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

Step 1: Part (a): Two effects of chlorine on benzene ring.
Chlorine (and halogens in general) exert two opposing effects on an attached aromatic ring: (i) Inductive effect ($-I$): Cl is more electronegative than C, withdrawing electrons through the sigma bond, overall deactivating the ring. (ii) Resonance effect ($+R$): Cl has lone pairs that can be donated into the pi-system of the ring through resonance, increasing electron density at ortho and para positions.
Step 2: Which effect determines the direction of electrophilic attack?
The $+R$ effect controls the direction: it specifically increases electron density at ortho and para positions via resonance. Electrophilic aromatic substitution (EAS) occurs where electron density is highest, i.e., at ortho and para positions. The $-I$ effect mostly affects the overall rate of substitution (making the ring slower to react than benzene), but the direction is still ortho/para due to $+R$.
Step 3: Explain the apparent paradox.
Even though Cl withdraws electrons inductively (deactivating), its lone-pair resonance donation specifically targets ortho/para positions. So Cl is an ortho/para director because the $+R$ effect governs the regioselectivity, even as the $-I$ effect lowers the overall reactivity. Cl is called a deactivating ortho/para director.
Step 4: Part (b): Why aryl halides cannot be prepared from phenol + HX or PX3.
For aliphatic alcohols, $-OH$ is easily converted to a leaving group: protonation gives $-OH_2^+$, and then $X^-$ attacks via SN1 or SN2 to give $R-X + H_2O$. This is feasible because the sp3 C-O bond can be broken.
Step 5: Why phenol's C-O bond cannot be broken this way.
In phenol, the oxygen lone pairs participate in resonance with the benzene ring (phenol resonance structures show C-O partial double-bond character): \[ C_6H_5-OH \leftrightarrow \text{resonance structures with }C=O \text{ character} \] This makes the C-O bond much stronger and shorter than in an aliphatic alcohol. Neither SN1 (would require an unstable aryl cation, sp2 carbon) nor SN2 (would require backside attack on an sp2 centre, geometrically impossible) can occur. HX and $PX_3$ therefore cannot cleave the phenolic C-O bond to give aryl halides.
Step 6: State both conclusions clearly.
(a) Cl is ortho-para directing because its lone-pair resonance ($+R$ effect) places extra electron density specifically at ortho/para positions for electrophilic attack, overriding the overall $-I$ deactivation in terms of regioselectivity. (b) In phenol, the C-O bond has partial double-bond character due to resonance; it is too strong to be broken by HX or $PX_3$ under normal conditions; SN1/SN2 at an sp2 aromatic carbon are not feasible. \[ \boxed{\text{(a) }+R\text{ directs ortho/para; (b) C-O resonance makes it uncleavable by HX/PX}_3} \]