Question

Which of the following is the most reactive towards clectrophilic nitration? (1) Benzene (2) Toluene (3) Benzoic acid (4) Nitrobenzene

Short answer

Toluene is the most reactive towards electrophilic nitration.

Step-by-step solution

Understand the Concept of Electrophilic Nitration

Electrophilic nitration is a reaction where an electrophile, typically the nitronium ion (o2^+), attacks an aromatic ring. The reactivity depends on the nature of substituents on the ring.

Identify Electron-Donating and Electron-Withdrawing Groups

Electron-donating groups (EDGs), such as alkyl groups, increase the electron density on the benzene ring, making it more reactive towards electrophiles. Electron-withdrawing groups (EWGs), such as carboxyl and nitro groups, decrease the electron density, making the ring less reactive.

Analyze Each Compound

1. Benzene has no substituents, so it has moderate reactivity. 2. Toluene has a methyl group (an EDG) that increases electron density, making it more reactive. 3. Benzoic acid has a carboxyl group (an EWG) that decreases electron density, making it less reactive. 4. Nitrobenzene has a nitro group (a strong EWG) that significantly decreases electron density, making it the least reactive.

Compare Reactivity

Given the information, the reactivity order towards electrophilic nitration from highest to lowest is: Toluene > Benzene > Benzoic acid > Nitrobenzene.

Determine the Most Reactive Compound

Based on the order of reactivity, Toluene is the most reactive towards electrophilic nitration.

Key concepts

Electron-Donating Groups

Electron-donating groups (EDGs) are groups attached to an aromatic ring that can release electrons into the ring via inductive or resonance effects. This process increases the electron density of the ring, making it more attractive to electrophiles, like in an electrophilic nitration reaction. Common examples of EDGs are:

• Alkyl groups (e.g., methyl, ethyl)
• Amino groups (-NH2)
• Hydroxyl groups (-OH)

Toluene is an example of a compound with an EDG since it contains a methyl group. The methyl group donates electrons to the benzene ring, making toluene more reactive towards electrophilic nitration compared to benzene with no substituents. Increased electron density makes it easier for the electrophile (nitronium ion) to attack the ring, facilitating the nitration process.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are substituents that pull electron density away from the aromatic ring. This decreases the electron density of the ring, making it less reactive towards electrophiles. EWGs achieve this via inductive or resonance effects. Common examples of EWGs include:

• Carboxyl groups (-COOH)
• Nitro groups (-NO2)
• Cyanide groups (-CN)

In the context of electrophilic nitration, compounds like benzoic acid and nitrobenzene contain strong EWGs. The carboxyl group in benzoic acid and the nitro group in nitrobenzene withdraw electrons from the benzene ring, making these compounds less reactive towards nitration. These groups reduce the electron density on the aromatic ring, hindering the approach of the electrophile.

Aromatic Compounds

Aromatic compounds, also known as arenes, are molecules that contain at least one aromatic ring. The most common example is benzene, a ring with alternating double bonds that creates a delocalized pi-electron system. This delocalized system is what gives aromatic compounds their unique chemical properties and reactivity. Aromatic compounds often undergo electrophilic substitution reactions, such as nitration. During nitration, the aromatic ring reacts with an electrophile (e.g., nitronium ion) and substitutes one of its hydrogen atoms. The reactivity of aromatic compounds in these reactions is greatly influenced by substituents attached to the ring. Substituents that are electron-donating increase reactivity, while electron-withdrawing groups decrease it. Therefore, understanding the nature of these substituents is crucial in predicting and explaining the behavior of different aromatic compounds in electrophilic reactions.