Question

The correct arrangement for decreasing order of electrophilic substitution for above compounds

• A. (IV) $>$ (I) $>$ (II) $>$ (III)
• B. (III) $>$ (I) $>$ (II) $>$ (IV)
• C. (II) $>$ (IV) $>$ (III) $>$ (I)
• D. (III) $>$ (IV) $>$ (II) $>$ (I)

Answer 1

The Correct Option is B

The objective is to order the provided substituted benzene compounds by their decreasing reactivity in electrophilic substitution reactions.

Concept Used:

The reactivity of benzene derivatives in electrophilic aromatic substitution (EAS) is determined by the electron density of the aromatic ring. The substituent present on the ring influences the reaction rate.

1. Activating Groups: These are electron-donating groups (EDGs) that increase the benzene ring's electron density, making it more susceptible to electrophiles. They enhance reactivity through positive resonance (\(+M\)), hyperconjugation, or positive inductive (\(+I\)) effects.
2. Deactivating Groups: These are electron-withdrawing groups (EWGs) that reduce the ring's electron density, diminishing its reactivity towards electrophiles. They decrease reactivity through negative resonance (\(-M\)) or negative inductive (\(-I\)) effects.

The general reactivity order is:
Benzene with a strong activating group > Benzene with a weak activating group > Benzene > Benzene with a weak deactivating group > Benzene with a strong deactivating group.

Step-by-Step Solution:

Step 1: Analyze the electronic effect of each substituent.

Compound (I) - Toluene: Characterized by a methyl group (\(-CH_3\)).

The \(-CH_3\) group donates electron density via:

• Hyperconjugation: Significant electron donation through C-H \(\sigma\)-electron delocalization.
• Inductive Effect: A minor electron-donating effect (\(+I\)).

Compound (II) - Benzene: The baseline compound without substituent effects.

Compound (III) - Anisole: Features a methoxy group (\(-OCH_3\)).

The \(-OCH_3\) group exhibits opposing effects:

• Resonance Effect: A powerful electron-donating effect (\(+M\)) from the oxygen lone pair delocalization.
• Inductive Effect: An electron-withdrawing effect (\(-I\)) due to oxygen's electronegativity.

Compound (IV) - Trifluoromethylbenzene: Contains a trifluoromethyl group (\(-CF_3\)).

The \(-CF_3\) group strongly withdraws electron density via:

• Inductive Effect: A potent electron-withdrawing effect (\(-I\)) from the highly electronegative fluorine atoms.
• Reverse Hyperconjugation: An electron-withdrawing resonance-like mechanism.

Step 2: Compare activating and deactivating strengths.

The relative electron-donating or withdrawing abilities determine the reactivity order.

• Comparing Activating Groups: Anisole (III) with its strong \(+M\) effect is more activating than toluene (I) with its hyperconjugation and \(+I\) effect. Hence, anisole (III) is more reactive than toluene (I).

Order: (III) > (I)

• Comparison with Benzene: Both \(-OCH_3\) and \(-CH_3\) are activating groups, making anisole (III) and toluene (I) more reactive than benzene (II).

Order: (III) > (I) > (II)

• Inclusion of Deactivating Group: The strongly deactivating \(-CF_3\) group in trifluoromethylbenzene (IV) renders it less reactive than benzene.

Step 3: Establish the final decreasing order of reactivity.

The consolidated analysis yields the following decreasing order of reactivity:

\[ \text{Anisole (III)} > \text{Toluene (I)} > \text{Benzene (II)} > \text{Trifluoromethylbenzene (IV)} \]

This corresponds to the sequence (III) > (I) > (II) > (IV).

The correct arrangement is presented in option (2).