Question

Nitrobenzene is used as a solvent for Friedel Crafts alkylation of bromobenzcne because (1) nitrobenzene cannot be used in alkylation (2) nitrobenzene is less reactive than bromobenzene (3) nitrobenzene is more reactive than bromobenzene (4) none of the above

Short answer

Nitrobenzene is used as a solvent because it is less reactive than bromobenzene (Option 2).

Step-by-step solution

- Understanding Nitrobenzene

Nitrobenzene is an aromatic compound with a nitro group (-NO2) attached to the benzene ring. This nitro group is electron-withdrawing and makes the ring less reactive towards electrophilic aromatic substitution reactions.

- Understanding Bromobenzene

Bromobenzene is another aromatic compound with a bromine atom attached to the benzene ring. The bromine atom is slightly electron-withdrawing but also has an electron-donating resonance effect, making the benzene ring moderately reactive towards electrophilic aromatic substitution reactions.

- Compare Reactivity

Nitrobenzene, having a strongly electron-withdrawing nitro group, is less reactive in electrophilic aromatic substitution reactions compared to bromobenzene. Bromobenzene is more reactive due to the partial electron-donating effect of the bromine atom.

- Conclusion

Since nitrobenzene is less reactive than bromobenzene, it can be used as a solvent in Friedel Crafts alkylation reactions. Its low reactivity prevents it from participating in the reaction, allowing bromobenzene to undergo alkylation more selectively.

Key concepts

Nitrobenzene

Nitrobenzene is an aromatic compound characterized by a benzene ring bonded to a nitro group (-NO₂). The nitro group is strongly electron-withdrawing. This means it pulls electron density away from the benzene ring, making it less reactive towards electrophilic aromatic substitution (EAS) reactions.

In detail, electrophilic aromatic substitution involves an aromatic ring (like benzene) reacting with an electrophile. The electron-withdrawing nitro group causes the benzene ring in nitrobenzene to become less nucleophilic, decreasing its reactivity with electrophiles.

As a result, nitrobenzene can be utilized as a solvent in specific reactions, such as Friedel-Crafts alkylation. Because it's less reactive, nitrobenzene does not compete with the substrates (like bromobenzene) that are intended to participate in the reaction.

Bromobenzene

Bromobenzene is another aromatic compound where a bromine atom is attached to a benzene ring. Unlike nitro groups, the bromine atom exerts both electron-withdrawing and electron-donating effects.

The electron-withdrawing effect comes from bromine's electronegativity, drawing some electron density away from the benzene ring. However, bromine also has lone pairs of electrons that can participate in resonance, donating electron density back into the ring. This makes the benzene ring in bromobenzene moderately reactive towards electrophilic aromatic substitution reactions.

Therefore, in reactions like the Friedel-Crafts alkylation, bromobenzene will actively participate, whereas nitrobenzene, being less reactive, serves only as a solvent.

Electrophilic Aromatic Substitution

Electrophilic aromatic substitution (EAS) is a fundamental reaction in organic chemistry where an electrophile replaces a hydrogen atom on an aromatic ring. Here's a step-by-step breakdown:

• First, the aromatic ring donates π-electrons to an electrophile, forming a carbocation intermediate called an arenium ion.
• This intermediate is stabilized by resonance.
• Finally, the loss of a proton regenerates the aromatic system, completing the substitution.

Reactivity in EAS can be markedly affected by substituents on the aromatic ring. Electron-donating groups (like hydroxyl or alkyl groups) make the ring more reactive towards electrophiles, while electron-withdrawing groups (like nitro or carbonyl groups) make it less reactive.

Understanding these principles clarifies why nitrobenzene (with its electron-withdrawing nitro group) is less reactive and suitable as a solvent, while bromobenzene, with its balance of electron-withdrawing and donating effects, is more reactive and undergoes EAS readily.