Question

Which of the following compound will most easily be attacked by an electrophile?

• A.
• B.
• C.
• D.

Answer 1

The Correct Option is D

The objective is to determine which of the provided aromatic compounds exhibits the highest reactivity towards electrophilic substitution. This reactivity is contingent upon the nature of the substituent present on the benzene ring.

Concept Used:

The reaction under consideration is Electrophilic Aromatic Substitution (EAS). In this process, an electrophile, an electron-deficient species, targets the electron-rich benzene ring. The reaction rate is dictated by the electron density of the aromatic ring. Substituents can influence this electron density:

• Activating Groups (Electron-Donating Groups, EDG): These groups augment the electron density within the benzene ring, rendering it more nucleophilic and thus more susceptible to electrophilic attack compared to unsubstituted benzene. Their electron-donating capacity stems from resonance (+R or +M effect) or hyperconjugation, alongside inductive effects (+I).
• Deactivating Groups (Electron-Withdrawing Groups, EWG): These groups diminish the electron density of the ring, reducing its reactivity relative to benzene. They exert this effect through resonance (-R or -M effect) or inductive effects (-I).

The compound most readily attacked will possess the most potent activating group.

Step-by-Step Solution:

Step 1: Examine the substituent in each compound.

• (a) Phenol: Characterized by a hydroxyl group (–OH).
• (b) Chlorobenzene: Features a chlorine atom (–Cl) as the substituent.
• (c) Benzene: Serves as the reference, with hydrogen (–H) as the substituent.
• (d) Toluene: Contains a methyl group (–CH₃).

Step 2: Categorize each substituent as activating or deactivating.

(a) Phenol (–OH group): The oxygen in the –OH group possesses lone pair electrons, which it donates to the benzene ring via a strong positive resonance effect (+R effect). Despite oxygen’s electronegativity causing a negative inductive effect (–I), the +R effect is dominant. Consequently, the –OH group is a potent activating group.

(b) Chlorobenzene (–Cl group): Chlorine, a halogen, is highly electronegative, leading to electron withdrawal from the ring through a strong negative inductive effect (–I). It also exhibits a positive resonance effect (+R) due to lone pairs. However, for halogens, the inductive effect overrides the resonance effect. Therefore, the –Cl group is deactivating.

(c) Benzene (–H group): This compound acts as the benchmark for reactivity comparisons.

(d) Toluene (–CH₃ group): The methyl group, an alkyl substituent, donates electrons through hyperconjugation and a weak positive inductive effect (+I). Thus, the –CH₃ group is classified as a weak activating group.

Step 3: Compare the activating strengths of the groups.

The objective is to identify the most reactive compound by comparing the activating power of –OH and –CH₃, and the deactivating power of –Cl.

• The –OH group is a strong activator, primarily due to its significant +R effect.
• The –CH₃ group is a weak activator, acting via hyperconjugation and the +I effect.
• The –Cl group is a weak deactivator.

The resonance effect (+R) of the –OH group substantially exceeds the hyperconjugation effect of the –CH₃ group, resulting in a greater increase in the benzene ring’s electron density for phenol.

Step 4: Establish the reactivity order.

Based on the substituent effects, the compounds rank in reactivity towards electrophilic attack as follows:

\[ \text{Phenol} > \text{Toluene} > \text{Benzene} > \text{Chlorobenzene} \] \[ \text{(a)} > \text{(d)} > \text{(c)} > \text{(b)} \]

Final Result:

The compound most susceptible to electrophilic attack is the one exhibiting the highest degree of activation. Phenol possesses the most powerful activating group (–OH) among the options. Consequently, phenol demonstrates the greatest reactivity.

The correct option is (b).