Replacement of $$Cl$$ of chlorobenzene to give phenol requires drastic conditions, but $$Cl$$ of $$2,4$$-dinitrochlorobenzene is readily replaced. This is because :A. $$ - N{O_2}$$ group makes the ring electron rich at ortho and para positions.B. $$ - N{O_2}$$ group withdraws electrons from meta position.C. $$ - N{O_2}$$ donate electrons at meta position.D. $$ - N{O_2}$$ withdraws electrons from ortho and para positions.

Question

Replacement of $$Cl$$ of chlorobenzene to give phenol requires drastic conditions, but $$Cl$$ of $$2,4$$-dinitrochlorobenzene is readily replaced. This is because :A. $$ - N{O_2}$$​ group makes the ring electron rich at ortho and para positions.B. $$ - N{O_2}$$ group withdraws electrons from meta position.C. $$ - N{O_2}$$​ donate electrons at meta position.D. $$ - N{O_2}$$​ withdraws electrons from ortho and para positions.

Vedantu Content Team · Accepted Answer

Hint: Replacement is a synonym of substitution (used in organic chemistry). Replacement is favoured when the bond strength is not so strong and after the removal of the concerned group, the system does become unstable.Complete step-by-step answer: In chlorobenzene, $$Cl$$ is attached to the carbon of the benzene ring which is highly aromatic. Removal of chloride is not easy and requires drastic conditions. This is because of the following two reasonsThe removal of chloride from the benzene ring leads to the formation of phenyl ion because the aromaticity of the benzene ring is disturbed.$$Cl$$ is attached to $s{p^2}$ hybridized carbon atom of benzene ring. Hence the bond between $$Cl$$ and carbon atom of the benzene ring is very strong and thus, not easily dissociated.In $$2,4$$-dinitrochlorobenzene, $$Cl$$ is attached to the benzene ring and all the above conditions are the same. But $$ - N{O_2}$$ group is an electron withdrawing group because of its high electronegativity than carbon of benzene ring. Hence, it withdraws the electron density from ortho and para positions and not meta position (this can be seen by the resonating structures of $$2,4$$-dinitrochlorobenzene). $$Cl$$ is present at ortho to one of the nitro group and para to other nitro group. Thus, the two groups extensively pull electron density making the release of chloride easy and promotes attack of nucleophile. This is called nucleophilic aromatic substitution. The substitution takes place when there is an electron withdrawing group present at ortho or para position to the leaving group.  Option ‘D’ is correctNote: Conversion of chlorobenzene from phenol is called Dow's Process. It takes place at drastic condition of $350$ degree C temperature and $300$ bar pressure in presence of $NaOH$.