08 Aromatic Compounds

Why Aromatic Rings Are Stable

Aromatic compounds are stabilized by cyclic systems of delocalized electrons. In benzene, six π electrons extend across the ring, making its carbon–carbon bonds equivalent. This stability helps explain why aromatic rings often react by replacing a ring atom—usually hydrogen—instead of adding across the ring as an alkene might.

Recognizing Aromaticity

For the introductory examples here, Aromaticity requires a cyclic and conjugated system with a continuous set of overlapping p orbitals. The system must also be planar or nearly planar so that the orbitals can overlap effectively. Hückel’s rule predicts aromaticity for a simple monocyclic system with $4n+2$ π electrons, where $n=0,1,2,\ldots$.

A planar, fully conjugated ring with $4n$ π electrons is antiaromatic and destabilized. A system that cannot remain planar or maintain continuous conjugation is nonaromatic rather than antiaromatic. These criteria are useful for introductory examples, although aromaticity is broader than the electron-counting rule alone.

The key distinction is whether the ring can sustain the geometry and orbital overlap needed for delocalization.

The EAS Mechanism

In electrophilic aromatic substitution (EAS), an electrophile replaces a hydrogen on an aromatic ring:

The mechanism has two main steps:

1. The ring’s π electrons form a bond to the electrophile. Aromaticity is temporarily lost, producing a resonance-stabilized carbocation called the σ complex, or arenium ion.

2. A base removes a proton from the carbon bonded to the electrophile. Electrons from the C–H bond restore the π system and the aromatic ring.

Forming the σ complex costs aromatic stabilization, so its formation is usually the slower step. A catalyst or strong reaction conditions may be needed to produce a sufficiently reactive electrophile. The ring’s return to aromaticity helps drive the second step.

Common Substitution Reactions

Common EAS reactions differ in the group introduced and the reagents used:

• Bromination introduces $\mathrm{-Br}$, typically using $\mathrm{Br_2/FeBr_3}$.

• Chlorination introduces $\mathrm{-Cl}$, typically using $\mathrm{Cl_2/FeCl_3}$.

• Nitration introduces $\mathrm{-NO_2}$, using $\mathrm{HNO_3/H_2SO_4}$. The electrophile is the nitronium ion, $\mathrm{NO_2^+}$; nitration of benzene forms nitrobenzene.

• Sulfonation introduces $\mathrm{-SO_3H}$, using $\mathrm{SO_3/H_2SO_4}$.

• Friedel–Crafts reaction includes alkylation, which introduces $\mathrm{-R}$ using reagents such as $\mathrm{R-Cl/AlCl_3}$, and acylation, which introduces $\mathrm{-COR}$ using reagents such as $\mathrm{RCOCl/AlCl_3}$.

In Friedel–Crafts reactions, a Lewis acid helps generate the electrophile. Alkylation can lead to carbocation rearrangements and repeated substitution because an alkyl group activates the ring. Acylation generally avoids rearrangement, and the acyl group deactivates the ring, limiting further substitution. Strongly deactivated rings and rings bearing basic amino groups often do not undergo these reactions successfully.

How Substituents Guide Ring Reactions

A group already attached to a ring affects both the rate and position of a later EAS reaction. Positions are described relative to that substituent: ortho means adjacent, meta means separated by one ring carbon, and para means opposite.

• Electron-donating groups usually activate the ring and direct incoming groups to ortho and para positions. Examples include $\mathrm{-OH}$, $\mathrm{-OR}$, $\mathrm{-NH_2}$, and alkyl groups. Donation stabilizes the σ complex formed by ortho or para attack.

• Electron-withdrawing groups usually deactivate the ring and direct substitution to the meta position. Examples include $\mathrm{-NO_2}$, $\mathrm{-CN}$, $\mathrm{-SO_3H}$, and carbonyl-containing groups attached through the carbonyl carbon, such as $\mathrm{-CHO}$ and $\mathrm{-CO_2H}$.

• Halogens are an exception: they deactivate the ring through an electron-withdrawing inductive effect but direct substitution to ortho and para positions because their lone pairs can donate by resonance.

To predict a product, classify each substituent as an activator or deactivator, then identify its directing effect. If substituents compete, the more strongly activating group often has greater influence, while steric crowding can favor the less hindered product. For example, nitration of toluene gives mainly ortho- and para-nitrotoluene because the methyl group activates the ring and directs substitution to those positions.

Side-Chain Reactions and SNAr

Not every reaction involving an aromatic compound replaces hydrogen on the ring. Some reactions occur at a side chain or replace a leaving group on the ring.

• Benzylic oxidation with a strong oxidant such as $\mathrm{KMnO_4}$ can convert an alkyl side chain into $\mathrm{-CO_2H}$, often regardless of side-chain length, if the side chain has at least one benzylic hydrogen. A side chain such as $\mathrm{-C(CH_3)_3}$, which has no benzylic hydrogen, resists this reaction.

• Benzylic bromination uses radical conditions such as NBS to replace a benzylic hydrogen with bromine. It occurs on the side chain, not by EAS on the ring.

• Nucleophilic aromatic substitution (SNAr) replaces a leaving group, often a halide, on an aromatic ring with a nucleophile. Electron-withdrawing substituents—especially those ortho or para to the leaving group—can stabilize the negatively charged intermediate. Unlike EAS, this reaction is favored by an electron-poor ring.

The location and type of substituent change help distinguish these reactions: oxidation and radical bromination act at the benzylic side chain, while SNAr changes a group directly attached to the aromatic ring.