AS Chemistry

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What is the difference between substitution and displacement reactions?
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Igloo writes ..........
 
In English, a “displacement” tends to lead to a “substitution”, but in chemistry the term “displacement” tends to be reserved to describe inorganic redox reactions where a more reactive metal 'displaces' a less reactive one, such as:
 
Mg(s) + CuO(s) ----> MgO(s) + Cu(s)
 
Although rarely used nowadays, the term “double displacement”, also reserved for inorganic reactions, involves a double “switch”, such as that occurring during a precipitation reaction:
 
eg Pb(NO₃)₂ + Na₂SO₄(aq) ----> PbSO₄(s) + 2NaNO₃(aq)
 
These days, however, we tend much to prefer thinking about what is really going on and give the ionic reaction instead:
 
Pb²⁺(aq) + SO₄^2-(aq) ----> PbSO₄(s)
 
In this way, the spectator ions, NO₃^- and Na⁺ are ignored, since they play no part in the process at all.
 
“Substitution” is the term predominantly used in organic chemistry, and describes a process in which one species (group) replaces another, e.g. the nucleophilic substitution occurring when halogenoalkanes react with the hydroxide ion:
 
eg OH^-(aq) + CH₃CH₂Br(l) ----> CH₃CH₂OH(aq) + Br^-(aq)
 
Substitution reactions are, of course, just one type of organic reaction. There are addition, elimination and addition/elimination reactions as well, together with other reactions, such as oxidations (eg primary alcohols to ketones) and reductions (eg carboxylic acids to primary alcohols).
 
In summary then, it is more a matter of chemical convention than anything else that we distinguish between “substitution” and “displacement” in this way.

Risk assessment
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updated: 29 April 2010