Aromaticity and Aromatic Reactivity: every key term you need (+ practice quiz)
25 flashcard terms for Organic Chemistry II Topic 2, written to match the course framework. Study them here, then drill them as interactive flashcards, or test yourself with the 8-question quiz — free, no account needed.
A cyclic, fully conjugated, planar molecule with a count of pi electrons that fills every bonding level. The result is unusual stability and reluctance to add across the ring.
Huckel count
The rule that aromatic rings hold two, six, ten or fourteen pi electrons, following the pattern of four times a whole number plus two.
Antiaromatic compound
A cyclic planar conjugated system with an electron count that leaves unpaired electrons in nonbonding levels. It is destabilized and distorts or reacts to escape planarity.
Nonaromatic compound
A ring that cannot delocalize fully because it is not planar or not fully conjugated, so it behaves like an ordinary alkene.
Benzene resonance energy
The extra stability of benzene over three isolated double bonds, measured by comparing hydrogenation energies and amounting to a large barrier to addition.
Heterocyclic aromatic
An aromatic ring containing nitrogen, oxygen or sulfur. Whether a lone pair counts toward the electron total depends on whether it lies in the pi system or in the ring plane.
Pyridine basicity
Pyridine's nitrogen lone pair sits in the ring plane, outside the pi system, so protonating it does not disturb aromaticity and the ring is basic.
Pyrrole basicity
Pyrrole's lone pair is part of the aromatic sextet, so protonating the nitrogen would destroy aromaticity, making the ring a very weak base.
Electrophilic aromatic substitution
The characteristic aromatic reaction: an electrophile attacks the ring, then a proton is lost so aromaticity is restored rather than an addition product formed.
Arenium ion
The delocalized cationic intermediate formed after the electrophile attacks. Its relative stability under different substituents is what explains all directing effects.
Halogenation of benzene
Requires a Lewis acid catalyst to polarize the halogen into a strong enough electrophile, since benzene is far less nucleophilic than an ordinary alkene.
Nitration
Uses a mixture of strong acids to generate the nitronium electrophile, and installs a group that can later be reduced to an amine.
Sulfonation
Installs a sulfonic acid group and is reversible under aqueous acid, which makes it useful as a temporary blocking group.
Friedel-Crafts alkylation
Attaches an alkyl group through a carbocation intermediate. It suffers from rearrangement and from repeat substitution because the product is more reactive than the starting ring.
Friedel-Crafts acylation
Attaches an acyl group through a resonance-stabilized acylium ion, so no rearrangement occurs and the deactivated product resists further attack.
Activating group
A substituent that donates electron density into the ring, speeding substitution and directing new groups to the positions adjacent and opposite to itself.
Deactivating group
A substituent that withdraws electron density, slowing substitution. Most such groups direct incoming electrophiles to the position across the ring.
Halogen directing anomaly
Halogens withdraw by induction yet donate by resonance, so they slow the reaction while still directing to the adjacent and opposite positions.
A strongly withdrawing group that destabilizes the intermediate for adjacent and opposite attack, leaving the position across the ring as the least bad option.
Steric influence on substitution
Bulky existing substituents suppress attack at the neighbouring position, shifting the product ratio toward the far position even when both are electronically allowed.
Synthetic order in aromatic synthesis
The sequence of installing groups determines the outcome, because each new substituent controls where the next one can go.
Nucleophilic aromatic substitution
Replaces a leaving group on a ring bearing strong electron-withdrawing groups, proceeding through an anionic intermediate stabilized by those groups.
Benzyne intermediate
A strained species formed by eliminating a proton and a leaving group from a ring under very strong base, giving substitution at either of two positions.
Benzylic reactivity
The position next to a ring forms stabilized radicals and cations, so oxidation and substitution occur there far more readily than at an ordinary carbon.
Side chain oxidation
Strong oxidants convert an alkyl side chain with at least one benzylic hydrogen into a carboxylic acid, regardless of the chain's original length.