Q.What are electrophilic substitution reactions? Explain its mechanism with an example of Aryl halide.

Phenol is highly activated toward electrophilic substitution, and dilute nitric acid at low temperature acts as a mild nitrating agent. The reaction yields a mixture of ortho- and para-nitrophenol, with the ortho isomer being the major product due to intramolecular hydrogen bonding.

Direct nitration of aniline is carried out in a strongly acidic medium, in which most of the aniline is protonated to the meta-directing anilinium ion. As a result a large amount of the meta isomer forms alongside para, with ortho suppressed: about 51% para, 47% meta, 2% ortho — option (C).

Friedel–Crafts acylation needs a genuine, water-free Lewis acid to activate the acyl halide into an electrophilic acylium ion; only anhydrous $AlCl_3$ qualifies among the options.

Mechanism: $RCOCl + AlCl_3 \rightarrow RCO^+ + AlCl_4^-$ The resonance-stabilised acylium ion $RCO^+$ is the actual electrophile that attacks the benzene ring (electrophilic aromatic substitution), giving an aryl ketone after loss of $H^+$ (which recombines with $AlCl_4^-$ to regenerate $HCl$ and the $AlCl_3$ catalyst).

Why the other options are wrong:

• (b) Aqueous $AlCl_3$: water reacts with and hydrolyses $AlCl_3$ (to $Al(OH)_3$ + $HCl$), destroying its Lewis-acid character; it also hydrolyses the acyl chloride reagent before it can react — the reaction cannot proceed. …

Groups that DONATE electron density into the benzene ring (via resonance or induction) are ortho/para directors; groups that WITHDRAW electron density are meta directors. -NH2 is a strong electron donor; the other three are all electron-withdrawing (carbonyl-type or nitrile) groups.

In electrophilic aromatic substitution, substituents already on a benzene ring direct where the next incoming electrophile attacks, based on how they affect the ring's electron density:

• Ortho/para-directing, activating groups: donate electron density into the ring (by resonance and/or induction), increasing electron density preferentially at the ortho and para positions. Examples: -NH2, -OH, -OCH3, -CH3, halogens (deactivating but still o/p-directing).
• Meta-directing, deactivating groups: withdraw electron density from the ring (usually via a carbonyl-type resonance-withdrawing structure, C=O or C#N conjugated to the ring), leaving the meta position relatively more electron-rich than ortho/para. Examples: -COOH, -CHO, -COR (including -COCH3), -CN, -NO2, -SO3H.

Check each option:

• -COOH (carboxylic acid): the C=O is conjugated with the ring and withdraws electron density -> meta director …

Halogenation of an aromatic ring (toluene) using $X_2$/$FeCl_3$ proceeds through attack of an electrophile on the electron-rich benzene ring, replacing a ring hydrogen — a textbook electrophilic aromatic substitution.

$FeCl_3$ is a Lewis acid catalyst that polarises the halogen molecule, generating an electrophile ($X^+$-like species). This electrophile attacks the electron-rich aromatic ring of toluene (activated further by the ring-activating, ortho/para-directing methyl group) to form an arenium (sigma-complex) intermediate, which then loses $H^+$ to restore aromaticity, giving ortho- and para-halotoluenes: …

Electron-withdrawing groups like -NO2 pull electron density away from the ortho/para positions, leaving the meta position comparatively electron-richer and thus favoured for electrophilic attack.

Substituents on a benzene ring are classified by how they direct further electrophilic aromatic substitution:

• Ortho/para directors: electron-donating groups (e.g. -OH, -NH2, -CH3 (option a, b, d) via +I or +M/resonance effects) increase electron density at the ortho and para positions. …

Friedel-Crafts alkylation of chlorobenzene with methyl chloride puts a methyl group ortho and para to the existing chlorine (since $-Cl$ is an o,p-director); the desired 2-chlorotoluene (ortho product) is then separated from the para isomer by fractional distillation.

Friedel-Crafts alkylation: Although chlorine is a net electron-withdrawing (deactivating) substituent due to its strong inductive effect, its lone pairs can still donate into the ring by resonance, which directs incoming electrophiles to the ortho and para positions (relative rates at those positions are higher than at meta, even though the ring overall reacts more slowly than benzene). Treating chlorobenzene with methyl chloride in the presence of anhydrous aluminium chloride (a Lewis acid catalyst that generates the electrophilic methyl carbocation, $CH_3^+$, from $CH_3Cl$) gives a mixture of ortho- and para-chlorotoluene:

$C_6H_5Cl \xrightarrow[\Delta]{CH_3Cl,\; \text{anhyd. } AlCl_3} o\text{-ClC}_6H_4CH_3 \; + \; p\text{-ClC}_6H_4CH_3$

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-OH is an ortho, para-directing group because its lone pairs donate electron density into the aromatic ring by resonance, increasing electron density specifically at the ortho and para positions; -CHO, -NO2 and -COOH are all electron-withdrawing groups that direct meta instead.

Substituent directing effects in electrophilic aromatic substitution depend on whether the group donates or withdraws electron density by resonance/induction:

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Introduce chlorine onto the ring before nitration — since $-Cl$ directs the incoming $-NO_2$ group to the ortho/para positions — then separate the desired para isomer from the ortho by-product.

Step 1 — Chlorination (electrophilic aromatic substitution):

$C_6H_6 + Cl_2 \xrightarrow[\text{dark, anhyd.}]{AlCl_3} C_6H_5Cl + HCl$

Benzene reacts with chlorine in the presence of anhydrous $AlCl_3$ (a Lewis-acid catalyst that polarizes $Cl_2$ to generate the electrophile $Cl^+$), substituting one ring hydrogen to give chlorobenzene.

Step 2 — Nitration:

$C_6H_5Cl \xrightarrow[\Delta]{\text{conc. }HNO_3 + \text{conc. }H_2SO_4} o\text{-chloronitrobenzene} + p\text{-chloronitrobenzene}$

The nitrating mixture generates the electrophile $NO_2^+$. Chlorine is an o,p-directing group (its lone pair donates into the ring by resonance, even though its inductive effect makes it net deactivating), so nitration occurs mainly at the ortho and para positions relative to $-Cl$, giving a mixture of the two isomers.

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