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Benzene Structure, Aromaticity, and Electrophilic Substitution

3 questions

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Question 1Question →

Benzene preferentially undergoes electrophilic substitution reactions rather than electrophilic addition reactions under standard conditions because addition reactions disrupt the aromatic resonance stabilization energy of the ring.

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Answer: True

Answer

The statement is True. Benzene undergoes electrophilic substitution instead of addition to preserve its aromatic resonance stabilization energy.
Benzene's delocalized π\pi-electron cloud makes it exceptionally stable. Electrophilic substitution allows the ring to react with electrophiles while retaining its planar, aromatic structure and resonance energy.

Step-by-Step Solution

1
Analyze the electronic structure and aromaticity of benzene.
Benzene is a cyclic, planar molecule with 66 delocalized π\pi-electrons, rendering it exceptionally stable due to aromatic resonance energy.
Determining structural stability dictates chemical reactivity.
2
Compare the outcome of substitution versus addition reactions on the ring.
Substitution retains the unbroken 6π6\pi-electron cloud (aromaticity), whereas addition destroys the cyclic delocalization, forming a non-aromatic cyclohexadiene derivative.
Preserving aromaticity lowers the activation energy and thermodynamic barrier for substitution relative to addition.
3
Evaluate the validity of the statement.
The statement accurately describes why substitution is preferred over addition.
Direct mapping between resonance stabilization energy and reaction pathway preference.

Key Concept

Aromatic Stability and Electrophilic Substitution Reactivity of Benzene
Question 2Question →

According to Hückel's rule, a planar cyclic conjugated compound exhibits aromatic stability if the number of delocalized π\pi-electrons in its conjugated ring system is equal to which expression, where nn is a non-negative integer?

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Answer: 4n+24n + 2

Answer

The expression for the number of delocalized π\pi-electrons in an aromatic compound is 4n+24n + 2.
Hückel's rule states that a cyclic, planar, fully conjugated molecule possesses aromatic stability when it contains (4n+2)(4n + 2) delocalized π\pi-electrons, where nn is an integer (0,1,2,3,…0, 1, 2, 3, \dots). For example, benzene has 66 π\pi-electrons, satisfying the rule for n=1n = 1.

Step-by-Step Solution

1
Recall the structural criteria for aromaticity.
An aromatic molecule must be cyclic, planar, fully conjugated, and possess a specific number of delocalized π\pi-electrons.
These structural conditions allow complete ring delocalization of π\pi-electrons.
2
Apply Hückel's rule to determine the required π\pi-electron count.
The total number of delocalized π\pi-electrons must equal (4n+2)(4n + 2), where n=0,1,2,3,…n = 0, 1, 2, 3, \dots
This formula predicts closed-shell electronic stability for planar conjugated rings.

Key Concept

Hückel's Rule of Aromaticity
Estimated Time:45s
Question 3Question →

In electrophilic aromatic substitution, benzene reacts with strong electrophiles generated by specific catalyst-reagent combinations. Match each benzene reaction system on the left with its corresponding active electrophile species generated during the reaction mechanism on the right.

Click a left item, then click its matching right item

Items

Nitration using concentrated HNO3\text{HNO}_3 and concentrated H2SO4\text{H}_2\text{SO}_4
Friedel-Crafts acylation using ethanoyl chloride (CH3COCl\text{CH}_3\text{COCl}) and anhydrous AlCl3\text{AlCl}_3
Catalytic bromination using Br2\text{Br}_2 and FeBr3\text{FeBr}_3
Sulfonation using fuming or concentrated H2SO4\text{H}_2\text{SO}_4

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Answer

Nitration produces the nitronium ion (NO2+\text{NO}_2^+); Friedel-Crafts acylation generates the acylium ion (CH3C+=O\text{CH}_3\text{C}^+=\text{O}); Catalytic bromination produces the bromonium ion (Br+\text{Br}^+); Sulfonation generates neutral sulfur trioxide (SO3\text{SO}_3).
Each benzene electrophilic substitution reaction relies on a specific reagent and catalyst mechanism to create a powerful electrophile capable of disrupting benzene's stable aromatic system. Nitration generates NO2+\text{NO}_2^+ via acid-base protonation of nitric acid by sulfuric acid. Friedel-Crafts acylation forms the acylium ion CH3C+=O\text{CH}_3\text{C}^+=\text{O} through chloride abstraction by the Lewis acid AlCl3\text{AlCl}_3. Bromination generates a polarized Br+\text{Br}^+ complex using FeBr3\text{FeBr}_3. Sulfonation relies on SO3\text{SO}_3, which features an electron-deficient sulfur atom due to polar S=O bonds.

Step-by-Step Solution

1
Identify the electrophile in nitration
Concentrated H2SO4\text{H}_2\text{SO}_4 acts as an acid to protonate HNO3\text{HNO}_3. Loss of H2O\text{H}_2\text{O} yields NO2+\text{NO}_2^+ (nitronium ion).
H2SO4\text{H}_2\text{SO}_4 is a stronger acid than HNO3\text{HNO}_3 and forces HNO3\text{HNO}_3 to act as a base.
2
Identify the electrophile in Friedel-Crafts acylation
The catalyst AlCl3\text{AlCl}_3 abstracts Cl−\text{Cl}^- from CH3COCl\text{CH}_3\text{COCl}, leaving the resonance-stabilized cations CH3C+=O\text{CH}_3\text{C}^+=\text{O}.
AlCl3\text{AlCl}_3 is an electron-deficient Lewis acid capable of coordinating chloride.
3
Identify the electrophile in bromination
FeBr3\text{FeBr}_3 coordinates with a bromine atom of Br2\text{Br}_2, polarising the bond to create an effective Br+\text{Br}^+ electrophile.
Benzene requires a Lewis acid catalyst to polarize halogen molecules sufficiently for reaction.
4
Identify the electrophile in sulfonation
Equilibrium in concentrated/fuming H2SO4\text{H}_2\text{SO}_4 produces neutral SO3\text{SO}_3, which has a highly electron-deficient sulfur atom.
The three electronegative oxygen atoms in SO3\text{SO}_3 withdraw electron density from the central sulfur atom.

Key Concept

Generation of Electrophiles in Benzene Electrophilic Substitution
Benzene Structure, Aromaticity, and Electrophilic Substitution Practice Questions — JAMB UTME | Examkin