Aromatic and Heteroaromatic Chemistry - Electrophilic Aromatic Substitution

This set of vocabulary flashcards covers the fundamentals of Electrophilic Aromatic Substitution (SEAr), including specific reaction mechanisms, intermediates, and the challenges of Friedel-Crafts reactions.

$$S_EAr$$

Electrophilic aromatic substitution; a reaction where an arene acts as a nucleophile to substitute a hydrogen atom with an electrophile, preserving its aromaticity.

Wheland intermediate

A resonance-stabilized cation (also called an arenium ion or $\sigma$-complex) formed during $S_EAr$ that is not aromatic due to one $sp^3$ carbon atom.

Hammond postulate

The principle used in $S_EAr$ to suggest that the first transition state (TS) is similar in structure to the Wheland intermediate because they are similar in energy.

Nitronium ion

The reactive cationic electrophile ($\text{NO}_2^+$) generated from a mixture of $\text{HNO}_3$ and $\text{H}_2 ext{SO}_4$ during nitration.

Oleum

A mixture of sulfur trioxide ($\text{SO}_3$) in sulfuric acid ($\text{H}_2 ext{SO}_4$) used as an electrophile source for sulfonation.

Sulfonic acid

The product of an $S_EAr$ sulfonation reaction, which is acidic enough to protonate the conjugate base of $\text{HCl}$.

Kinetic product

The product that forms faster due to having a lower energy transition state to the Wheland intermediate; in naphthalene sulfonation at $80^{ ext{o}} ext{C}$, this is the 1-substituted product.

Thermodynamic product

The most stable product formed in a reversible reaction; in naphthalene sulfonation at $160^{ ext{o}} ext{C}$, this is the 2-substituted product which avoids $1,8$-peri strain.

Desulfonation

An entropically driven process ($\Delta S angle angle 0$) at high temperatures where the sulfonic acid group is removed by treatment with acid and heat.

Lewis acid-base adduct

The complex formed in halogenation between a halogen ($ext{X}_2$) and a Lewis acid catalyst ($ext{MX}_3$) to activate the electrophile.

CAN

Ceric ammonium nitrate ($( ext{NH}_4)_2[ ext{Ce(NO}_3)_6]$), an oxidant used as a source of electrophilic iodine in aromatic substitution.

Acylium ion

A linear, resonance-stabilized electrophile ($ext{R-C}^+ = ext{O} ightleftharpoons ext{R-C} ext{ extequiv} ext{O}^+$) formed in Friedel-Crafts acylation.

Friedel-Crafts alkylation

An $S_EAr$ reaction introducing an alkyl group using an alkyl halide and a Lewis acid catalyst, often limited by carbocation rearrangements.

Hyperconjugation

Also called $\sigma$-conjugation; the donation of electron density from a C-H bond into an adjacent empty p-orbital, which stabilizes carbocations.

Polyalkylation

A common problem in Friedel-Crafts alkylation where the addition of an alkyl group makes the aromatic ring more electron-rich and reactive, leading to multiple substitutions.