The question asks to identify which of the given aromatic compounds is the most reactive towards an electrophilic attack. This reactivity is determined by the nature of the substituent attached to the benzene ring.
Concept Used:
The reaction in question is an Electrophilic Aromatic Substitution (EAS). In this reaction, an electrophile (an electron-deficient species) attacks the electron-rich benzene ring. The rate of this reaction depends on the electron density of the aromatic ring. Substituents on the ring can modify this electron density:
Activating Groups (Electron-Donating Groups, EDG): These groups increase the electron density in the benzene ring, making it more nucleophilic and thus more reactive towards electrophiles than benzene itself. They donate electrons primarily through the resonance effect (+R or +M) or hyperconjugation and the inductive effect (+I).
Deactivating Groups (Electron-Withdrawing Groups, EWG): These groups decrease the electron density in the ring, making it less reactive than benzene. They withdraw electrons through the resonance effect (-R or -M) or the inductive effect (-I).
The compound that is "most easily attacked" will be the one with the most powerful activating group.
Step-by-Step Solution:
Step 1: Analyze the substituent group in each compound.
(a) Phenol: The substituent is a hydroxyl group (–OH).
(b) Chlorobenzene: The substituent is a chlorine atom (–Cl).
(c) Benzene: There is no substituent other than hydrogen (–H), which serves as the reference compound.
(d) Toluene: The substituent is a methyl group (–CH₃).
Step 2: Classify each substituent as activating or deactivating.
(a) Phenol (–OH group): The oxygen atom in the –OH group has lone pairs of electrons. It donates these electrons to the benzene ring through a strong positive resonance effect (+R effect). Although oxygen is electronegative and exerts a negative inductive effect (–I), the +R effect is far more dominant. Therefore, the –OH group is a strong activating group.
(b) Chlorobenzene (–Cl group): Chlorine is a halogen. It is highly electronegative, so it withdraws electron density from the ring via a strong negative inductive effect (–I). It also has lone pairs that it can donate through a positive resonance effect (+R). However, for halogens, the –I effect outweighs the +R effect. Thus, the –Cl group is a deactivating group.
(c) Benzene (–H group): This is our baseline for comparing reactivity.
(d) Toluene (–CH₃ group): The methyl group is an alkyl group. It is an electron-donating group through hyperconjugation and a weak positive inductive effect (+I). Therefore, the –CH₃ group is a weak activating group.
Step 3: Compare the activating strength of the groups.
We need to find the compound that is most reactive. We compare the activating power of the activating groups (–OH and –CH₃) and the deactivating power of the deactivating group (–Cl).
The –OH group is a strong activator due to its powerful +R effect.
The –CH₃ group is a weak activator due to hyperconjugation and the +I effect.
The –Cl group is a weak deactivator.
The resonance effect (+R) of the –OH group is significantly stronger than the hyperconjugation effect of the –CH₃ group. Therefore, the –OH group increases the electron density of the benzene ring much more than the –CH₃ group does.
Step 4: Determine the overall reactivity order.
Based on the effects of the substituents, the order of reactivity of the compounds towards electrophilic attack is:
The compound that will be most easily attacked by an electrophile is the one that is most activated. Phenol has the most strongly activating group (–OH) among the given options. Therefore, phenol is the most reactive.
The correct option is (b).
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