Q66E - Organic Chemistry (Mcmurry)

Question

Phenylboronic acid, ${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}\mathbf{B}{\mathbf{\left(}\mathbf{OH}\mathbf{\right)}}_{\mathbf{2}}$ , is nitrated to give 15% ortho substitution product and 85% meta. Explain the meta-directing effect of the $\mathbf{B}{\mathbf{\left(}\mathbf{OH}\mathbf{\right)}}_{\mathbf{2}}$ group.

Step-by-Step Solution

Answer

The ortho and para positions of phenylboronic acid are the most electron-deficient, therefore the substitution primarily occurs at the meta position.

Step 1: Ortho para and meta directing groups

If an electron withdrawing groups also known as deactivating group present on the aromatic ring then the electrophilic substitution takes place at meta position. Example of meta directing groups are cyano group (CN), aldehyde group (CHO), nitro group (NO₂) and any unsaturated group attached to the aromatic ring with conjugation.

If an electron donating groups present on the aromatic ring then the electrophilic substitution takes place at ortho and para positions.Example for ortho-para directing groups are alkyl group, hydroxyl group (OH).

Step 2: Nitration

Nitration is a chemical process for the addition or introduction of nitro group into an organic compound. For example conversion of benzene to 1,3,5-trinitrobenzene.

Nitration of benzene

Step 3: Explain the meta-directing effect of the B ( OH ) 2 group.

Nitration of phenylboronic acid

The trivalent boron atom in phenyl boronic acid has only six-outer shell electrons and is a Lewis acid. It is possible to write resonance forms for phenylboronic acid in which an electron pair from the phenyl ring is delocalized onto boron. In these resonance forms, the ortho and para positions of phenylboronic acid are the most electron deficient, and substitution occur primarily at meta position.

                Resonating structure for meta-directing effect of the $\mathrm{B}{\left(\mathrm{OH}\right)}_2$ group