Question

Predict the major product(s) of nitration of the following substances. Which react faster than benzene, and which slower? (a) Bromobenzene (b) Benzonitrile (c) Benzoic acid (d) Nitrobenzene (e) Benzenesulfonic acid (f) Methoxybenzene

Short answer

Methoxybenzene reacts faster; bromobenzene, benzonitrile, benzoic acid, nitrobenzene, and benzenesulfonic acid react slower than benzene.

Step-by-step solution

Understanding Electrophilic Aromatic Substitution

Nitration is a type of electrophilic aromatic substitution (EAS) where a nitro group ( O_2^+ ) replaces one of the hydrogen atoms on a benzene ring. The reactivity depends on the substituents present on the benzene ring.

Analyze Substituent Effects

Substituents on an aromatic ring can be either activating (increasing the reactivity) or deactivating (decreasing the reactivity) towards EAS. Activating groups usually donate electrons (e.g., -OCH_3), while deactivating groups withdraw electrons (e.g., -NO_2). They also direct the incoming electrophile to specific positions: ortho/para or meta.

Predict Reactivity for Bromobenzene

Bromobenzene contains a bromine atom, which is a weakly deactivating and ortho/para-directing group due to its ability to donate electron density through resonance. Thus, it will be less reactive than benzene, and the nitration will occur at the ortho and para positions.

Evaluate Reactivity for Benzonitrile

Benzonitrile has a cyano group (-CN), which is a strong electron-withdrawing group. It is meta-directing and deactivates the benzene ring significantly. Therefore, benzonitrile is less reactive to nitration than benzene, with nitration occurring at the meta position.

Consider Reactivity for Benzoic Acid

Benzoic acid has a carboxylic acid group (-COOH), another strong electron-withdrawing group, and is also meta-directing. This means benzoic acid will react more slowly than benzene under nitration conditions, with substitution at the meta position.

Determine Reactivity for Nitrobenzene

Nitrobenzene has a nitro group (-NO_2), which is a very strong electron-withdrawing and meta-directing group. This makes nitrobenzene much less reactive than benzene, with nitration occurring at the meta position, if nitration occurs at all under normal conditions.

Analyze Reactivity for Benzenesulfonic Acid

Benzenesulfonic acid contains a sulfonic acid group (-SO_3H), another strong electron-withdrawing group and meta-directing. It will thus be less reactive than benzene, with nitration occurring at the meta position.

Assess Reactivity for Methoxybenzene

Methoxybenzene (anisole) contains a methoxy group (-OCH_3), which is a strong electron-donating and ortho/para-directing group. This would make methoxybenzene more reactive than benzene, with the nitration likely at the ortho and para positions.

Key concepts

Nitration

Nitration is an important chemical reaction in organic chemistry, especially when dealing with aromatic compounds like benzene. During nitration, a nitro group (\( \text{-NO}_2 \)) is introduced into the aromatic ring. This process is a type of electrophilic aromatic substitution (EAS) where the aromatic ring maintains its pi-electron cloud while substituting a hydrogen atom.

The electrophile, nitronium ion (\( \text{NO}_2^+ \)), is typically generated from nitric acid and sulfuric acid. The reaction conditions are crucial, often requiring controlled temperatures to avoid over-reaction or ring degradation.

Nitration is widely used for synthesizing nitroaromatics, which can serve as precursors for dyes, pharmaceuticals, and explosives.

Substituent Effects

Substituents on a benzene ring significantly affect its reactivity and the position of incoming substituents during reactions like nitration. These effects are categorized into activating and deactivating effects, as well as ortho/para-directing and meta-directing influences.

• Activating Substituents: Often electron-donating, these groups increase the electron density of the ring, enhancing reactivity. Examples are hydroxy (\(-\text{OH}\)) and methoxy (\(-\text{OCH}_3\)) groups.
• Deactivating Substituents: Often electron-withdrawing, these groups decrease reactivity by pulling electron density away from the ring. Nitro (\(-\text{NO}_2\)), cyano (\(-\text{CN}\)), and carboxylic acid (\(-\text{COOH}\)) are typical examples.

Substituent effects dictate the overall orientation and reactivity of the nitration process by influencing the electron distribution across the aromatic system.

Benzene Reactivity

Benzene is a relatively stable molecule due to its delocalized pi-electron system, making it less reactive than other unsaturated compounds. When undergoing reactions like nitration, benzene's inherent stability can be altered by substituents attached to the ring.

• Substituents that donate electron density generally increase the reactivity of benzene towards electrophilic substitutions.
• Electron-withdrawing groups do the opposite, making the ring less reactive.

It's important to consider these effects to understand how substitutions change the reactivity order and conditions required for nitration.

Ortho/Para-Directing Groups

Ortho/para-directing groups are substituents on a benzene ring that guide incoming substituents to the ortho and para positions. These groups usually have lone pairs or pi-bonds that can donate electron density through resonance, stabilizing the positive charge formed during intermediate stages of the electrophilic substitution.

Common examples include:

• Alkyl groups like methyl (\(-\text{CH}_3\))
• Ether groups like methoxy (\(-\text{OCH}_3\))
• Amine groups like amino (\(-\text{NH}_2\))

These groups generally increase the rate of reaction compared to benzene, due to their electron-donating nature, which enhances the electrophile's attraction to the aromatic ring.

Meta-Directing Groups

Meta-directing groups are substituents that direct electrophiles to the meta position on a benzene ring during electrophilic aromatic substitution. These groups are typically strong electron-withdrawing and effectively reduce the electron density of the ring through inductive and resonance effects.

Key examples are:

• Nitro group (\(-\text{NO}_2\))
• Carboxylic acid group (\(-\text{COOH}\))
• Cyano group (\(-\text{CN}\))

These strong deactivators make the benzene less reactive towards electrophiles, and the electrophilic substitution that does occur happens predominantly at the meta position.