Nucleophilic Substitution Reactions
Haloarenes are less reactive towards nucleophilic substitution reaction (why? discussed in chemical properties of haloarenes). However, under drastic conditions they do undergo nucleophilic substitution reaction.
Some of the reactions are shown below :
Replacement by OH−
When haloarenes are heated with an aqueous solution of NaOH at 623K and under 300 atmospheric pressure, sodium phenoxides (C6H5−ONa) are formed which upon acidification yield phenols.
The presence of electron withdrawing groups such as NO2, CN etc., at ortho and para positions increase the reactivity of haloarenes.
Only warm water is required in the formation of picric acid
2,4,6-trinitrophenol (Picric acid)
Question : Arrange 4-nitrochlorobenzene; 2,4-dinitrochlorobenzene and 2,4,6-trinitrochlorobenzene according to reactivity towards nucleophilic substitution reaction with CH3ONa.
Answer : Electron-withdrawing groups increase the reactivity towards nucleophilic substitution
reaction; hence, the order is:
2,4,6-trinitrochlorobenzene > 2,4-dinitrochlorobenzene > 4-nitrochlorobenzene