

  1. Cl+ O3 -> O2 + ClO

  2. ClO+ O3 -> 2O2 + Cl

  Termination:

  Cl+ Cl-> Cl2
• give the overall equation for ozone depletion

  2O3-> 3O2

  Cl* is the catalyst