Glossary of Key Terms

Term	Definition
Aromaticity	Property of a cyclic, planar, fully conjugated compound with (4n+2) pi electrons; confers exceptional thermodynamic stability
Huckel's Rule	A compound is aromatic if planar, cyclic, fully conjugated, and has (4n+2) pi electrons (n = 0,1,2,...)
Anti-aromatic	Planar, cyclic, fully conjugated compound with 4n pi electrons; thermodynamically destabilized
Non-aromatic	Compound that fails one or more of the aromaticity criteria; neither stabilized nor destabilized by aromaticity
Resonance hybrid	The actual structure of a molecule that is a weighted average of all valid Lewis resonance contributors; not any single contributor
Electrophilic Aromatic Substitution (EAS)	Reaction in which an electrophile replaces a hydrogen on an aromatic ring, preserving aromaticity
Arenium ion	The carbocation intermediate in EAS; also called sigma complex or Wheland intermediate; formed when $E^{+}$ bonds to the ring carbon
Sigma complex	See arenium ion; named because $E^{+}$ is attached by a sigma bond to the ring carbon, disrupting the pi system
Wheland intermediate	Synonymous with arenium ion / sigma complex; named after chemist George Wheland
Rate-determining step (RDS)	The slowest step in a multi-step mechanism that controls the overall reaction rate; in EAS, step 2 ( $E^{+}$ attack on pi cloud)
Electrophile	Electron-pair acceptor; species with a positive charge or electron deficiency that attacks electron-rich sites
Nitronium ion ( $NO_{2}^{+}$ )	The active electrophile in nitration; generated by $H_{2}SO_{4}$ protonation of $HNO_{3}$ followed by dehydration
Acylium ion (R $CO^{+}$ )	The electrophile in FC acylation; resonance-stabilized (R- $C^{+}$ =O ↔ R-C≡ $O^{+}$ ); does not rearrange
Ortho-para director	A substituent that directs incoming electrophiles to positions 2 and 4 (ortho and para) on the ring
Meta director	A substituent that directs incoming electrophiles to position 3 (meta) on the ring
Activating group	A substituent that increases the ring's reactivity toward EAS compared to benzene (increases electron density)
Deactivating group	A substituent that decreases the ring's reactivity toward EAS compared to benzene (decreases electron density)
Inductive effect (+I/-I)	Electron donation or withdrawal through sigma bonds along the carbon chain
Mesomeric effect (+M/-M)	Electron donation or withdrawal through pi bonds / conjugation / resonance
Hyperconjugation	Overlap of C-H sigma bonding orbital with adjacent empty or pi orbital; responsible for +I effect of alkyl groups
Delocalization	Spreading of electron density over multiple atoms/bonds via pi conjugation; reduces energy of the system
Lewis acid	An electron-pair acceptor; $AlCl_{3}$ , $FeCl_{3}$ , $FeBr_{3}$ are Lewis acid catalysts in EAS reactions
Sulfonation	EAS reaction of benzene with fuming $H_{2}SO_{4}$ ( $SO_{3}$ electrophile) to give benzenesulfonic acid; the only reversible EAS
Rearrangement	Migration of a carbocation to a more stable structure via hydride or methyl shift; occurs in FC alkylation, NOT acylation
Polyalkylation	Successive alkylation of the ring beyond monosubstitution; problem in FC alkylation because each alkyl group activates the ring further
Resonance energy	The extra stability (≈36 kcal/mol for benzene) due to electron delocalization compared to a hypothetical localized structure
Cyclooctatetraene (COT)	$C_{8}H_{8}$ ; 8 pi electrons (4n); NON-aromatic (not anti-aromatic) because it adopts a non-planar tub conformation
Cyclobutadiene	$C_{4}H_{4}$ ; 4 pi electrons (4n); planar; fully conjugated; ANTI-aromatic — extremely unstable
Benzenesulfonic acid	Ar $SO_{3}$ H; product of benzene sulfonation; SMILES: `OS(=O)(=O)c1ccccc1`; can be reversed to benzene by heating with dilute $H_{2}SO_{4}$