Q32E - Organic Chemistry (Mcmurry)

Question

The nitroso group, - NO, is one of the few non-halogens that is an ortho- and para-directing deactivator. Explain this behaviour by drawing resonance structures of the carbocation intermediates in ortho, meta, and para electrophilic reaction on nitroso benzene,${\mathbf{C}}_{\mathbf{6}}{\mathbf{H}}_{\mathbf{5}}\mathbf{N}\mathbf{=}\mathbf{O}$.

Step-by-Step Solution

Answer

Ortho directing:

Para directing:

Meta directing:

Step 1: Nitroso benzene

The aromatic ring is deactivated toward electrophilic aromatic substitution due to the electron-withdrawing inductive effect of nitrogen and oxygen.

Step 2: Nitroso benzene ortho attack

The aromatic ring deactivates toward electrophilic substitution. Due to the presence of lone nitrogen of nitroso group, it shows delocalization with a benzene ring and activates the ortho position for electrophilic substitution. But it is less stable due to the steric hindrance occurring at the ortho position due to adjacent nitroso group.

Ortho-directing

Step 3: Nitroso benzene para-attack

The aromatic ring deactivates toward electrophilic substitution. Due to the presence of lone nitrogen of nitroso group, it shows delocalization with a benzene ring and activates the para position for electrophilic substitution. It is more stable than ortho due to presence of electrophile at 4^th position to nitroso group.

Para-directing

Step 4: Nitroso benzene meta-attack

The aromatic ring deactivates toward electrophilic substitution. Due to the electron-withdrawing inductive effect of nitrogen and oxygen, it deactivates the meta-position, and only ortho and para are activated, so meta-attack is not possible.

Meta-directing