03 Aromaticity and Electrophilic Aromatic Substitution Free Online FlashCards

What structural features define an aromatic ring?

An aromatic ring is cyclic, planar, and fully conjugated, with a p orbital on every ring atom.

What electron-count rule does benzene satisfy?

Hückel’s rule requires $4n + 2$ π electrons. Benzene has six π electrons, so it is aromatic.

Why does benzene typically substitute rather than add?

Substitution replaces a ring hydrogen and restores aromaticity. Addition would interrupt conjugation and lose aromatic stabilization.

What forms when an aromatic ring first bonds to an electrophile?

The ring’s π system bonds to an electrophile, temporarily forming a resonance-stabilized carbocation called the arenium ion, or σ complex.

Which reagents and group are associated with aromatic nitration?

Nitration uses $\mathrm{HNO_3}$ and $\mathrm{H_2SO_4}$ to introduce a nitro group, $\mathrm{NO_2}$, onto the ring.

How do electron-donating groups generally affect EAS?

Electron-donating groups generally stabilize the positively charged arenium-ion intermediate, activate the ring, and direct new substituents to ortho and para positions.

Why are halogens deactivating yet ortho/para-directing?

Halogens deactivate the ring by inductive withdrawal but direct ortho/para by donating lone-pair electrons through resonance.

What are the main products of toluene nitration?

The methyl group activates the ring and directs ortho/para, so the main products are ortho- and para-nitrotoluene; meta substitution is minor.

Which step is usually rate-determining in EAS?

Formation of the arenium ion disrupts aromaticity and is usually the rate-determining step of EAS.

How is aromaticity restored after the arenium ion forms?

A base removes the hydrogen from the carbon bearing the electrophile. The electrons restore the π system and aromaticity.

How can aromatic sulfonation be reversed?

Sulfonation uses $\mathrm{SO_3}$ and $\mathrm{H_2SO_4}$ to introduce $\mathrm{SO_3H}$. Hot, dilute aqueous acid can remove this group.

How do most electron-withdrawing groups affect EAS?

Most electron-withdrawing groups destabilize the arenium-ion intermediate, deactivate the ring, and direct substitution mainly to meta positions.