08 Aromatic Compounds Free Online FlashCards

What conditions make a simple monocyclic system aromatic?

A simple monocyclic aromatic system is cyclic, conjugated, planar or nearly planar, and has $4n+2$ π electrons, where $n=0,1,2,\ldots$.

Why does benzene usually undergo substitution rather than addition?

Aromatic rings commonly undergo substitution rather than addition because substitution can restore the aromatic system, while addition would destroy it.

What happens during the two steps of electrophilic aromatic substitution?

The ring’s π electrons bond to an electrophile, temporarily losing aromaticity and forming a resonance-stabilized σ complex. Loss of a proton then restores the π system and aromaticity.

What groups do Friedel–Crafts alkylation and acylation introduce?

Friedel–Crafts alkylation uses $\mathrm{R-Cl/AlCl_3}$ to add an alkyl group; acylation uses $\mathrm{RCOCl/AlCl_3}$ to add an acyl group.

How do electron-donating groups usually affect EAS?

Electron-donating groups usually activate the ring and direct further substitution to ortho and para positions by stabilizing the σ complex formed in those attacks.

What is required for benzylic oxidation of an alkyl side chain?

Strong oxidants such as $\mathrm{KMnO_4}$ can convert an alkyl side chain with at least one benzylic hydrogen into $\mathrm{-CO_2H}$, often regardless of chain length.

What favors nucleophilic aromatic substitution?

In nucleophilic aromatic substitution, a nucleophile replaces a leaving group on an electron-poor ring. Electron-withdrawing groups, especially ortho or para to the leaving group, stabilize the negatively charged intermediate.

How do antiaromatic and nonaromatic rings differ?

A planar, fully conjugated ring with $4n$ π electrons is antiaromatic and destabilized. A ring that cannot maintain planarity or continuous conjugation is nonaromatic.

What does electron delocalization do for benzene?

Benzene’s six π electrons are delocalized around the ring. This gives its carbon–carbon bonds equivalent character and provides extra stability.

Why is σ-complex formation usually the slow step in EAS?

Formation of the σ complex is usually the slower step because it costs the ring aromatic stabilization.

Which reagents and electrophile are used in benzene nitration?

Benzene nitration uses $\mathrm{HNO_3/H_2SO_4}$ and introduces a nitro group, $\mathrm{-NO_2}$. The electrophile is the nitronium ion, $\mathrm{NO_2^+}$.

How do common limitations of Friedel–Crafts alkylation and acylation differ?

Friedel–Crafts alkylation can cause carbocation rearrangements and repeated substitution. Acylation generally avoids rearrangement, and its deactivating acyl group limits further substitution.