Addition reaction is a type of reaction in which two or more molecules combine with each other to form a single substance. This type of reaction occurs only with those molecules which have either a double or a triple bond between their carbon atoms.Due to having double bond,these molecules can break one of the carbon-carbon bonds and can ADD other atoms by sharing those electrons with other atoms(which were previously bonded to carbon atoms). Thus, addition reaction makes an unsaturated compound into a saturated compound. Ex--C2H4 + H2 = C2H6
Substitution reaction is a type of reaction in which one atom of any other element substitutes or replaces one of the hydrogen atoms of a given hydrocarbon.Substitution reaction does not add any atom to the hydrocarbon because it takes place only in saturated hydrocarbons(containing only single bonds).Thus if any atom has to be added in the hydrocarbon,one of the hydrogen atoms needs to be replaced by that atom.
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