Lesson 5, Topic 1
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Bond breaking in organic reactions notes

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Organic compounds contain covalent bonds, for example between

  • Two carbon atoms
  • A carbon atom and a hydrogen atom
  • A carbon atom and a halogen atom

There are two different ways for the covalent bond to break. These are homolytic fission and heterolytic fission

  • In homolytic fission, the shared pair of electrons in the covalent bond divide equally between the two atoms
  • Each product species keeps one of the electrons from the covalent bond.
  • These species are called free radicals
  • Each free radical has an unpaired electron and is uncharged
  • Homolytic fission usually occurs when the two atoms bonded together are identical or when they have similar electronegativities 
  • In heterolytic fission, both electrons of the shared pair in the covalent bond are kept by one of the atoms. This can be shown as :
  • One of the products keeps both of the electrons from the covalent bond.
  • This species is a negative ion
  • The right hand product species does not keep either of the electrons from the covalent bond. 
  • This species is a positive ion
  • heterolytic fission usually occurs when the two atoms bonded together have different electronegativities
  • The atom with the higher electronegativity is the one that keeps both electrons from the bond
  • In addition reactions, two reactant species combine together to form a single species
  • Usually all the species are molecules 
  • A general equation for this type of reaction is :

A + B → C 

  • In a substitution reaction, two reactant species combine together to form two product species.
  • A general equation for this type of reaction is :

A + B → C + D

Example: 

  • In this reaction, the OH group has taken the place of or substituted the Br atom
  • In an oxidation reaction, one organic compound is oxidised, usually by an inorganic reagent. 
  • This means that the organic compound can either lose hydrogen or gain oxygen
  • Usually the oxygen atoms produced by an oxidising agent are shown using the symbol [O]

Example: Oxidation of ethanol using a mixture of sulfuric acid and potassium dichromate (VI) [O]

  • It is important to use [O] to indicate an oxidation agent rather than O2 as that would have indicated that molecular oxygen was used  
  • In a reduction reaction, one organic compound is reduced, sometimes by hydrogen gas and a catalyst and sometimes by an inorganic reagent.
  • This means that the organic compound can either gain hydrogen or loose oxygen

Example: Reduction of an alkene to an alkane by hydrogen gas and nickel catalyst

  • This is a reduction reaction because two hydrogen’s have been gained.
  • It is also an example of an addition reaction
  • In addition polymerisation very large numbers of a reactant molecule react together to form one very large product molecule.
  • A general equation for this is:
  • A familiar example of this type of reaction is the polymerisation of ethene to poly(ethene)