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Halogenoalkane lesson


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Halogenoalkane lesson

  1. 1. Halogenoalkane Version 1.4
  2. 2. <ul><li>Halogenoalkanes are compounds in which one </li></ul><ul><li>or more hydrogen atoms in an alkane have </li></ul><ul><li>been replaced by halogen atoms (fluorine, </li></ul><ul><li>chlorine, bromine or iodine). </li></ul>
  3. 3. The different kinds of halogenoalkanes <ul><li>Halogenoalkanes fall into different classes depending on how the halogen atom is positioned on the chain of carbon atoms. There are some chemical differences between the various types. </li></ul><ul><li>Primary </li></ul><ul><li>Secondary </li></ul><ul><li>Tertiary </li></ul>
  4. 4. Reactions of the halogenoalkanes <ul><li>Nucleophilic substitution </li></ul><ul><li>Elimenation reaction </li></ul>
  5. 5. <ul><li>Nucleophilic Substitution </li></ul><ul><li>In a substitution reaction, one atom or group of atoms, takes the place of another in a molecule. </li></ul><ul><li>A nucleophile is a molecule or ion that has a high electron density. It is attracted to atoms in molecules with a lower electron density. </li></ul><ul><li>The general form for the reaction is </li></ul><ul><li>Nu: - + R-X  R-Nu + X: </li></ul>
  6. 6. 1.With aqueous alkali <ul><li>When an aqueous solution of NaOH or KOH added to haloalkane alcohol is produced. </li></ul><ul><li>propan-2-ol </li></ul>
  7. 7. The mechanism
  8. 8. 2.With Cyanide ion <ul><li>The halogenoalkane is boiled under reflux with potassium cyanide dissolved in ethanol. </li></ul><ul><li>Notice : </li></ul><ul><li>Reaction added an extra carbon atom to the chain. </li></ul>+ CN - (aq) propanenitrile CH 3 CH 2 CN + Br - CH 3 CH 2 Br
  9. 9. 3.With alcoholic ammonia (NH 3 ) <ul><li>The nitrogen atom carries a partial negative charge and has alone </li></ul><ul><li>pair of electrons. This makes it nucleophile. </li></ul><ul><li>concentrated ammonia (in excess) in ethanol solution.heat in a sealed tube. </li></ul>
  10. 10. Reaction : <ul><li>CH 3 CH 2 Br + NH 3 ---------> CH 3 CH 2 NH 2 + H Br Ethylamine (amine) </li></ul><ul><li>The amine formed is also a nucleophile. </li></ul><ul><li>It will attack any unreacted bromoethane in the mixture. </li></ul><ul><li>CH 3 CH 2 Br + CH 3 CH 2 NH 2  ( CH 3 CH 2 ) 2 NH + HBr </li></ul><ul><li>diethylamine </li></ul><ul><li>(a secondary amine) </li></ul><ul><li>The secondary amine formed can also act as a nucleophile </li></ul><ul><li>CH 3 CH 2 Br + ( CH 3 CH 2 ) 2 NH  ( CH 3 CH 2 ) 3 N + HBr </li></ul><ul><li>triethylamine </li></ul><ul><li>( a tertiary amine) </li></ul><ul><li>If any unreacted bromoethyane remain , the tertiary amine attack it. </li></ul><ul><li>CH 3 CH 2 Br + ( CH 3 CH 2 ) 3 N    (CH 3 CH 2 ) 4 N + Br - </li></ul><ul><li>tetraethylammonium bromide </li></ul>
  11. 11. <ul><li>Elimination reaction </li></ul><ul><li>Halogenoalkanes also undergo elimination reactions in </li></ul><ul><li>the presence of sodium or potassium hydroxide which </li></ul><ul><li>dissolved in ethanol. </li></ul>
  12. 12. The mechanism <ul><li>In elimination reactions, the hydroxide ion acts as a base - removing a hydrogen as a hydrogen ion from the carbon atom next door to the one holding the bromine. </li></ul><ul><li>The resulting re-arrangement of the electrons expels the bromine as a bromide ion and produces propene. </li></ul>
  13. 13. <ul><li>In </li></ul><ul><li>the substitution reaction between a halogenoalkane and OH - ions, the hydroxide ions are acting as nucleophiles. For example, one of the lone pairs on the oxygen can attack the slightly positive carbon. This leads on to the loss of the bromine as a bromide ion, and the -OH group becoming attached in its place. </li></ul>The role of the hydroxide ion in a substitution reaction
  14. 14. The role of the hydroxide ion in an elimination reaction <ul><li>Hydroxide ions have a very strong tendency to combine with hydrogen ions to make water - in other words, the OH - ion is a very strong base. In an elimination reaction, the hydroxide ion hits one of the hydrogen atoms in the CH 3 group and pulls it off. This leads to a cascade of electron pair movements resulting in the formation of a carbon-carbon double bond, and the loss of the bromine as Br - . </li></ul>
  15. 15. Application <ul><li> </li></ul>