Question

Total number of deactivating groups in aromatic electrophilic substitution reaction among the following is

Answer 1

Correct Answer: 2

This problem requires the identification and enumeration of all functional groups that deactivate aromatic electrophilic substitution reactions from a provided set.

Concept Used:

Substituent groups on a benzene ring influence its reactivity toward electrophiles by altering electron density. Activating groups increase electron density, making the ring more nucleophilic, while deactivating groups decrease electron density, rendering the ring less nucleophilic. This effect is mediated by resonance and inductive effects, directing electrophiles typically to ortho/para or meta positions, respectively (with exceptions).

• Activating Groups: These groups enhance the benzene ring's reactivity by donating electron density, usually through +R or +I effects. Examples include \(-\text{OH}\), \(-\text{OR}\), \(-\text{NH}_2\), \(-\text{NHR}\), and alkyl groups (\(-\text{R}\)).
• Deactivating Groups: These groups diminish the benzene ring's reactivity by withdrawing electron density, typically through -R or -I effects. Examples include \(-\text{NO}_2\), \(-\text{CN}\), \(-\text{SO}_3\text{H}\), and carbonyl groups (\(-\text{CHO}\), \(-\text{COR}\)).

Step-by-Step Solution:

Step 1: Compile a list of the distinct functional groups presented.

The functional groups provided are:

1. \(-\text{C}(=\text{O})\text{CH}_3\) (Acetyl)
2. \(-\text{OCH}_3\) (Methoxy)
3. \(-\text{NHC}(=\text{O})\text{CH}_3\) (Acetamido)
4. \(-\text{NHCH}_3\) (Methylamino)
5. \(-\text{C}\equiv\text{N}\) (Cyano)

Step 2: Evaluate each group to ascertain its activating or deactivating nature.

1. Acetyl group (\(-\text{C}(=\text{O})\text{CH}_3\)):

The carbonyl moiety exhibits strong electron-withdrawing properties via both inductive (-I) and resonance (-R) effects, thereby deactivating the ring.

2. Methoxy group (\(-\text{OCH}_3\)):

Despite a -I effect from oxygen, the +R effect from lone pair delocalization into the ring dominates, making it an activating group.

3. Acetamido group (\(-\text{NHC}(=\text{O})\text{CH}_3\)):

The nitrogen's lone pair participates in resonance donation to the ring (+R effect), classifying it as an activating group, albeit moderated by the adjacent carbonyl.

4. Methylamino group (\(-\text{NHCH}_3\)):

The strong +R effect from the nitrogen's lone pair makes this a potent activating group.

5. Cyano group (\(-\text{C}\equiv\text{N}\)):

This group is strongly electron-withdrawing through both inductive (-I) and resonance (-R) effects, classifying it as deactivating.

Step 3: Tally the count of deactivating groups.

The classification is as follows:

• Activating: \(-\text{OCH}_3\), \(-\text{NHC}(=\text{O})\text{CH}_3\), \(-\text{NHCH}_3\).
• Deactivating: \(-\text{C}(=\text{O})\text{CH}_3\), \(-\text{C}\equiv\text{N}\).

Two deactivating groups are identified.

Final Result:

The total count of deactivating groups from the provided list is 2.