Electrophilic Aromatic Substitution and Related Reactions (Aromatic Chemistry)

A set of vocabulary-style flashcards covering key terms and concepts from the lecture notes on electrophilic aromatic substitution, directing effects, and related named reactions.

Aromatic hydrocarbons

Unusually stable cyclic delocalised molecules (e.g., benzene); they resist addition reactions and undergo electrophilic substitution rather than addition.

Aromatic substitution

Replacement of a ring hydrogen by an electrophile, with aromaticity restored in the product.

Electrophilic substitution

A reaction where an electrophile substitutes onto an aromatic ring, preserving aromaticity after the hydrogen is replaced.

Electrophile

An electron‑deficient species that accepts electrons from the aromatic ring; examples include nitronium NO2+, acylium ions R‑CO+, and alkyl carbocations.

Arenium ion (sigma complex)

The non‑aromatic, resonance‑stabilized cation formed after the first attack of an electrophile on the ring.

Rate-determining step (RDS) in EAS

The first step (formation of the arenium ion) is slow and endothermic; aromaticity is lost and the step sets the overall rate.

Two-step mechanism of EAS

Step 1: formation of the arenium ion (slow, endothermic). Step 2: deprotonation to restore aromaticity (fast, exothermic).

Activating groups

Electron‑donating substituents that increase the rate of EAS and direct to ortho/para positions.

Strongly activating groups

Donor groups with N or O atoms (e.g., ‑NH2, ‑NHR, ‑NR2, ‑OH, ‑OR) that greatly accelerate EAS and direct to ortho/para.

Moderately activating groups

Groups that donate electrons moderately (e.g., alkyl/aryl groups) and direct to ortho/para.

Weakly activating groups

Groups with a mild activating effect; still direct to ortho/para but lessen the rate increase.

Deactivating groups

Electron‑withdrawing substituents that slow EAS and often direct to meta.

Strongly deactivating groups

Powerfully electron‑withdrawing groups (e.g., NO2, SO3H, CN) that greatly reduce reactivity and direct meta.

Moderately deactivating groups

Groups withdrawing electrons to a moderate extent, also often meta directing.

Weakly deactivating groups

Mildly electron‑withdrawing groups; decrease rate but not as strongly.

Ortho/para directing groups

Activating groups that direct electrophilic attack to the ortho and para positions relative to the substituent.

Meta directing groups

Deactivating or strongly withdrawing groups that direct electrophilic attack to the meta position.

Regioselectivity

Preference for substitution at one position over others in an aromatic ring during EAS.

Nitration

Electrophilic substitution introducing a NO2 group; typically uses conc. HNO3/H2SO4 to generate the nitronium ion NO2+.

Nitronium ion

NO2+; the active electrophile in most nitration reactions of arenes.

Halogenation

Electrophilic substitution of a halogen (Cl, Br) using X2 in the presence of a Lewis acid (e.g., FeX3, AlX3).

Friedel–Crafts alkylation

Introduction of an alkyl group onto an aromatic ring using an alkyl halide and a Lewis acid (e.g., AlCl3); can rearrange and cause polyalkylation.

Friedel–Crafts acylation

Introduction of an acyl group via an acyl chloride and a Lewis acid to give an aryl ketone; usually does not rearrange the carbon skeleton.

Fries rearrangement

Rearrangement of acylated phenyl esters under Lewis acid to give ortho/para acetylated phenols.

Gattermann reaction

Formylation of benzene to benzaldehyde using CO and HCl in the presence of ZnCl2.

Gattermann–Koch reaction

Formylation of benzene using CO/HCl with AlCl3 to give benzaldehyde.

Sandmeyer reaction

Replacement of a diazonium salt by CuX (X = Cl, Br, CN) to give aryl halides or cyanides.

Balz–Schiemann reaction

Conversion of a diazonium tetrafluoroborate to the corresponding aryl fluoride upon heating.

Diazonium salts

Aryl diazonium salts formed from primary amines via nitrous acid; stable at 0–5°C and used in multiple coupling reactions.

Azo dyes

Colored compounds formed by coupling diazonium salts with activated aromatics (e.g., phenols, naphthols) forming the N=N azo linkage.

Coupling reactions (diazonium chemistry)

Reaction of diazonium salts with activated aromatics (phenols, naphthols, amines) under basic or neutral conditions to give azo dyes.

Nitration of phenol

Phenol is strongly activated; nitration can proceed with dil. HNO3, often via nitroso phenol as an intermediate.

Sulphonation

Introduction of a sulfonic acid group (SO3H) on benzene/phenol using conc. H2SO4 or oleum; often reversible and temperature‑dependent.

Desulphonation

Removal of sulfonic acid group (SO3H) by hydrolysis or heating to regenerate the unsubstituted ring or relocate substitution.

Kolbe reaction (Kolbe–Schmitt context)

Preparation of salicylic acid (2‑hydroxybenzoic acid) from phenoxide with CO2 under high pressure; leads to aspirin after acetylation.

Reimer–Tiemann reaction

Formylation of phenols with chloral (CHCl3/NaOH) giving o‑ and p‑hydroxybenzaldehydes (ortho predominant).

Naphthalene EAS

EAS on naphthalene occurs preferentially at the α (1‑position) due to higher electron density and stability of the arenium ion.

EAS in heterocycles (pyrrole, furan, thiophene)

Electron‑rich heterocycles undergo EAS with high reactivity; typical order: pyrrole > furan > thiophene; attack often at the α (2‑) position.

Gammexane (chlorinated cycloalkane) formation

Product from halogenation of benzene under UV light giving multiple chlorinated cycloalkane isomers (historical example in halogenation section).

Gattermann reaction vs. nitration context

Formylation processes using CO/HCl (Gattermann) or CO/HCl with AlCl3 (Gattermann–Koch) to introduce formyl groups on benzene.

Phenol and aniline halogenation (bromination)

Phenol/aniline are highly activated; bromination can give tribromophenols; solvent and conditions influence major products.

Desulphonation applications

Desulphonation used to block para position temporarily or to reveal other substitution patterns after sulfonation.

Oxidation of benzylic side chains (KMnO4/K2Cr2O7)

Oxidation of alkyl side chains on arenes to carboxylic acids (benzoic acid formation) when benzylic C–H bonds are present.

Aryl to carbonyl transformations (oxidation/reagent families)

Diverse oxidants (KMnO4, Cr(VI), V2O5, etc.) convert activated rings or side chains to carbonyl or quinone structures depending on substrate.