STPM Chemistry Form 6 – The Alkanes (Part 2)

by BerryBerryTeacher

in Berry Reference (Notes)

This Part 2 of notes on The Alkanes from Berry Berry Easy for STPM Form 6 Chemistry is focussed on the general reactions of alkanes. The focus in this part is given to reactions of alkanes with halogens. Typically, alkanes react with halogens in the “free radical halogenation” reaction where H-atoms of alkanes are replaced one-by-one by halogen atoms. Free radical halogenation of alkanes usually undergo three steps which is the initiation, propagation and termination. Read more about it below.

[Tips: Reactions of alkanes with halogens can potentially be explosive in nature. Why would this be so? Make a guess before reading further. Try to think in terms of energy released or energy gain. So have you guess the answer? Simple, because the reaction is highly exothermic. Did you guess it correct the first time?]

STPM Chemistry Form 6 Note – The Alkanes (Part 2)

Petroleum Crude

General Reaction

Substitute reaction (Free radical reaction)

  • Halogenation – (reactant; chlorine, Cl2 / bromine, Br2 in the presence of heat or light)

Mechanism of halogenations:

1) Chain Initiation:

  • Cl2 –> 2Cl•
  • Halogen radicals are formed by homolysis.
  • Energy is required in this initiation process, usually through the provision of heat or light.

2) Chain Propagation:

  • CH4 + Cl• –> •CH3 + HCl
  • •CH3 + Cl2 –> CH3Cl + Cl•
  • Also known as chain reaction.
  • Here, halogen radical abstracts a H-atom from the alkane to give it to an alkyl radical.

3) Chain Termination:

  • Cl• + Cl• –> Cl2
  • •CH3 + Cl• –> CH3Cl
  • •CH3 + •CH3 –> CH3CH3
  • In the chain termination step, radicals recombined and they ceased to be radicals.
  • The end products are stable structures.

This marks the end of the Part 2 of notes on The Alkanes for STPM Form 6 Chemistry by Berry Berry Easy.

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