Question

Draw all resonance structures for the carbocation formed by ortho attack of the electrophile ${}^{\mathbf{+}}\mathbf{NO}_{\mathbf{2}}$ on each starting material. Label any resonance structures that are especially stable or unstable.

Short answer

The ortho-para directing groups substitute the electrophile at ortho and para positions. The electron-donating groups stabilize the positive charge (delocalizing in the ring) by donating electrons.

The electron-withdrawing groups destabilize the positive charge (delocalizing in the ring) by donating electrons.

Step-by-step solution

Ortho-para directing groups

The ortho-para directing groups substitute the electrophile at ortho and para positions. The electron-donating groups stabilize the positive charge (delocalizing in the ring) by donating electrons.

The electron-withdrawing groups destabilize the positive charge (delocalizing in the ring) by donating electrons.

Examples of ortho-para and meta-directing groups

Ortho-para directing groups: OH, R-, -OR,${\text{NH}}_{\text{2}}$ , etc.

Meta directing groups: CHO, COOH, ${\text{NO}}_{\text{2}}$, etc.

Explanation

a. The substituent on the benzene ring is $\text{C}{\left({\text{CH}}_{\text{3}}\right)}_{\text{3}}$ , which is electron-donating. The positive charge obtained by the substitution of the electrophile is stabilized by $\text{C}{\left({\text{CH}}_{\text{3}}\right)}_{\text{3}}$.

b. The substituent on the benzene ring is OH, which is electron-donating. The positive charge obtained by the substitution of the electrophile is stabilized by OH.

c. The substituent on the benzene ring is CHO, which is electron-withdrawing. The positive charge obtained by the substitution of the electrophile is not stabilized by CHO.