Chapter 17 – Reactions of Aromatic Compounds

Vocabulary flashcards summarizing key terms, reagents, mechanisms, and directing effects for Chapter 17 aromatic chemistry, suitable for exam review.

Electrophilic Aromatic Substitution (EAS)

A reaction in which an electrophile replaces a hydrogen on an aromatic ring, maintaining aromaticity after deprotonation.

Sigma Complex (Arenium Ion)

The positively charged, non-aromatic carbocation intermediate formed after an electrophile adds to benzene during EAS.

Halogenation (EAS)

Introduction of Cl or Br onto benzene using X2 with a Lewis acid catalyst such as FeBr3 or AlCl3.

Bromination of Benzene

Reaction of benzene with Br2/FeBr3 giving bromobenzene and HBr; rate-limiting step is formation of the sigma complex.

Chlorination of Benzene

Reaction of benzene with Cl2/AlCl3 to yield chlorobenzene; proceeds analogously to bromination.

Iodination of Benzene

Formation of iodobenzene using I2/HNO3, often needing an oxidant to generate I+.

Nitration of Benzene

Reaction of benzene with HNO3/H2SO4 producing nitrobenzene via the nitronium ion (NO2+).

Nitronium Ion (NO2⁺)

The electrophile generated from HNO3 in strong acid that attacks benzene during nitration.

Reduction of Nitro Group

Conversion of an –NO2 group to –NH2 using Zn, Sn, or Fe in aqueous HCl; yields anilines.

Sulfonation of Benzene

Reaction with SO3/H2SO4 producing benzenesulfonic acid; reversible by desulfonation with dilute acid/heat.

Benzenesulfonic Acid

Product of benzene + SO3/H2SO4; serves as a blocking or directing group that can be removed by acid/heat.

Friedel-Crafts Alkylation

Attachment of an alkyl group to benzene using an alkyl halide and a Lewis acid (AlCl3); proceeds via a carbocation.

Friedel-Crafts Acylation

Introduction of an acyl group (RCO–) onto benzene using an acyl chloride and AlCl3, forming a phenyl ketone.

Acylium Ion (R–C≡O⁺)

Resonance-stabilized electrophile generated in Friedel-Crafts acylation.

Limitations of Friedel-Crafts Alkylation

Fails on strongly deactivated rings, can give carbocation rearrangements, and often leads to polyalkylation.

Clemmensen Reduction

Zn(Hg)/aq HCl reduction that converts carbonyl groups of acylated benzenes to alkyl chains.

Gattermann-Koch Formylation

Generation of a formyl cation (HCO⁺) from CO/HCl/AlCl3–CuCl to install a CHO group on benzene.

Activating Group

Substituent that increases benzene’s reactivity toward EAS by donating electron density; usually ortho/para-directing.

Deactivating Group

Substituent that decreases benzene’s reactivity toward EAS by withdrawing electron density; many are meta-directing.

Ortho/Para Director

Group that directs incoming electrophiles to the 2- and 4-positions of the ring due to resonance or inductive effects.

Meta Director

Group that directs electrophilic attack to the 3-position because ortho/para attack would place positive charge adjacent to the EWG.

Halogen Directing Effect

Halogens are deactivating due to –I effect yet ortho/para-directing because of lone-pair π donation.

Inductive Effect (–I or +I)

Electron withdrawal or donation through σ-bonds affecting ring reactivity and directing behavior.

Resonance Effect (+M or –M)

Electron donation or withdrawal through π-bond conjugation, crucial in determining activation and orientation.

Alkyl Group (–R)

Weak activator and ortho/para director via electron-releasing inductive effect.

Methoxy Group (–OCH3)

Strong activator; donates electron density by resonance, strongly promoting ortho/para substitution.

Amino Group (–NH2)

Very strong activator and ortho/para director; can lead to polyhalogenation without catalyst.

Nitro Group (–NO2)

Strong deactivator and meta director due to powerful –I and –M effects.

Sulfonic Acid Group (–SO3H)

Strongly deactivating, meta-directing substituent introduced by sulfonation.

Carbonyl-Containing Group

General class (–COR, –CHO, –COOR) that deactivates benzene and directs meta.

