Question

The position least nitrated, when m-bromochlorobenzene is nitrated is (1) position ortho to bromine (2) position ortho to chlorine (3) position ortho to bromine and chlorine (4) position meta to chlorine

Short answer

(4) position meta to chlorine

Step-by-step solution

- Understand the Structure

Firstly, observe the structure of m-bromochlorobenzene. It is a benzene ring with a bromine (Br) and a chlorine (Cl) atom attached to meta positions.

- Analyze Directing Effects

Both bromine and chlorine are ortho/para directing groups but are deactivating in nature. The nitration will be influenced by the positions relative to these atoms.

- Position Evaluation

Determine the positions in the benzene ring: ortho, meta, and para relative to bromine and chlorine. Specifically, identify positions where nitration is least favorable.

- Least Nitrated Position

Considering the deactivating effect of both bromine and chlorine, the least favorable position for nitration will be meta to the positions where chlorine is present.

- Conclusion

From the analysis, position meta to chlorine (without other influence) will be the least nitrated due to deactivation from chlorine.

Key concepts

electrophilic aromatic substitution

Electrophilic Aromatic Substitution (EAS) is a reaction where an electrophile replaces a hydrogen atom in an aromatic ring. It's a fundamental way to modify benzene and other aromatic compounds. Here, a nitration process is essentially an EAS reaction where a nitro group \(NO_2\) is introduced.

For an EAS reaction, the aromatic ring reacts with an electrophile. Electrophiles are attracted to the electron-rich aromatic ring, making it possible for them to replace a hydrogen atom. Examples of other reactions similar to nitration include halogenation, sulfonation, and acylation.

It's important to understand the behavior of substituents on the ring, as they can either activate or deactivate the ring towards electrophilic attack and direct the incoming group to specific positions (ortho, meta, or para). Knowing these effects is crucial in predicting the outcome of the reaction accurately.

ortho/para directors

Ortho/para directors are substituents on a benzene ring that influence where the next substituent will attach during an electrophilic aromatic substitution. Substituents that donate electrons through resonance or inductive effects typically direct new substituents to the ortho (adjacent) and para (opposite) positions relative to themselves.

In the case of m-bromochlorobenzene:

• Bromine (Br) and chlorine (Cl) are both ortho/para directors due to their lone pairs that can participate in resonance, thus donating electron density to the ring.
• Despite being electron-withdrawing by induction, their ability to donate electrons through resonance outweighs this effect when determining their directing role.

Therefore, when nitrating m-bromochlorobenzene, these groups make the ortho and para positions more favorable for nitration.

deactivating groups

Deactivating groups are substituents that decrease the reactivity of the aromatic ring towards electrophilic aromatic substitution. They do this by withdrawing electron density from the ring, making it less attractive to electrophiles.

For example, both bromine and chlorine are deactivating groups because they have high electronegativity and withdraw electron density through an inductive effect, even though they also act as ortho/para directors.

In the nitration of m-bromochlorobenzene, the presence of these deactivating groups means that the ring is generally less reactive than an unsubstituted benzene ring. However, because they also direct new substituents to the ortho and para positions, the meta positions (relative to these groups) still get less electron density and thus are least favored for a nitration reaction.

• Therefore, in the problem, the position least nitrated is meta to chlorine.