Question

Which of the following substituents is ortho-, para-directing but deactivate the benzene ring? (A) \(-\mathrm{CCl}_{3}\) (B) \(-\mathrm{NHCOCH}_{3}\) (C) \(-\mathrm{OCH}_{3}\) (D) \(-\mathrm{Cl}\)

Short answer

The substituent that is ortho-, para-directing and deactivating for the benzene ring is (B) \(-\mathrm{NHCOCH}_{3}\). This group has a lone pair on the Nitrogen atom that allows it to direct ortho-, para-positions, while the carbonyl group is electron-withdrawing, reducing the electron density in the benzene ring.

Step-by-step solution

Identify the ortho-, para-directing substituents

We will first identify which of the given substituents are ortho-, para-directing by examining their electron-donating capabilities: (A) \(-\mathrm{CCl}_{3}\) - Carbon in this group has no lone pairs to donate. (B) \(-\mathrm{NHCOCH}_{3}\) - Nitrogen in this group has a lone pair that it can donate to the benzene ring. (C) \(-\mathrm{OCH}_{3}\) - Oxygen in this group has lone pairs to donate to the benzene ring. (D) \(-\mathrm{Cl}\) - Chlorine in this group has several lone pairs, but it is a weak electron-donating group. From the above analysis, we can see that (B) and (C) are ortho-, para-directing groups due to the lone pairs available on the Nitrogen and Oxygen atoms, respectively.

Identify the deactivating substituents

Now, we will identify which of the previously identified ortho-, para-directing substituents (B and C) are deactivating by examining their electron-withdrawing properties: (B) \(-\mathrm{NHCOCH}_{3}\) - The carbonyl group is electron-withdrawing, which can reduce the overall electron density in the benzene ring. (C) \(-\mathrm{OCH}_{3}\) - The methoxy group, in contrast, is electron-donating due to resonance and does not reduce the electron density of the benzene ring. Therefore, among the ortho-, para-directing substituents, only (B) \(-\mathrm{NHCOCH}_{3}\) is also a deactivating substituent.

Choose the correct option

Based on our analysis in Steps 1 and 2, we have found that the substituent which is ortho-, para-directing and deactivating is: (B) \(-\mathrm{NHCOCH}_{3}\)

Key concepts

Electron-Donating Groups

Electron-donating groups (EDGs) are substituents that increase the electron density on a benzene ring. They do this by donating electrons either through inductive effect or resonance. Inductive effect involves the donation of electrons through sigma bonds, while resonance involves the overlap of p-orbitals allowing for electron delocalization.

Common examples of electron-donating groups include hydroxyl (\(-\mathrm{OH}\)), methoxy (\(-\mathrm{OCH}_{3}\)), and amino (\(-\mathrm{NH}_2\)) groups. They typically possess lone pairs of electrons that can interact with the pi system of the benzene ring.

• These groups tend to activate the benzene ring, making it more reactive in electrophilic aromatic substitution reactions.
• The presence of EDGs leads to electron density being concentrated at the ortho and para positions of the benzene ring.

EDGs are important in determining the directing effects during a chemical reaction, leading to substitution mostly at the ortho and para positions on the benzene ring.

Electron-Withdrawing Groups

Electron-withdrawing groups (EWGs) have the opposite effect of electron-donating groups. They decrease the electron density of a benzene ring by withdrawing electrons. This can occur via an inductive effect or resonance.

Inductive EWGs, such as trifluoromethyl (\(-\mathrm{CF}_3\)), pull electrons through sigma bonds due to their electronegativity. Resonance EWGs, such as the nitro group (\(-\mathrm{NO}_2\)), decrease electron density through pi-bonding interactions.

• They typically deactivate the benzene ring, making it less reactive to electrophilic aromatic substitution.
• EWGs can direct the incoming substituents to the meta position of the benzene ring because they decrease electron density at ortho and para positions.

It's essential to understand that while some substituents might have both electron-donating and withdrawing characteristics, their overall effect will dictate their behavior in chemical reactions.

Deactivating Substituents

Deactivating substituents are those that reduce the reactivity of the benzene ring towards electrophilic aromatic substitution. They achieve this by withdrawing electron density from the benzene ring, often through electronegative atoms or by resonance with electron-withdrawing groups.

Common deactivating substituents include nitro (\(-\mathrm{NO}_2\)), carboxyl (\(-\mathrm{COOH}\)), and acyl (\(-\mathrm{C(=O)R}\)) groups. These groups decrease the reactivity of the benzene ring by:

• Creating a partial positive charge on the benzene ring, making it less attractive to electrophiles.
• Directing electrophilic substitutions to the meta position, as the ortho and para positions are less electron-dense.

In some cases, a substituent can be both ortho-, para-directing and deactivating, such as the acetamido group (\(-\mathrm{NHCOCH}_3\)).This unique behavior is due to their ability to donate electrons by resonance while simultaneously withdrawing electrons through their electron-withdrawing parent group, deactivating the ring overall.