Question

Give reasons for the following: \[ (a)\;\text{Although chlorine is an electron-withdrawing group, yet it is ortho-, para-directing in electrophilic aromatic substitution reactions.} \] \[ (b)\;\text{Aryl halides cannot be prepared by reacting phenol with concentrated halogen acids or phosphorus halides.} \] \[ (c)\;\text{Chloroform is stored in closed dark coloured bottles.} \]

Hint:

Halogens are deactivating but ortho-para directing because \(-I\) controls reactivity and \(+R\) controls orientation.

Answer 1

Concept:
Halogens show two effects in aromatic compounds: \[ -I\text{ effect} \] and \[ +R\text{ effect} \] The \(-I\) effect withdraws electrons and deactivates the ring. The \(+R\) effect donates lone pair electrons to the ring and directs substitution to ortho and para positions.

Step 1: Reason for ortho-para direction by chlorine.
Chlorine withdraws electrons through inductive effect. \[ Cl \Rightarrow -I \] Therefore, it deactivates the benzene ring. But chlorine also has lone pairs. These lone pairs are donated to the benzene ring by resonance. \[ Cl \Rightarrow +R \] Due to resonance, electron density increases at ortho and para positions. Therefore, chlorine is ortho-para directing.

Step 2: Reason aryl halides cannot be prepared from phenol using halogen acids or phosphorus halides.
In phenol, the \(C-O\) bond has partial double bond character due to resonance. \[ C-O \Rightarrow \text{partial double bond character} \] Therefore, the \(C-O\) bond is strong and difficult to break. So phenol does not react with concentrated halogen acids or phosphorus halides to form aryl halides.

Step 3: Reason for storage of chloroform.
Chloroform undergoes oxidation in presence of air and light to form phosgene. \[ 2CHCl_3+O_2 \rightarrow 2COCl_2+2HCl \] Phosgene \((COCl_2)\) is highly poisonous. Therefore, chloroform is stored in closed dark coloured bottles. Hence: \[ \boxed{\text{Chlorine is ortho-para directing due to resonance donation.}} \] \[ \boxed{\text{Phenol has strong }C-O\text{ bond due to resonance, so aryl halides are not formed.}} \] \[ \boxed{\text{Chloroform is stored in dark bottles to prevent formation of phosgene.}} \]