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 electron-withdrawing \(-\mathrm{B}(\mathrm{OH})_{2}\) group decreases electron density at ortho/para positions, favoring meta substitution.

Step-by-step solution

Understand the Reactants and Products

Phenylboronic acid has the chemical formula \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~B}(\mathrm{OH})_{2}\). When it undergoes nitration, it primarily forms a meta-substituted product (85%) with some ortho substitution (15%). We need to understand why the meta position is favored.

Analyze the Substituent Effect

The \(-\mathrm{B}(\mathrm{OH})_{2}\) group is an electron-withdrawing group. It pulls electron density away from the aromatic ring due to its ability to stabilize negative charges, reducing electron density mainly at the ortho and para positions.

Explain the Meta-directing Nature

Due to the electron-withdrawing nature of the \(-\mathrm{B}(\mathrm{OH})_{2}\) group, the ortho and para positions become electron-deficient or deactivated compared to the meta position. This results in electrophilic substitution (like nitration) occurring more readily at the meta position where there is relatively higher electron density.

Discuss the Electronic Influence

Through resonance and inductive effects, the \(-\mathrm{B}(\mathrm{OH})_{2}\) group's electron-withdrawing characteristic reduces the availability of electrons for substitution at the ortho and para positions, favoring meta substitution.

Key concepts

Understanding Phenylboronic Acid

Phenylboronic acid is a fascinating compound with the formula \( \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{~B}(\mathrm{OH})_{2} \). In simpler terms, it is an aromatic compound, meaning it contains a phenyl group attached to a boronic acid moiety. This configuration is unique because the boronic acid group impacts how the molecule reacts with other chemicals.

A key aspect to note is that the \(-\mathrm{B}(\mathrm{OH})_{2} \) group connected to the benzene ring plays a significant role in its reactivity. When it is part of an aromatic system, like phenylboronic acid, it changes the way the ring interacts with incoming reactants. Understanding these changes can help us predict how this compound will behave in various chemical reactions.

Phenylboronic acids are helpful in many applications, particularly in organic chemistry synthesis and as building blocks in pharmaceuticals. They are often used in reactions where the precise position of substitution on the benzene ring matters, like in the production of specific types of nitration products.

Role of Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) are atoms or groups of atoms attached to a molecule that pull electron density towards themselves. The \(-\mathrm{B}(\mathrm{OH})_{2} \) group attached to the phenyl ring in phenylboronic acid acts as an electron-withdrawing group.

This pulling of electron density reduces the electron cloud over certain areas of the benzene ring. Specifically, in the context of phenylboronic acid, the ortho and para positions, which are directly adjacent and opposite to the boronic acid group, experience a decrease in electron availability. This makes these positions less favorable for reactions involving electrophiles, like nitration.

Electron-withdrawing groups can stabilize negative charges due to their ability to accept or disperse additional electron density. This stabilization leads to a preference for specific types of chemical reactions to occur at sites that are not as electron-deficient, which is why the meta position often becomes the reaction hotspot.

Nitration of Aromatic Compounds

Nitration is a common reaction involving aromatic compounds, where a nitro group \((\mathrm{-NO}_2)\) is introduced into the molecule. In the case of phenylboronic acid, this reaction predominantly leads to a substitution at the meta position.

The process generally involves mixing the aromatic compound with concentrated nitric and sulfuric acids. The nitronium ion \((\mathrm{NO}_2^+)\), formed during this process, acts as the electrophile that attacks the benzene ring. Since phenylboronic acid has an electron-withdrawing \(-\mathrm{B}(\mathrm{OH})_{2}\) group, the electron density at the ortho and para positions is lower, making these sites less reactive to the nitronium ion.

Consequently, the meta position, with relatively more electron density, becomes the preferred site for nitration. This example perfectly illustrates how substituents on an aromatic ring can guide the outcome of reactions by altering the distribution of electron density and influencing chemical reactivity.