Question

Predict the major product(s) of mononitration 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

(f) Methoxybenzene reacts faster than benzene; (b), (c), (d), (e) react slower; major products are directed by substituents.

Step-by-step solution

Understand the Reaction

Mononitration refers to the introduction of a nitro group (-NO2) into an aromatic ring like benzene under nitrating conditions (usually using a mixture of concentrated nitric acid and sulfuric acid). The reaction rate is influenced by substituents already present on the benzene ring. Electron-donating groups increase the rate, while electron-withdrawing groups decrease it.

Analyze each compound

Identify the substituent present on each compound: (a) Bromobenzene - has a bromo group (Br-), an ortho/para-director and weakly deactivating. (b) Benzonitrile - contains a cyano group (-CN), a strongly deactivating meta-director. (c) Benzoic acid - has a carboxyl group (-COOH), a moderately deactivating meta-director. (d) Nitrobenzene - contains a nitro group (-NO2), a strongly deactivating meta-director. (e) Benzenesulfonic acid - features a sulfonic acid group (-SO3H), a strongly deactivating meta-director. (f) Methoxybenzene - has a methoxy group (-OCH3), a strongly activating ortho/para-director.

Determine Reactivity

Compare the reactivities based on the substituents: - (a) Bromobenzene reacts slower than benzene due to weak deactivation. - (b) Benzonitrile reacts much slower than benzene due to strong deactivation. - (c) Benzoic acid reacts slower than benzene due to moderate deactivation. - (d) Nitrobenzene reacts much slower than benzene due to strong deactivation. - (e) Benzenesulfonic acid reacts much slower than benzene due to strong deactivation. - (f) Methoxybenzene reacts faster than benzene due to strong activation.

Predict Major Products

Determine the position of the new nitro group based on the directing effect of the substituents: - (a) Bromobenzene: Nitro group at ortho and para positions relative to Br. - (b) Benzonitrile: Nitro group at the meta position relative to CN. - (c) Benzoic acid: Nitro group at the meta position relative to COOH. - (d) Nitrobenzene: Another nitro group at the meta position. - (e) Benzenesulfonic acid: Nitro group at the meta position. - (f) Methoxybenzene: Nitro group at ortho and para positions relative to OCH3.

Key concepts

Reaction Reactivity

In chemistry, reaction reactivity refers to how quickly or slowly a chemical reaction occurs. When considering mononitration, which involves adding a nitro group \((-NO_2)\) to an aromatic compound, the existing substituents on the aromatic ring can greatly affect the reactivity. The nature of these substituents determines whether the reaction happens faster or slower than it would with benzene alone.

Aromatic compounds with electron-donating groups (EDGs) tend to have increased reactivity in mononitration. These groups donate electrons into the aromatic ring, making it more electron-rich and more reactive toward electrophiles. Conversely, electron-withdrawing groups (EWGs) pull electrons away from the aromatic ring, making it less reactive by stabilizing the intermediate carbocations formed during the reaction.

In the case of the given compounds:

• Methoxybenzene has an EDG that increases reactivity.
• Compounds like nitrobenzene and benzenesulfonic acid have EWGs, decreasing their reactivity.

Aromatic Compounds

Aromatic compounds, like benzene, are characterized by ring-shaped molecules with alternating single and double bonds. This structure provides stability due to resonance, where the electrons are delocalized across the ring.

The stability and unique bonding of aromatic compounds make them important starting materials in many chemical reactions, including mononitration. Knowing the specific properties of these aromatic systems helps predict how they will respond to new substituents or functional groups.

Substituents already present on an aromatic molecule can alter its electronic characteristics, either enhancing or diminishing its reactivity. The position of the nitro group in the mononitration product is often influenced by whether the substituent is electron-donating or withdrawing, directing new substituents to ortho, meta, or para positions.

Electron-Donating Groups (EDGs)

EDGs are atoms or groups attached to an aromatic ring that increase its electron density by donating electrons through resonance or inductive effects. This increase in electron density makes the ring more reactive to electrophiles, which are electron-deficient species looking to accept electrons.

Examples of electron-donating groups include:

• Alkoxy groups, such as methoxy \((-OCH_3)\), which were found in methoxybenzene.
• Alkyl groups \((-CH_3, -C_2H_5, etc.)\), although they are weaker activators compared to other EDGs.

These groups often direct incoming substituents to the ortho and para positions, which are the most electron-rich areas of the aromatic ring. For example, in methoxybenzene, during mononitration, the nitro group is mainly added to the ortho and para positions because the methoxy group strongly activates these positions.

Electron-Withdrawing Groups (EWGs)

EWGs are substituents that reduce the electron density of an aromatic ring by withdrawing electrons through resonance or inductive effects. Lowering the electron density decreases the reactivity of the ring toward electrophilic substitution reactions like mononitration.

Common electron-withdrawing groups include:

• Nitro \((-NO_2)\) group, as seen in nitrobenzene, which pulls electrons away from the ring via resonance and induction, making the ring less reactive.
• Carboxyl \((-COOH)\) group, like in benzoic acid, which is a moderately deactivating group and directs new substituents to the meta position.
• Cyano \((-CN)\) and sulfonic \((-SO_3H)\) groups are also strong EWGs found in benzonitrile and benzenesulfonic acid, respectively.

These groups direct electrophiles chiefly to the meta positions, in part because the ortho and para positions are less favorable due to the reduced electron density. For instance, benzonitrile undergoes mononitration predominantly at the meta position.