Question

Arrange the following in decreasing order of rate of electrophilic aromatic substitution. (A) \(\mathrm{I}>\mathrm{II}>\mathrm{III}\) (B) III > II > I (C) II \(>\mathrm{I}>\mathrm{III}\) (D) III > I > II

Short answer

The correct arrangement of the given molecules in decreasing order of their rate of electrophilic aromatic substitution is (C) II > I > III. This is because the -CH3 group in molecule II is an electron-donating group that stabilizes the carbocation intermediate, while the -NO2 group in molecule III is an electron-withdrawing group that destabilizes the carbocation intermediate. In molecule I, there are no substituents, resulting in a lower rate of electrophilic aromatic substitution than molecule II, but higher than molecule III.

Step-by-step solution

Identify the molecules

The molecules given are: I: \(\mathrm{I}\) (no substituent on the benzene ring) II: \(\mathrm{II}\) (benzene ring with a -CH3 group, which is an electron-donating group) III: \(\mathrm{III}\) (benzene ring with a -NO2 group, which is an electron-withdrawing group)

Analyze electrophilic aromatic substitution

Electrophilic aromatic substitution takes place mostly at the ortho or para position when a substituent is present on the benzene ring. The rate of electrophilic aromatic substitution depends on the stability of the carbocation intermediate formed during the reaction.

Determine stability of carbocations

Let's consider the stability of the carbocation intermediates formed during the reactions: - In molecule I, since there are no substituents present, the carbocation intermediate is relatively less stable. This results in a lower rate of electrophilic aromatic substitution. - In molecule II, the -CH3 group is an electron-donating group which increases the electron density at the ortho and para positions, making the carbocation intermediate more stable. This results in a higher rate of electrophilic aromatic substitution. - In molecule III, the -NO2 group is an electron-withdrawing group which decreases the electron density at the ortho and para positions, making the carbocation intermediate less stable. This results in a lower rate of electrophilic aromatic substitution compared to molecule I.

Arrange the molecules in the decreasing order of the rates

As discussed above, the rate of electrophilic aromatic substitution is: II > I > III

Choose the correct option

From the given options, the correct arrangement is: (C) II > I > III

Key concepts

Carbocation Stability

In electrophilic aromatic substitution, the stability of the carbocation intermediate plays a crucial role in determining the reaction rate. A carbocation is a positively charged ion that has only six electrons in its outer shell, making it electron-deficient and highly reactive. This is why stabilizing a carbocation is vital for the reaction to proceed efficiently.

Several factors influence carbocation stability:

• Resonance: A carbocation is more stable when it can delocalize its positive charge across multiple atoms. This delocalization increases electron sharing and decreases the electron deficiency at the carbocation center.
• Inductive effects: Atoms or groups that pull electrons toward themselves can help stabilize positive charges through inductive effects.
• Hyperconjugation: Overlapping of sigma bonds with the empty p-orbital in the carbocation can also add to the stability through electron donation.

Bringing this back to our examples, molecule II with an electron-donating group provides additional electrons, enhancing carbocation stability. Thus, more stable carbocations correspond to higher reaction rates in electrophilic aromatic substitution.

Electron-Donating Groups

Electron-donating groups (EDGs) like the -CH3 group in molecule II enhance the reactivity of benzene rings in electrophilic aromatic substitution. These groups increase the electron density on the benzene, especially at the ortho and para positions, by pushing electrons into the aromatic system.

Key characteristics of electron-donating groups include:

• Activating the benzene ring: EDGs make the benzene ring more reactive toward electrophiles by contributing additional electrons.
• Ortho/Para directivity: These groups direct incoming electrophiles to ortho and para positions, where the enhanced electron density makes the positions more nucleophilic and more capable of stabilizing an incoming positive charge.

In the context of our example, molecule II with the -CH3 substituent shows a higher rate of reaction since it enhances stability and reactivity of the carbocation formed during the reaction by pushing electron density towards the ortho and para positions.

Electron-Withdrawing Groups

In contrast to electron-donating groups, electron-withdrawing groups (EWGs) such as the -NO2 group in molecule III reduce the rate of electrophilic aromatic substitution by decreasing the electron density on the benzene ring. This effect renders the benzene less reactive toward electrophiles.

Main effects of electron-withdrawing groups include:

• Deactivating the benzene ring: EWGs pull electron density away from the benzene, making the molecule less nucleophilic and less likely to stabilize an incoming electrophile.
• Meta directivity: These groups tend to direct electrophiles to the meta position rather than the more electrophile-attractive ortho or para positions.

For molecule III, the presence of the -NO2 group leads to a decrease in the rate of electrophilic substitution reactions. This is because it destabilizes the carbocation intermediate, resulting in a less favorable reaction pathway compared to molecules without such withdrawing effects.