Problem 40 - General Chemistry: Principles and Modern Applications

Question

Predict the main product(s) of (a) the mononitration of benzoic acid; (b) the monosulfonation of phenol; (c) the monobromination of 2 -nitrobenzaldehyde.

Step-by-Step Solution

Answer

The products are (a) 3-nitrobenzoic acid, (b) para-sulfophenol, and (c) 3-bromo-2-nitrobenzaldehyde.

Step 1: Nitration of Benzoic Acid

When benzoic acid undergoes nitration, the nitro group (-NO2) will be added at meta-position due to the carboxyl (-COOH) group's deactivating and meta-directing effect. This will result in 3-nitrobenzoic acid.

Step 2: Sulfonation of Phenol

In the sulfonation of phenol, the strong activating effect of the hydroxyl group (-OH) will direct the sulfonic acid group (-SO3H) to the ortho and para positions. However, since ortho product would be sterically hindered, the major product is para product, which is para-sulfophenol.

Step 3: Bromination of 2-Nitrobenzaldehyde

2-nitrobenzaldehyde has two groups attached to the benzene ring: an aldehyde group (-CHO) and a nitro group (-NO2). Both are deactivating and meta-directing. Thus, the bromine atom will be added to the meta position relative to both groups, which results in 3-bromo-2-nitrobenzaldehyde.

Key Concepts

Nitration of Benzoic Acid

Nitration is a type of electrophilic aromatic substitution (EAS) reaction, which involves the introduction of a nitro group ( NO_2 ) into the aromatic ring. It is a key step in synthesizing a wide variety of compounds that have applications in medicinal chemistry and materials science.

When benzoic acid is subjected to nitration, the carboxyl group ( COOH ) present exerts a strong deactivating and meta-directing influence on the reaction. This is because the carboxyl group withdraws electron density from the aromatic ring, making it less reactive towards electrophiles. In benzoic acid nitration, the result is the formation of 3-nitrobenzoic acid where the nitro group occupies the meta position relative to the carboxyl group.

Understanding the influence of substituents on the aromatic ring is crucial for predicting the outcome of the reaction. The carboxyl group, being an electron-withdrawing group (EWG), slows down the reaction and directs new substituents to the meta position. Here are the simple steps of the reaction mechanism:

• Generation of a powerful nitronium ion ( NO_2^+ ) electrophile.
• Attack of the electrophile on the meta position of the aromatic ring.
• Formation of a sigma complex, followed by a loss of a proton to restore aromaticity.

Sulfonation of Phenol

Sulfonation is another example of an electrophilic aromatic substitution reaction where a sulfonic acid group ( SO_3H ) replaces a hydrogen atom on an aromatic ring. In phenol, the hydroxyl group ( OH ) is strongly activating and ortho, para-directing due to its ability to donate electron density through resonance.

During the sulfonation of phenol, the hydroxyl group enhances the reactivity of the ring by increasing its electron density, thereby facilitating the attack of the electrophilic sulfur trioxide ( SO_3 ). Despite the activation towards both ortho and para positions, the major product of this reaction is para-sulfophenol. The ortho position is less favored due to steric hindrance, which makes the para position the site of preferential attack for the sulfonic acid group.

The practicality of sulfonation lies in both the relative ease of modifying the properties of aromatic compounds and the reversibility of the reaction. Products of sulfonation are useful in designing detergents, dyes, and pharmaceuticals. For students, understanding the role of activating groups and steric factors in directing the outcome of sulfonation is significant in mastering aromatic chemistry.

Bromination of 2-Nitrobenzaldehyde

Bromination is a specific type of electrophilic aromatic substitution where a bromine atom is incorporated into an aromatic system. When 2-nitrobenzaldehyde undergoes bromination, it illustrates the combined electronic effects of two substituents on the aromatic ring - an aldehyde group ( CHO ) and a nitro group ( NO_2 ).

Both the aldehyde and the nitro groups are electron-withdrawing, which decreases the electron density of the benzene ring and makes it less reactive towards electrophiles. As a consequence, these groups are meta-directing, and thus the bromine atom is positioned at the meta location to each group. The outcome of this reaction is 3-bromo-2-nitrobenzaldehyde.

This intricate dance between the substituents' positioning and their electronic effects provides rich learning opportunities. Appreciating the concept of deactivation and the meta-directing nature of certain groups is essential for correctly predicting the product structures in complex aromatic bromination reactions. Brominated aromatics have extensive utility, including in the synthesis of flame retardants and pharmaceuticals.