Question

Identify the correct order of reactivity in electrophilic substitution reactions of the following compounds: c1ccccc1 Cc1ccccc1 Clc1ccccc1 O=[N+]([O-])c1ccccc1 (I) (II) (III) (IV) (a) \(\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}\) (b) IV > III > II > I (c) \(\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}\) (d) \(\mathrm{II}>\mathrm{III}>\mathrm{I}>\mathrm{IV}\)

Short answer

(c) \(\mathrm{II} > \mathrm{I} > \mathrm{III} > \mathrm{IV}\)

Step-by-step solution

Identify the Compounds

We have four compounds to analyze: (I) benzene \( c_1ccccc_1 \) (II) toluene \( Cc_1ccccc_1 \) (III) chlorobenzene \( Clc_1ccccc_1 \) (IV) nitrobenzene \( O=[N+]([O-])c_1ccccc_1 \). These compounds differ by their substituents which affect their reactivity in electrophilic substitution reactions.

Analyze the Substituents and Their Effects

The electrophilic substitution reaction rate is influenced by substituents on the benzene ring. - Toluene (II) has a methyl group, which is an electron-donating group, increasing the electron density on the ring and enhancing the reactivity. - Benzene (I) has no substituent, so it is the baseline. - Chlorobenzene (III) has chlorine, which is electron-withdrawing by inductive effect but weakly donating by resonance, lowering reactivity compared to toluene. - Nitrobenzene (IV) has a nitro group, a strong electron-withdrawing group by both inductive and resonance effects, which greatly reduces the reactivity.

Determine the Order of Reactivity

Based on the effects of the substituents: - Toluene (II) is the most reactive due to the electron-donating nature of the methyl group. - Benzene (I) follows with no substituent effect. - Chlorobenzene (III) is less reactive due to the halogen effect. - Nitrobenzene (IV) is the least reactive due to the strong electron-withdrawing nature of the nitro group.

Key concepts

Reactivity Order

In electrophilic substitution reactions, the reactivity of aromatic compounds is greatly influenced by the nature of substituents attached to the aromatic ring. The order of reactivity depends on whether these substituents donate or withdraw electrons from the benzene ring. In our exercise, four compounds are analyzed: Toluene (II), Benzene (I), Chlorobenzene (III), and Nitrobenzene (IV). Let's compare their reactivities:

• Toluene (II): This is the most reactive compound because the methyl group acts as an electron donor, enhancing the reactivity.
• Benzene (I): With no substituents, benzene acts as a neutral reference point in reactivity.
• Chlorobenzene (III): Less reactive than benzene due to the opposing effects of chlorine.
• Nitrobenzene (IV): The least reactive with a strongly electron-withdrawing nitro group.

Determining the correct reactivity order involves assessing these electron effects of substituents.

Substituent Effects

Substituents play a crucial role in determining the reactivity of benzene rings in electrophilic substitution reactions. The influence they exert is categorized into electron-donating or electron-withdrawing effects.

• Electron-donating groups (EDGs): These increase the electron density on the aromatic ring, making it more reactive towards electrophiles by stabilizing the transition state. For example, the methyl group in toluene.
• Electron-withdrawing groups (EWGs): These decrease the electron density on the aromatic ring, making it less reactive by destabilizing the transition state. Such is the case with the nitro group in nitrobenzene.

Thus, understanding the substituents' nature and their effects help predict the compounds' behavior in reactions.

Aromatic Compounds

Aromatic compounds are a class of cyclic, planar molecules known for their stability and unique electronic structure. The core structure, benzene, serves as a classic example. The stability of aromatic compounds comes from the delocalized electrons within the pi (π) bonds across the ring. This delocalization offers benzene its characteristic resonance, making it less reactive than alkenes but capable of specific reactions like electrophilic substitution. These reactions involve an electrophile attacking the electron-rich aromatic ring. However, the outcome and speed of these reactions depend significantly on any substituents present on the ring, showcasing the interplay between structure and reactivity.

Electron-Donating Groups

Electron-donating groups (EDGs) enhance the electron density on the aromatic ring, facilitating reactions with electrophiles. These groups push electrons into the ring through resonance or inductive effects. Examples & Effects:

• Methyl group: Present in toluene, this group is a representative EDG. By hyperconjugation, it donates electron density, increasing the ring's reactivity.
• -OH, -NH₂ groups: These are even stronger EDGs due to their ability to partake in resonance.

The presence of EDGs preferentially directs incoming electrophiles to positions ortho and para to the donating group, enhancing substitution reactions further.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) typically decrease the reactivity of aromatic rings by drawing electron density away from the ring via inductive or resonance effects. Examples & Effects:

• Nitro group: Found in nitrobenzene, it's a powerful EWG. The nitro group pulls electrons out of the ring through both inductive effects due to the electronegative oxygen atoms and resonance, leaving the ring less nucleophilic.
• -CF₃, -NO₂ groups: They are severe deactivators and tend to direct electrophilic attacks to the meta position due to their electron-withdrawing nature.

Understanding these groups' roles allows chemists to predict and explain the outcomes of electrophilic substitution reactions accurately.