Chapter 15 – Conjugated Systems, Orbital Symmetry & UV Spectroscopy

Vocabulary flashcards summarizing essential terms, definitions, and concepts from Chapter 15 covering conjugated systems, allylic chemistry, Diels–Alder reactions, pericyclic symmetry, and UV spectroscopy.

Conjugated diene

A molecule containing two double bonds separated by one single bond (C=C–C=C); more stable than isolated or cumulated dienes.

Cumulated diene

A compound with consecutive double bonds (C=C=C); generally the highest-energy, least-stable diene type.

Isolated diene

A molecule in which the two double bonds are separated by two or more single bonds; intermediate in stability between conjugated and cumulated systems.

Heat of hydrogenation (ΔH°)

The enthalpy change when a compound is fully hydrogenated; lower (less exothermic) values indicate greater alkene stability.

s-cis conformation

The conformation of a conjugated diene in which the two double bonds lie on the same side of the single bond—required for Diels–Alder reactivity.

s-trans conformation

The conformation of a conjugated diene with the two double bonds on opposite sides of the single bond—usually more stable than s-cis.

Molecular orbital (MO) diagram

A representation that shows the relative energies and phases of atomic orbitals combined into bonding, non-bonding, and antibonding molecular orbitals.

HOMO

Highest Occupied Molecular Orbital; electron donor orbital in pericyclic reactions.

LUMO

Lowest Unoccupied Molecular Orbital; electron acceptor orbital in pericyclic reactions.

Node

A region of zero electron density in an orbital; the number of nodes increases with orbital energy.

Allylic position

The carbon adjacent to a C=C double bond; forms stabilized cations, anions, and radicals through resonance.

Allylic cation

A positively charged species with the charge delocalized over an allylic framework; more stable than a typical 1° carbocation.

Allylic radical

A radical in which the unpaired electron is delocalized over an allylic π system—bond dissociation energy ≈ 372 kJ mol⁻¹ (89 kcal).

Allylic anion

A negatively charged allylic species with four π electrons delocalized over three atoms.

N-bromosuccinimide (NBS)

A brominating reagent that maintains low [Br₂] for selective allylic radical halogenation.

SN2 acceleration (allylic)

Increased SN2 rate on allylic halides/tosylates due to transition-state charge dispersal into the adjacent π system.

1,2-addition

Addition across the first double bond of a conjugated diene; kinetic product formed faster at low temperature.

1,4-addition

Addition that yields the conjugated product of a diene, placing new atoms on C-1 and C-4; thermodynamic product favored at higher temperature.

Kinetic control

Reaction conditions where the product distribution is determined by relative activation energies (lower Eₐ wins).

Thermodynamic control

Conditions where the most stable product predominates because reactions are reversible and equilibrate.

Diels–Alder reaction

A concerted [4+2] cycloaddition between a conjugated diene (HOMO donor) and a dienophile (LUMO acceptor) forming a six-membered ring.

Dienophile

The alkene or alkyne component in a Diels–Alder reaction; reactivity enhanced by electron-withdrawing substituents.

Endo rule

In bicyclic Diels–Alder adducts, electron-withdrawing groups on the dienophile prefer the endo (inside) orientation due to secondary orbital overlap.

Regiochemistry (1,4 vs 1,2)

The orientation of new bonds in a Diels–Alder adduct; predicted by matching the largest π-electron coefficients of diene and dienophile.

Pericyclic reaction

A concerted process involving a cyclic flow of electrons through overlapping orbitals; includes cycloadditions and sigmatropic shifts.

Symmetry-allowed cycloaddition

A pericyclic reaction whose interacting MOs have matching phases, giving constructive overlap and low activation energy.

Symmetry-forbidden reaction

A reaction where HOMO–LUMO overlap is antibonding under thermal conditions, preventing concerted bond formation (e.g., thermal [2+2]).

Photochemical [2+2] cycloaddition

A UV-induced [2+2] reaction made allowed by promoting one alkene to its excited state, reversing orbital symmetry.

Ultraviolet-visible (UV-Vis) spectroscopy

Analytical technique measuring electronic transitions (π→π, n→π) to probe conjugation; reports λmax values.

λmax

The wavelength at which maximum absorbance occurs; shifts to longer wavelength (bathochromic shift) with increased conjugation.

π→π* transition

Excitation of electrons from a bonding π MO to an antibonding π* MO; energy gap decreases with conjugation length.

Bathochromic shift

A red shift (longer λmax) observed as the π system length or substituent conjugation increases.

β-Carotene

A highly conjugated tetraene (11 C=C) absorbing at 454 nm, giving an orange color by absorbing blue light.

Orbital symmetry

The relative phase alignment of interacting orbitals that determines whether pericyclic reactions are allowed or forbidden.

Conjugation energy (resonance stabilization)

The extra stability (≈15 kJ mol⁻¹ in butadiene) arising from delocalization of π electrons across adjacent double bonds.

Allylic halogenation

Selective replacement of an allylic hydrogen by halogen via radical mechanism, often using NBS under hv.

Curved-arrow mechanism

Notation illustrating electron flow in reaction steps; essential for explaining 1,2- vs 1,4-addition pathways.

Thermodynamic vs kinetic product diagram

Energy profile showing lower Eₐ (kinetic) pathway versus lower ΔG (thermodynamic) product with possible reversibility at higher T.

Visible region (400–750 nm)

Portion of the EM spectrum where highly conjugated molecules absorb, giving rise to perceived colors.

End-to-end overlap (π bonding)

Side-by-side interaction of parallel p-orbitals forming a delocalized π bond across multiple atoms in conjugated systems.