Aromaticity Chemistry Of Aromatic Compounds

(i) Chlorobenzene to Picric Acid:
(ii) Chlorobenzene to Dichlorobenzene:
(iii) Chlorobenzene to Chlorobenzene Sulphonic Acid:
Conclusion:
Chlorobenzene undergoes nitration to give picric acid, halogenation to give dichlorobenzene, and sulphonation to give chlorobenzene sulphonic acid.

Step 1: Recall nitration mechanism.
Nitration is an electrophilic substitution reaction where an electrophile attacks the benzene ring.
Step 2: Electrophile formation.
In nitration, concentrated HNO reacts with concentrated H SO to form the nitronium ion (NO ).
Step 3: Role of electrophile.
This nitronium ion attacks the benzene ring of benzaldehyde to carry out nitration.
Step 4: Conclusion.
Thus, the electrophile is NO (nitronium ion).

Step 1: Understanding Resonance in Haloarenes.
In haloarenes (like chlorobenzene), the lone pair of electrons on the halogen atom interacts with the -electrons of the benzene ring. This delocalization leads to resonance structures.
Step 2: Resonance Structures.
The -orbital of the halogen atom overlaps with the -orbitals of the benzene ring, giving rise to multiple resonance structures. As a result, the C–Cl bond acquires partial double bond character.
Step 3: Effect on Nucleophilic Substitution.
Because of resonance: - The C–Cl bond in haloarenes is shorter and stronger than a normal C–Cl bond.
- This makes the bond difficult to break, thereby making nucleophilic substitution reactions of haloarenes much less reactive compared to haloalkanes.
Conclusion:
Resonance stabilizes the C–Cl bond in haloarenes, giving it partial double bond character and reducing the tendency for nucleophilic substitution.

Step 1: Nature of chlorine as a substituent.
Chlorine is an electron withdrawing group due to its strong –I (inductive) effect because of high electronegativity. This tends to decrease electron density on the benzene ring.
Step 2: Resonance effect of chlorine.
Chlorine possesses lone pairs of electrons which it can donate to the benzene ring through resonance (+R effect). This resonance increases electron density particularly at the ortho and para positions of the benzene ring.
Step 3: Combined effect.
- The –I effect makes chlorine a deactivating group (reaction rate decreases).
- The +R effect directs the incoming electrophile to ortho and para positions.
Conclusion:
Although chlorine is overall deactivating due to its –I effect, it acts as an
ortho-para directing group in electrophilic substitution reactions because of its strong +R effect.

Step 1: Reactivity comparison.
- In chloroethane, the C–Cl bond is a simple polar covalent bond; chlorine can easily undergo nucleophilic substitution. - In chlorobenzene, the lone pair of chlorine interacts with the benzene ring ( -resonance). This delocalisation gives partial double bond character to the C–Cl bond. Step 2: Effect of resonance.
Due to resonance: 1. Bond length of C–Cl decreases. 2. Bond strength increases. 3. Nucleophilic substitution becomes difficult. Step 3: Reaction with Cl .
In presence of FeCl , chlorobenzene undergoes electrophilic substitution: Products: 1,2-dichlorobenzene (ortho) and 1,4-dichlorobenzene (para). Step 4: Reaction with conc. H SO .
Sulphonation occurs: Product: p-chlorobenzene sulphonic acid (major). Step 5: Conclusion.
Thus, chlorobenzene is less reactive than chloroethane due to resonance, and it undergoes electrophilic substitution with Cl and H SO .

(A) Electrophilic substitution in halobenzene Step 1: Nature of halogen substituent.
In halobenzene, halogen atoms are deactivating due to the –I effect (electron withdrawing inductive effect). However, they also exhibit +R effect (electron donating by resonance), which increases electron density at ortho and para positions. Step 2: Directive influence.
Because of this dual effect, halogens are ortho/para directing but overall deactivate the ring towards electrophilic substitution. Step 3: Example reaction.
Chlorobenzene + Cl (in presence of FeCl ): Step 4: Conclusion.
Thus, halobenzene undergoes electrophilic substitution at ortho and para positions, but at a slower rate than benzene.

Step 1: Identify compound A.
Since the product (C) has molecular formula C H N, it must be an aromatic amine (aniline). The intermediate step suggests that compound B is amide, which upon Hoffmann bromamide reaction gives amine. Hence, A must be an aromatic acid derivative.
Thus, compound A is
benzamide (C H CONH ).
Step 2: Reaction with aqueous ammonia.
An aromatic acid chloride (benzoyl chloride, C H COCl) reacts with aqueous ammonia to form
benzamide (B).
Step 3: Hoffmann bromamide reaction.
When benzamide (B) is treated with Br and KOH, Hoffmann bromamide reaction occurs. The –CONH group is converted into –NH . Thus, the product is
aniline (C H NH ).
Step 4: Structures of compounds.
\begin{itemize} \item Compound A: Benzoyl chloride (C H COCl) \item Compound B: Benzamide (C H CONH ) \item Compound C: Aniline (C H NH ) \end{itemize} Conclusion:
- A = Benzoyl chloride (C H COCl)
- B = Benzamide (C H CONH )
- C = Aniline (C H NH )

(i) Chlorobenzene to Benzene:
By reduction with Ni/Alkali.
(ii) Chlorobenzene to Phenol:
By hydrolysis under high temperature and pressure.
(iii) Chlorobenzene to Toluene:
Through Friedel–Crafts alkylation.
(iv) Chlorobenzene to Aniline:
Via nitration followed by reduction.
Then by Sandmeyer reaction: (v) Chlorobenzene to Diphenyl:
Through Ullmann reaction.
Conclusion:
Chlorobenzene undergoes reduction, hydrolysis, substitution, or coupling reactions under suitable conditions to give benzene, phenol, toluene, aniline, and diphenyl.