The problem asks to arrange the given substituted benzene compounds in the decreasing order of their reactivity towards electrophilic substitution.
Concept Used:
The reactivity of a benzene derivative towards electrophilic aromatic substitution (EAS) depends on the electron density of the aromatic ring. The rate of the reaction is influenced by the nature of the substituent already present on the ring.
Activating Groups: Electron-donating groups (EDGs) increase the electron density in the benzene ring, making it more nucleophilic and thus more reactive towards electrophiles. These groups are called activating groups. They exhibit positive resonance (\(+M\)), hyperconjugation, or positive inductive (\(+I\)) effects.
Deactivating Groups: Electron-withdrawing groups (EWGs) decrease the electron density in the ring, making it less nucleophilic and less reactive towards electrophiles. These groups are called deactivating groups. They exhibit negative resonance (\(-M\)) or negative inductive (\(-I\)) effects.
The general order of reactivity 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: The substituent is a methyl group (\(-CH_3\)).
The \(-CH_3\) group is an alkyl group. It donates electron density to the ring through two effects:
Hyperconjugation: Delocalization of C-H \(\sigma\)-electrons, which is a significant electron-donating effect.
Inductive Effect: A weak electron-donating effect (\(+I\)).
Compound (II) - Benzene: This is the reference compound with no substituent effects.
Compound (III) - Anisole: The substituent is a methoxy group (\(-OCH_3\)).
The \(-OCH_3\) group has an oxygen atom with lone pairs of electrons directly attached to the ring. It exhibits two opposing effects:
Resonance Effect: A strong electron-donating effect (\(+M\)) due to the delocalization of oxygen's lone pair into the ring.
Inductive Effect: An electron-withdrawing effect (\(-I\)) due to the high electronegativity of oxygen.
Compound (IV) - Trifluoromethylbenzene: The substituent is a trifluoromethyl group (\(-CF_3\)).
The \(-CF_3\) group has a carbon atom attached to three highly electronegative fluorine atoms. It strongly withdraws electron density from the ring through:
Inductive Effect: A very strong electron-withdrawing effect (\(-I\)) due to the fluorine atoms.
Reverse Hyperconjugation: An electron-withdrawing resonance-like effect.
Step 2: Compare the activating and deactivating strengths.
We compare the electron-donating and withdrawing abilities of the substituents to establish the order of reactivity.
Comparing Activating Groups: We compare \(-OCH_3\) (in III) and \(-CH_3\) (in I). The \(+M\) effect of \(-OCH_3\) is significantly stronger than the hyperconjugation and \(+I\) effect of \(-CH_3\). Therefore, anisole (III) is much more reactive than toluene (I).
Order: (III) > (I)
Comparing with Benzene: Both \(-OCH_3\) and \(-CH_3\) are activating groups, so both anisole (III) and toluene (I) are more reactive than benzene (II).
Order: (III) > (I) > (II)
Including the Deactivating Group: The \(-CF_3\) group in compound (IV) is strongly deactivating, making the ring much less electron-rich than benzene. Therefore, trifluoromethylbenzene (IV) is the least reactive compound.
Step 3: Establish the final decreasing order of reactivity.
Combining the analyses, the decreasing order of reactivity towards electrophilic substitution is:
