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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)