Lesson 10, Topic 1
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Halogenoalkane notes

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What are halogenoalkanes ?

  • A Halogenoalkane is an alkane with at least one halogen atom in place of a hydrogen atom

Halogenoalknes can be primary, secondary or tertiary

On the carbon with the halogen attached :

  1. Primary : 1 alkyl groups attached
  2. Secondary : 2 alkyl groups attached
  3. Tertiary : 3 alkyls groups attached

Forming alcohols from halogenoalkanes by hydrolysis

Halogenoalkanes can be hydrolysed (bonds broken using water) to alcohols in a nucleophilic substitution reaction. 

Comparing the reactivities of halogenoalkanes

  • Mix halogenoalkanes with water to form an alcohol

 

 

  • Add silver nitrate solution to the mixture. The silver ions react with the halide ions as soon as they form, giving a silver halide precipitate

  • Set up 3 test tubes each containing a different halogenoalkane, ethanol and aqueous silver ntrate solution
  • Time how long it takes for a precipitate to form in each test tube. The  more quickly a precipitate forms, the faster the rate of hydrolysis is for that halogenoalkane
  • You can use the colours of the precipitates to distinguish between chloro, bromo and iodo halogenoalkanes

explaining the reactivity of the halogenoalkanes

  • In order to hydrolyse a halogenoalkane you have to break the carbon-halogen bond
  • How quickly different halogenoalkanes are hydrolysed depends on the cabon-halogen bond enthalpy

  • Weaker carbon-halogen bonds break more easilly – 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 the bond enthalpy
  • The size of the halogen increase down group 7, so iodoalkanes have the weakest bonds and are hydrolyesd the fastest. Fluoroalkanes have the strongest bonds so they are the slowest at hydrolysing
  • You can compare the reactivity of chloroalkanes, bromoalkanes and iodoalkanes by experiment

halogenoalkanes and nucleophilic substitution

  • Halogens are generally more electronegative than carbon.
  • So, the carbon – halogen bond is polar
  • The δ+ carbon doesnt have enough electrons. This means it can be attacked by a nucelophile. A nucleophiles an electron pair donor. It donates an electron pair to somewhere without enough electrons
  • OH-, NH3 and CN- are examples of nucleophiles that react readily with halogenoalkanes. Water is also a weak nucleophile
  • A nucleophile can bond with the δ+ carbon of a halogenoalkane and be substituted for the halogen
  • This is called nucleophilic substitution:
  • X is the halogen. Nuc is the nucleophile, which provides a pair of electrons for the Cδ+
  • The C-X bond breaks heterolytically – both electrons from the bond are taken by the halogen
  • The halogen falls off as the nucleophile bonds to the carbon