16.2 Other Aromatic Substitutions

Electrophilic Aromatic Substitution Reactions

Electrophilic aromatic substitution (EAS) is the central reaction type for functionalizing benzene rings. A hydrogen on the aromatic ring gets replaced by an electrophile, and the ring's aromaticity is preserved in the final product. The specific electrophile determines whether you get halogenation, nitration, sulfonation, or a Friedel-Crafts product.

What makes EAS so important is that it lets you build complexity onto aromatic rings in a controlled way. The substituents already on the ring dictate where the next group goes and how fast the reaction proceeds.

Mechanism of Electrophilic Aromatic Substitution

Every EAS reaction follows the same three-step mechanistic framework:

1. Generation of the electrophile ($E^+$): The reagents react (often with a catalyst) to produce a strong electrophile.
2. Electrophilic attack on the ring: The $\pi$ electrons of the aromatic ring attack $E^+$, breaking aromaticity and forming a resonance-stabilized carbocation intermediate called the arenium ion (also called a sigma complex or Wheland intermediate).
3. Deprotonation: A base removes the hydrogen from the carbon bearing the electrophile, restoring aromaticity and giving the substituted product.

The arenium ion is the key intermediate. It's stabilized by resonance (the positive charge is delocalized over three carbons of the ring), but it has lost aromaticity. That's why the final deprotonation step is so favorable: it restores the aromatic system.

Each specific EAS reaction differs mainly in how the electrophile is generated:

• Nitration ($E^+ = NO_2^+$): The nitronium ion forms when concentrated $HNO_3$ is protonated by concentrated $H_2SO_4$, followed by loss of water.
• Sulfonation ($E^+ = SO_3$): Sulfur trioxide acts as the electrophile, generated from fuming sulfuric acid ($H_2SO_4 \cdot SO_3$). Technically $SO_3$ is a strong electrophile rather than a cation, but it reacts with the ring in the same way.
• Halogenation ($E^+ = X^+$): Discussed in detail below.

Halogenation Methods for Aromatic Rings

Halogenation introduces $F$, $Cl$, $Br$, or $I$ onto the ring, but each halogen requires different conditions because their reactivity varies enormously.

Halogen reactivity follows the order $F_2 > Cl_2 > Br_2 > I_2$:

• Fluorination is so exothermic that it's essentially uncontrollable by standard EAS. In practice, aromatic fluorides are made by other routes (e.g., the Balz-Schiemann reaction or nucleophilic aromatic substitution with $F^-$).
• Chlorination uses $Cl_2$ with a Lewis acid catalyst like $FeCl_3$ (or $AlCl_3$). The Lewis acid polarizes the $Cl{-}Cl$ bond, generating the electrophilic chlorine species.
• Bromination works the same way: $Br_2$ with $FeBr_3$ (or $AlBr_3$). For activated rings (those with strong electron-donating groups like $-OH$ or $-NH_2$), bromination can occur without a catalyst.
• Iodination is the least reactive and requires an oxidant ($HNO_3$ or $H_2O_2$) alongside $I_2$ to generate the electrophilic $I^+$ species. Without the oxidant, the reaction doesn't proceed because $I_2$ alone is too weak an electrophile.

Milder halogenating agents like NCS ($N$-chlorosuccinimide) and NBS ($N$-bromosuccinimide) can also be used for aromatic halogenation under certain conditions, though NBS is more commonly associated with benzylic/allylic bromination in introductory courses.

Regioselectivity and Substituent Effects

When a benzene ring already carries a substituent, the position where the next electrophile attaches is not random. This regioselectivity is governed by two properties of the existing substituent: whether it activates or deactivates the ring, and whether it directs to the ortho/para or meta positions.

Activating groups donate electron density into the ring (through resonance or induction), making it more reactive than unsubstituted benzene. Most activating groups are ortho/para directors. Examples include $-OH$, $-NH_2$, $-OCH_3$, and alkyl groups ($-CH_3$, $-C_2H_5$).

Deactivating groups withdraw electron density from the ring, making it less reactive. Most deactivating groups are meta directors. Examples include $-NO_2$, $-CN$, $-COOH$, and $-SO_3H$.

The one major exception to memorize: halogens ($-F$, $-Cl$, $-Br$, $-I$) are deactivating (they're electronegative, pulling electron density away by induction) but ortho/para directing (they donate electron density back through lone-pair resonance). This combination makes them unique among substituents.

Biological Significance of Aromatic Substitution

Aromatic Reactions in Hormone Biosynthesis

EAS-like chemistry isn't just a lab reaction. Enzyme-catalyzed aromatic substitutions are central to how your body makes critical signaling molecules.

Catecholamine and serotonin biosynthesis both depend on aromatic hydroxylation:

• Tyrosine hydroxylase converts tyrosine to L-DOPA by adding a hydroxyl group to the aromatic ring. L-DOPA is then decarboxylated to produce dopamine, which can be further converted to norepinephrine and epinephrine.
• Tryptophan hydroxylase converts tryptophan to 5-hydroxytryptophan, which is decarboxylated to form serotonin.

Thyroid hormone biosynthesis relies on aromatic iodination:

• The enzyme thyroid peroxidase uses $H_2O_2$ as an oxidant to generate reactive iodine species (similar in principle to the $I_2$/oxidant system used in the lab).
• This reactive iodine iodinates tyrosine residues within the protein thyroglobulin.
• Coupling of two iodinated tyrosine residues then produces the thyroid hormones thyroxine ($T_4$, with four iodines) and triiodothyronine ($T_3$, with three iodines).

Other Aromatic Substitution Reactions

Friedel-Crafts Reactions

Friedel-Crafts reactions are a specific subset of EAS that form new carbon-carbon bonds on the aromatic ring. There are two types:

• Friedel-Crafts Alkylation: An alkyl halide ($RX$) reacts with the aromatic ring in the presence of a Lewis acid catalyst (typically $AlCl_3$). The Lewis acid generates a carbocation ($R^+$), which acts as the electrophile. A major limitation is that the product is more reactive than the starting material (the alkyl group is activating), so polyalkylation is hard to avoid. Carbocation rearrangements can also lead to unexpected products.
• Friedel-Crafts Acylation: An acyl halide ($RCOCl$) reacts with the ring using $AlCl_3$ to generate an acylium ion ($RCO^+$). This produces an aryl ketone. Unlike alkylation, acylation doesn't suffer from polysubstitution because the product ketone is deactivated (the $C{=}O$ group is electron-withdrawing). Acylium ions also don't rearrange, giving you cleaner regiochemistry.

Nucleophilic Aromatic Substitution and Benzyne

Not all aromatic substitutions are electrophilic. Two other pathways are worth knowing:

Nucleophilic aromatic substitution ($S_NAr$) involves a nucleophile replacing a leaving group on the ring. This requires strong electron-withdrawing groups (especially $-NO_2$) ortho or para to the leaving group. These groups stabilize the negatively charged intermediate (called a Meisenheimer complex). Without them, the reaction doesn't proceed because the aromatic ring is too electron-rich to accept a nucleophile.

Benzyne mechanism: When a strong base (like $NaNH_2$) reacts with an aryl halide that lacks electron-withdrawing groups, the reaction can proceed through a benzyne intermediate. The base removes a proton adjacent to the leaving group, and the halide departs to form a highly strained, reactive triple bond within the six-membered ring. A nucleophile then attacks this benzyne. Because the triple bond spans two carbons, the nucleophile can add to either carbon, leading to a mixture of regioisomers.