Question

Which of the following groups would enhance the reactivity of electrophilic aromatic substitution? (1) \(-\mathrm{CN}\) (2) - CHO (3) \(-\mathrm{CH}_{3}\) (4) \(-\mathrm{NO}_{2}\)

Short answer

\(-\mathrm{CH}_{3}\) enhances the reactivity of electrophilic aromatic substitution.

Step-by-step solution

Understand Electrophilic Aromatic Substitution (EAS)

Electrophilic Aromatic Substitution (EAS) is a reaction in which an atom, typically hydrogen, attached to an aromatic system is replaced by an electrophile. The reactivity in EAS is influenced by substituents already present on the aromatic ring.

Identify Activating and Deactivating Groups

Substituents on an aromatic ring can either activate (increase reactivity) or deactivate (decrease reactivity) the ring toward EAS. Activating groups typically donate electrons, enhancing the nucleophilicity of the ring. Deactivating groups withdraw electrons, making the ring less reactive.

Categorize the Given Groups

Classify each of the given groups as either electron-donating or electron-withdrawing: (1) \(-\mathrm{CN}\) is an electron-withdrawing group, (2) - CHO is an electron-withdrawing group, (3) \(-\mathrm{CH}_{3}\) is an electron-donating group, (4) \(-\mathrm{NO}_{2}\) is an electron-withdrawing group.

Determine the Most Activating Group

Electron-donating groups activate the aromatic ring toward EAS. Among the given choices, the \(-\mathrm{CH}_{3}\) group is the only electron-donating group, making it the one that enhances reactivity the most for EAS.

Key concepts

activating groups

In Electrophilic Aromatic Substitution (EAS), substituents on the aromatic ring play a crucial role in the reaction's overall speed and outcome. Activating groups are substituents that increase the reactivity of the aromatic ring. They do this by donating electrons into the ring, usually through resonance or inductive effects. When the electron density of the aromatic ring is increased, it becomes more attractive to electrophiles, thus enhancing the reactivity.

Some common activating groups include:

• Hydroxyl groups (-OH)
• Amino groups (-NH₂)
• Methyl groups (-CH₃)

These groups generally have lone pairs or electron-donating characteristics that make the ring more nucleophilic. For example, -OH and -NH₂ groups can donate electrons through resonance, significantly boosting the reactivity towards electrophiles.

deactivating groups

Deactivating groups, on the other hand, have the opposite effect on an aromatic ring in Electrophilic Aromatic Substitution (EAS). These groups pull electron density away from the ring, making it less reactive towards electrophiles. The aromatic ring becomes less nucleophilic, meaning it is less attractive to the positively charged entities looking to react.

Common deactivating groups include:

• Nitro groups (-NO₂)
• Cyanides (-CN)
• Carbonyl groups (-CHO)

These groups generally either have a strong electronegativity or form partial positive charges on the ring through resonance or induction. This electron-withdrawing nature makes the aromatic ring less likely to participate in EAS, thus decreasing its reactivity.

electron-donating groups

Electron-donating groups (EDGs) are vital for understanding reactivity in aromatic chemistry. EDGs donate electrons into the aromatic ring, either through resonance structures or inductive effects. They make the ring more electron-rich and more nucleophilic, which increases the ring's reactivity towards electrophiles.

Examples of electron-donating groups include:

• Alkyl groups (-CH₃)
• Alcohol groups (-OH)
• Amine groups (-NH₂)

For instance, in the given exercise, the -CH₃ group is an electron-donating group. It pushes electrons via hyperconjugation, increasing the electron density on the ring. This makes the ring more reactive, especially in Electrophilic Aromatic Substitution (EAS).

electron-withdrawing groups

Electron-withdrawing groups (EWGs) are functionally the opposite of electron-donating groups. EWGs pull electron density away from the aromatic ring, making it less electron-rich and less nucleophilic. This makes the aromatic ring less reactive to electrophiles, impacting the outcome of Electrophilic Aromatic Substitution (EAS) negatively.

Some common electron-withdrawing groups include:

• Nitro groups (-NO₂)
• Cyano groups (-CN)
• Carbonyl groups (such as -CHO and -COOH)

In the exercise, -CN, -CHO, and -NO₂ are all electron-withdrawing groups. They decrease the electron density of the aromatic ring through induction or resonance withdrawal. This makes the ring less reactive in EAS, as the ring is less likely to attract electrophiles.