Nucleophilic Aromatic Substitution (NAS)

Replacement of a leaving group on an aromatic ring by a nucleophile, often requiring EWG ortho/para to LG.

Meisenheimer Complex

Resonance-stabilized anionic σ-complex formed during NAS before loss of the leaving group.

Benzyne Mechanism

Elimination–addition NAS pathway via a highly reactive benzyne intermediate, giving mixture of regioisomers.

Leaving Group Position in NAS

LG must be ortho or para to at least one strong EWG (e.g., NO2) for addition–elimination NAS to occur rapidly.

Organocuprate Coupling

Reaction of aryl or vinyl copper reagents (R2CuLi) with aryl halides to form C–C bonds without rearrangement.

Heck Reaction

Pd-catalyzed coupling of aryl/vinyl halides with alkenes in presence of base to give substituted alkenes.

Suzuki Reaction

Pd-catalyzed cross-coupling between aryl/vinyl halides and boronic acids in base, forming biaryls or Csp2–Csp2 bonds.

Birch Reduction

Dissolving-metal reduction (Na/NH3/ROH) converting benzene to 1,4-cyclohexadienes; substituent effects are regiochemical.

Hydrogenation of Benzene

Conversion of benzene to cyclohexane using H2, high pressure, and metal catalysts such as Pt or Pd.

Benzene Hexachloride (BHC)

Product formed when benzene is chlorinated under UV light or heat to give C6H6Cl6 (lindane).

Side-Chain Oxidation

KMnO4 or Na2Cr2O7 oxidation of alkylbenzenes converting side chains to benzoic acids irrespective of chain length.

Benzylic Bromination

Selective substitution at the benzylic position using Br2 or NBS under radical conditions (hv).

Benzylic Carbocation

Resonance-stabilized cation that makes benzyl halides highly reactive in SN1 reactions.

SN1 Reactivity of Benzyl Halides

Benzylic halides ionize readily forming stabilized carbocations, leading to fast solvolysis or substitution.

SN2 Reactivity of Benzyl Halides

Benzylic halides undergo rapid backside attack because the adjacent π-system stabilizes the transition state.

Quinone

Conjugated cyclic diketone produced by oxidation of phenols; functions in redox biology (e.g., coenzyme Q).

Hydroquinone

Benzene-1,4-diol reduced form of quinone; can be oxidized back to quinone in redox cycles.

Coenzyme Q (Ubiquinone)

Isoprenoid quinone that shuttles electrons in mitochondrial respiration via reversible redox with hydroquinone form.

Directing Effect Hierarchy (Activators)

–NH2 > –OH > –OR > –NHCOR > alkyl for increasing activation strength.

Directing Effect Hierarchy (Deactivators)

–NO2 > –SO3H > –COR/–CN > –X (halogens) for increasing deactivation.

Hydrogen–Deuterium Exchange

Acid-catalyzed replacement of benzene hydrogens with deuterium using D2SO4/D2O, demonstrating reversibility of EAS.

Curved Arrow Mechanism (EAS)

Shows π electrons attacking the electrophile, forming the sigma complex, and deprotonation restoring aromaticity.

Meta-Directing Rule

Electrophiles avoid positions where the σ-complex would place positive charge adjacent to a strong EWG.

Polyalkylation Problem

Alkyl groups activate the ring, causing successive Friedel-Crafts alkylations unless conditions are controlled.

Rearrangement in Friedel-Crafts

Carbocations generated can rearrange (hydride or alkyl shifts), leading to isomeric alkylated products.

Desulfonation

Removal of SO3H from benzene by heating in aqueous acid, regenerating the parent ring.

Tribromination of Anisole

Methoxy group activation causes anisole to undergo rapid, catalyst-free addition of three bromine atoms.

Energy Diagram of Director Effects

Lower activation energy for ortho/para attack with activators; higher for meta attack, explaining product ratios.

Activated Position in NAS

Carbon ortho or para to an EWG where negative charge is stabilized during Meisenheimer complex formation.

Benzylic Radical

Resonance-stabilized radical intermediate formed during benzylic bromination, leading to selective substitution.

Side-Chain SN1 vs SN2

Benzylic halides can undergo either mechanism; SN1 favored in polar protic solvents, SN2 in strong nucleophiles/aprotic.