Question

Phenylboronic acid, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~B}(\mathrm{OH})_{2},\) is nitrated to give \(15 \%\) orthosubstitution product and \(85 \%\) meta. Explain the meta-directing effect of the \(-\mathrm{B}(\mathrm{OH})_{2}\) group.

Short answer

The -B(OH)^{2} group is electron-withdrawing, directing nitration to the meta position by stabilizing the meta intermediate.

Step-by-step solution

Understand the Nitration Reaction

In a nitration reaction, an aromatic compound is treated with nitric acid and sulfuric acid to introduce a nitro group ( O_{2}) into an aromatic ring. The position at which the nitro group is introduced is influenced by the substituents already present on the benzene ring.

Analyze the Influence of Substituents

Substituents on a benzene ring can be classified as either activating or deactivating, and they can direct incoming groups to ortho, meta, or para positions. The -B(OH)^{2} group present in phenylboronic acid is a deactivating group which typically directs new substituents to the meta position.

Recognize the Electron-Withdrawing Nature

The -B(OH)^{2} group is electron-withdrawing. This means it pulls electron density away from the aromatic ring, making it less reactive. Electron-withdrawing groups often stabilize intermediates formed during substitution at the meta position.

Stabilization of the Meta Intermediate

When electrophilic substitution occurs, the carbocation intermediate formed is stabilized by the electron-withdrawing nature of the -B(OH)^{2} group at the meta position. This leads to a high percentage (85%) of nitration occurring at the meta position, compared to the ortho position (15%).

Conclude with Meta Directing Effect

Due to its electron-withdrawing nature, the -B(OH)^{2} group acts as a meta director in electrophilic substitution processes, favoring substitution at the meta position of the aromatic ring.

Key concepts

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution (EAS) is a key reaction in organic chemistry where an electrophile replaces a hydrogen atom in an aromatic ring like benzene. This process involves a multi-step mechanism:

• The aromatic ring with its delocalized π electrons acts as a nucleophile and attracts an electrophile.
• A complex is formed where the aromaticity of the ring is temporarily lost, creating a positively charged carbocation intermediate known as the sigma complex or arenium ion.
• Finally, deprotonation occurs to restore aromaticity, replacing the original hydrogen with the electrophile on the ring.

The position where the electrophile adds to the aromatic ring depends greatly on the existing substituents present on the ring. Different substituents influence the reactivity and positioning of new groups introduced in these reactions.

Meta-Director Groups

Meta-Director Groups on an aromatic ring are substituents that direct incoming electrophiles predominantly to the meta position, which is the position one carbon away on either side of the substituent already present. These groups are typically characterized by:

• Being electron-withdrawing, meaning they pull electron density away from the benzene ring, thus decreasing its reactivity.
• Not stabilizing the sigma complex intermediates at the ortho and para positions as effectively as at the meta position.

The $-B(OH)_2$ group in phenylboronic acid is an example of such a group, because its electron-withdrawing effect stabilizes the meta position more effectively. This stabilization happens because the positive charge of the intermediate that forms during electrophilic aromatic substitution aligns favorably when the electron-withdrawing group is meta, resulting in less reactive ortho and para reactions.

Nitration Reaction

The Nitration Reaction is a common example of an electrophilic aromatic substitution where a nitro group, $NO_2$, is introduced into an aromatic ring. This reaction typically uses a mixture of nitric acid ($HNO_3$) and sulfuric acid ($H_2SO_4$) to generate the nitronium ion ($NO_2^+$), the effective electrophile.

• In this reaction, the nitronium ion forms by the protonation and subsequent dehydration of nitric acid.
• Once formed, the $NO_2^+$ ion attacks the electron-rich aromatic ring to form a sigma complex.
• Subsequent deprotonation leads to the substitution of $NO_2$ onto the aromatic ring, restoring its aromatic character.

The position of the $NO_2$ group on the benzene depends on the nature of substituents such as electron-withdrawing and donating groups present, which control the orientation and the rate of nitration.

Electron-Withdrawing Groups

Electron-Withdrawing Groups (EWGs) are substituents that attract electrons toward themselves and away from other parts of the molecule like an aromatic ring. These groups play a crucial role in dictating the outcome of reactions like electrophilic aromatic substitution.

• By reducing electron density in the aromatic ring, they make the ring less reactive compared to unsubstituted benzene.
• Throughout the process, EWGs stabilize carbocations formed during substitution at specific positions, usually favoring meta substitution due to their alignment with the positive charge of intermediates.
• An example is the $-B(OH)_2$ group, which exemplifies electron-withdrawing behavior resulting in significant meta-directing influence during the nitration of phenylboronic acid.

Understanding how EWGs work allows chemists to predict and manipulate chemical reactions, making them an essential concept in synthetic chemistry.