Problem 4 - Chemistry in 30 days

Question

Which of the following compounds reacts slower than benzene in electrtophilic bromination in the benzene ring? (a) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{NO}_{2}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{2} \mathrm{NH}_{2}\) (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{3}\)

Step-by-Step Solution

Answer

Compound (a) 3H3NO2.

Step 1: Understanding Electrophilic Bromination

Electrophilic bromination involves the substitution of a bromine atom onto an aromatic ring, which is an electrophilic aromatic substitution reaction. The rate of this reaction depends on the electron density of the aromatic ring: more electron-rich rings react faster with electrophiles like bromine.

Step 2: Identifying Electron Withdrawing and Donating Groups

Compounds with electron-donating groups (EDGs) like \(-\mathrm{OH}\) and \(-\mathrm{NH}_{2}\) make the aromatic ring more electron-rich, increasing reactivity towards electrophilic bromination. Electron-withdrawing groups (EWGs) like \(-\mathrm{NO}_{2}\) make the ring less electron-dense, decreasing reactivity.

Step 3: Analyzing the Given Compounds

(a) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{NO}_{2}\) contains the \(\mathrm{NO}_{2}\) group, which is a strong electron-withdrawing group. (b) \(\mathrm{C}_{6} \mathrm{H}_{2} \mathrm{NH}_{2}\) contains the \(\mathrm{NH}_{2}\) group, a strong electron-donating group. (c) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\) contains an \(\mathrm{OH}\), another electron-donating group. (d) \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{CH}_{3}\) contains the \(\mathrm{CH}_{3}\), which further donates electrons to the ring.

Step 4: Comparing to Benzene

Benzene by itself is a neutral reference for reactivity in electrophilic bromination. Any compound with electron-withdrawing groups like \(\mathrm{NO}_{2}\) will be less reactive towards electrophiles compared to benzene.

Step 5: Conclusion

Since \(\mathrm{NO}_{2}\) is an electron-withdrawing group, \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{NO}_{2}\) reacts slower than benzene in electrophilic bromination of the benzene ring.

Key Concepts

Electron-Withdrawing Groups

In chemistry, electron-withdrawing groups (EWGs) play a crucial role in determining how aromatic rings behave in reactions such as electrophilic aromatic substitution. These groups pull electron density away from the benzene ring, making it less reactive. This lack of reactivity is because electrophiles, or electron-loving species, prefer to attack regions of higher electron density.

Common examples of electron-withdrawing groups include:

• Nitro group (\(- ext{NO}_2\) )
• Carbonyl groups like ketones and aldehydes
• Cyanides (\(- ext{CN}\))

The presence of these groups on a benzene ring can significantly decrease the rate of electrophilic substitution reactions like bromination. The nitro group is especially strong in this regard, as it can stabilize the ring even further through resonance structures, drawing even more electron density away.

In practice, if a benzene ring has an EWG such as \(- ext{NO}_2\) , the overall reactivity of the molecule in terms of electrophilic bromination will be greatly reduced compared to plain benzene.

Electron-Donating Groups

Electron-donating groups (EDGs) have the opposite effect of electron-withdrawing groups. Instead of pulling electrons away, they push electron density onto the aromatic ring. This makes the ring more electron-rich, increasing its reactivity toward electrophilic aromatic substitution processes.

Such groups enhance the ring's reactivity by stabilizing the intermediates formed during the reaction. This stabilization is often achieved through resonance, where the electron-donating group shares its lone pair of electrons with the ring. Examples of effective EDGs include:

• Hydroxyl group (\(- ext{OH}\))
• Amino group (\(- ext{NH}_2\))
• Alkyl groups like methyl (\(- ext{CH}_3\))

These groups, especially \(- ext{NH}_2\) and \(- ext{OH}\) , are known to impart significant electron density via resonance, which makes the benzene ring they are attached to considerably more reactive to electrophiles like bromine. If you think of the reaction as a game of tag, EDG-equipped rings like those with \(- ext{OH}\) or \(- ext{NH}_2\) groups are far better equipped to "tag" bromine than plain benzene is.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a common and important type of reaction in organic chemistry. It involves the replacement of a hydrogen atom on an aromatic ring, such as benzene, with an electrophile. This process is fundamentally a dance between the electron-rich aromatic ring and the electron-seeking electrophile.

The core stages of EAS include:

• Formation of the electrophile
• Intermediate formation, where the electrophile temporarily bonds to the aromatic system, resulting in an arenium ion
• Deprotonation, where a hydrogen atom is removed to restore aromaticity

Reactivity in EAS is largely influenced by the substituents already present on the aromatic ring. Electron-donating groups make the ring more reactive by increasing electron density, which makes it easier for electrophiles to attack. Conversely, electron-withdrawing groups do the opposite by reducing electron density, thus slowing down the reaction.

Understanding EAS is essential to mastering aromatic chemistry, as it helps predict how different substituted benzene compounds will behave in various bromination or other electrophilic substitution scenarios. This kind of insight is key, especially when analyzing which compounds react faster or slower than benzene in such reactions.