Conjugation, Resonance and Dienes: every key term you need (+ practice quiz)
25 flashcard terms for Organic Chemistry II Topic 1, 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 run of adjacent parallel p orbitals that lets electrons delocalize over several atoms. Delocalization lowers energy, which is why conjugated isomers dominate at equilibrium.
Resonance contributor
One valid way of drawing electrons in a delocalized system. No single drawing is real; the true structure is a weighted blend of all of them.
Major resonance contributor
The drawing with the most complete octets, the fewest charges, and any negative charge on the most electronegative atom. It carries the most weight in the blend.
Allylic position
The carbon next to a double bond. A radical or cation formed there is stabilized by delocalization, which is why allylic sites are attacked selectively.
Allylic bromination
Uses a reagent that keeps bromine concentration very low, favouring radical substitution at the allylic carbon over addition across the double bond.
Conjugated diene
Two double bonds separated by one single bond. It is more stable than an isolated diene, which shows in a smaller heat of hydrogenation.
Cumulated diene
Two double bonds meeting at one carbon. The two pi systems lie in perpendicular planes, so the arrangement is strained and unusually reactive.
s-cis conformation
The coplanar arrangement of a diene with both double bonds on the same side of the connecting single bond. Only this arrangement can undergo cycloaddition.
Kinetic product
The product formed fastest through the lowest activation barrier. It dominates at low temperature where the first step cannot reverse.
Thermodynamic product
The most stable product, which dominates at high temperature because reactions become reversible and the system settles at equilibrium.
Conjugate addition to a diene
Addition across the ends of a conjugated system rather than across one double bond, giving the more substituted and more stable alkene.
Diels-Alder reaction
A one-step cycloaddition between a diene and an alkene that makes two carbon-carbon bonds and a six-membered ring, with stereochemistry preserved on both partners.
Dienophile
The alkene partner in a cycloaddition. Electron-withdrawing groups lower its orbital energy and accelerate the reaction sharply.
Endo preference
The transition state in which the dienophile's substituent points under the forming ring is favoured by secondary orbital overlap, giving the endo product under kinetic control.
Stereospecificity of cycloaddition
Substituent relationships in the starting alkene are carried into the ring, so a cis alkene gives cis substituents in the product.
Ultraviolet absorption
Conjugated systems absorb ultraviolet light by promoting an electron between pi levels. Longer conjugation narrows the gap and shifts absorption to longer wavelength.
Chromophore
The part of a molecule responsible for its light absorption, usually an extended conjugated system rather than the molecule as a whole.
Molecular orbital picture of conjugation
Combining several p orbitals produces a ladder of bonding and antibonding levels; filling only the lower levels is the source of delocalization stability.
The most energetic filled level, which behaves as the electron donor. Its energy predicts how readily a molecule acts as a nucleophile.
Lowest unoccupied orbital
The least energetic empty level, which behaves as the electron acceptor. Lowering it with electron-withdrawing groups makes a molecule a stronger electrophile.
Hyperconjugation
Overlap between a filled adjacent single bond and an empty or partly filled orbital. It is the reason more substituted carbocations and alkenes are more stable.
Inductive effect
Charge shifted through sigma bonds by electronegativity. It falls off sharply with distance, unlike resonance which can act across a whole conjugated system.
Alkene stability trend
More substituted alkenes are more stable, and conjugated alkenes more stable still, which is why elimination and rearrangement drift in that direction.
Heat of hydrogenation
The energy released on adding hydrogen across a double bond. Comparing values for isomers measures how much extra stability delocalization provides.
Isoprene unit
A five-carbon branched building block whose repetition explains the carbon skeletons of terpenes and many natural products.