Question

Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, \(\mathrm{E}\) (a) Chlorobenzene, 2,4-dinitrochlorobenzene, \(p\) -nitrochlorobenzene (b) Toluene, \(p-\mathrm{H}_{3} \mathrm{C}-\mathrm{C}_{6} \mathrm{H}_{4}-\mathrm{NO}_{2}, p-\mathrm{O}_{2} \mathrm{~N}-\mathrm{C}_{6} \mathrm{H}_{4}-\mathrm{NO}_{2}\).

Short answer

(a) p-Nitrochlorobenzene > Chlorobenzene > 2,4-dinitrochlorobenzene (b) Toluene > p-methylnitrobenzene > p-dinitrobenzene

Step-by-step solution

Understand Electrophilic Aromatic Substitution

In electrophilic aromatic substitution, an electrophile replaces a hydrogen atom on an aromatic ring. The reactivity of the compounds depends on the substituents already present on the ring.

Review the Nature of Substituents

Evaluate whether substituents such as nitro (-NO₂) or methyl (-CH₃) groups are electron-donating or electron-withdrawing. Electron-donating groups (EDGs) like -CH₃ increase the electron density on the ring, enhancing reactivity, while electron-withdrawing groups (EWGs) like -NO₂ decrease the electron density, reducing reactivity.

Analyze the Reactivity of Compounds

(a) Chlorobenzene has no electron-donating or strongly electron-withdrawing groups. 2,4-dinitrochlorobenzene has two -NO₂ groups, making it highly deactivated. p-Nitrochlorobenzene has one -NO₂ group, thus less deactivated compared to 2,4-dinitrochlorobenzene. (b) Toluene has a -CH₃ group, activating the ring. p-H₃C-C₆H₄-NO₂ (para-methylnitrobenzene) has one -NO₂ group, making it less active than toluene but more so than a dinitro compound. p-O₂N-C₆H₄-NO₂ (para-dinitrobenzene) has two -NO₂ groups, the most deactivating in this set.

Rank the Compounds

(a) p-Nitrochlorobenzene (less deactivated compared to two -NO₂ groups) > Chlorobenzene (neutral) > 2,4-dinitrochlorobenzene (highly deactivated) (b) Toluene (-CH₃ activated) > para-methylnitrobenzene (one -NO₂ less deactivated) > para-dinitrobenzene (most deactivated)

Key concepts

Electron-Donating Groups

Electron-Donating Groups (EDGs) are molecules or substituents that increase the electron density of an aromatic ring through the donation of electrons. They typically have lone pairs or pi bonds that can be delocalized into the aromatic system. This increased electron density makes the ring more reactive towards electrophilic aromatic substitution reactions.

Common examples of EDGs include the methyl group (-CH₃) and hydroxyl group (-OH). These groups can "activate" the ring, making it more attractive to electrophiles. When an electrophile approaches an aromatic ring with EDGs, these groups facilitate the substitution process, often increasing the rate of reaction.

• EDGs are generally ortho/para-directing, meaning they favor the electrophile attaching itself at positions adjacent or opposite to the EDG.
• Due to their activating properties, compounds such as toluene (which contains an -CH₃ group) are more reactive compared to benzene without such groups.

Electron-Withdrawing Groups

Electron-Withdrawing Groups (EWGs), on the other hand, are substituents that decrease the electron density of an aromatic ring. These groups typically have characteristics that pull electron density away from the ring, making it less reactive to electrophilic aromatic substitution.

Nitro groups (-NO₂) represent a classic example of EWGs. They withdraw electrons through resonance and inductive effects, effectively "deactivating" the ring and making it less attractive to electrophiles. As such, aromatic compounds with EWGs usually respond slower or require more forcing conditions for electrophilic aromatic substitution to take place.

• EWGs are commonly meta-directing, which means they cause the electrophile to attach to the meta position relative to the EWG.
• For instance, para-dinitrobenzene, having two nitro groups, shows lower reactivity compared to compounds with fewer or no nitro groups.

Reactivity Order

In electrophilic aromatic substitution, the reactivity of aromatic compounds towards electrophiles is greatly influenced by the presence and nature of substituents on the aromatic ring. The order of reactivity is a direct reflection of how electron-donating or electron-withdrawing these substituents are.

For example, in a set of compounds:

• A compound like toluene, with an electron-donating methyl group, will typically have increased reactivity compared to compounds that lack activating groups.
• Conversely, a compound such as 2,4-dinitrochlorobenzene, which carries strong electron-withdrawing substituents (two -NO₂ groups), will be significantly less reactive.

The key to predicting reactivity order lies in assessing the combined effects of all present substituents. Consider both their activating and deactivating influences, as well as their positions on the aromatic ring. To determine reactivity order correctly, students should focus on how substituents modify the electron density and stability of the carbocation intermediate formed during the reaction.