Chapters 16-18: Conjugation, Aromaticity, and Aromatic Reactions

Flashcards covering key vocabulary and concepts from Chapters 16-18, focusing on conjugated diene stability, molecular orbital theory, aromaticity, Electrophilic Aromatic Substitution (EAS), Nucleophilic Aromatic Substitution (SNAr), and pericyclic reactions.

Conjugated Diene Stability

Conjugated dienes exhibit greater stability compared to non-conjugated alkenes, typically with about 15 kJ/mol stabilization energy, due to delocalization of π electrons.

Delocalization of π electrons

The spreading of pi electrons across a conjugated system, lowering the overall energy and increasing thermodynamic stability of the molecule.

Valence Bond (VB) Theory

A theory that describes bonds as localized between pairs of atoms, using resonance to indirectly describe delocalization and emphasizing discrete structures.

Molecular Orbital (MO) Theory

A theory that describes electrons as delocalized over the entire molecule, where atomic orbitals combine to form molecular orbitals, inherently building in delocalization.

VB Theory (Key Difference)

Describes bonds as localized and relies on resonance forms to describe electron distribution.

MO Theory (Key Difference)

Describes bonds as delocalized over the molecule and relies on constructive/destructive overlap of atomic orbitals (AOs) to form molecular orbitals.

Molecular Orbital Theory of Ethylene

Involves two atomic p orbitals overlapping to form one bonding π MO and one antibonding π* MO, with both electrons occupying the bonding π MO.

Molecular Orbital Theory of Butadiene

Involves the overlap of four unhybridized p orbitals in a conjugated diene, forming molecular orbitals where nodes increase with energy, and lowest energy MOs are filled first.

Molecular Orbital Theory of Hexatriene

Describes a conjugated system involving six p orbitals and six π electrons across a six-atom system, forming six molecular orbitals that are filled from lowest energy first.

Frontier Orbitals

The Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO), which are critical for predicting chemical reactivity, especially in photochemistry and cycloadditions.

HOMO → LUMO Transition

An electronic transition where electrons move from the Highest Occupied Molecular Orbital to the Lowest Unoccupied Molecular Orbital, typically requiring energy in the UV/visible region and dictating a molecule's absorption properties.

UV-Vis Spectroscopy

A technique used to study conjugated π systems which absorb UV or visible light through HOMO → LUMO electronic transitions, providing structural information based on absorbance and wavelength.

Absorbance (A)

A measure of the amount of light absorbed by a sample, calculated as A = log(I0/I), where I0 is incident intensity and I is transmitted intensity.

Molar Absorptivity (ε)

A measure of how strongly a chemical species absorbs light at a given wavelength, calculated as ε = A / (cl), where A is absorbance, c is concentration, and l is path length.

λmax

The wavelength at which a substance exhibits its maximum absorption of light, indicating the energy required for the HOMO→LUMO transition; increases with more conjugation.

Hydrohalogenation of Conjugated Dienes

The addition of H-X across a conjugated diene, which can yield two regioisomeric products (1,2- or 1,4-adducts) due to the formation of a resonance-stabilized allylic carbocation intermediate.

1,2-addition

An electrophilic addition pattern to conjugated dienes where the electrophile and nucleophile add to adjacent carbons on one of the original double bonds, typically forming faster (kinetically favored).

1,4-addition

An electrophilic addition pattern to conjugated dienes where the electrophile and nucleophile add to the 1st and 4th carbons of the conjugated system, creating a new double bond between C2 and C3, usually more stable (thermodynamically favored).

Kinetic Control

A reaction pathway where the product distribution is determined by the relative rates of competing reactions (lower activation barrier), typically observed at lower temperatures, favoring the faster-forming 1,2-adduct.

Thermodynamic Control

A reaction pathway where the product distribution is determined by the relative stabilities of competing products, typically observed at higher temperatures or with longer reaction times, favoring the more stable 1,4-adduct.

Pericyclic Reactions

A class of organic reactions characterized by a concerted mechanism without ionic or radical intermediates, involving a cyclic transition state where electrons move around a closed loop.

Diels–Alder Reaction

A type of [4+2] cycloaddition pericyclic reaction where a conjugated diene reacts with a dienophile (alkene or alkyne) to form a substituted cyclohexene ring in a concerted manner.

[4+2] Cycloaddition

A specific class of cycloaddition reactions, exemplified by the Diels-Alder reaction, where one component contributes 4 π electrons (e.g., the diene) and the other contributes 2 π electrons (e.g., the dienophile) to form a new ring.

s-cis Conformation (Diels-Alder)

Refers to the conformation of the diene where the two double bonds are on the same side of the connecting single bond, which is required for the Diels-Alder reaction to occur readily.

Electron-Withdrawing Groups (Diels-Alder)

Substituents on the dienophile that accelerate the Diels-Alder reaction by lowering the LUMO energy, making the dienophile more electrophilic and reacting faster with the diene's HOMO.

Stereospecificity (Diels-Alder)

A characteristic of the Diels-Alder reaction where the stereochemistry (cis/trans relationships) of the starting diene and dienophile is preserved in the final cyclohexene product.

Regioselectivity (Diels-Alder)

The preference for forming one constitutional isomer over another in the Diels-Alder reaction, which is predicted by aligning the partial charges or electron-rich and electron-poor regions of the diene and dienophile.

Endo Cycloadduct

The major bicyclic product formed when a cyclic diene undergoes a Diels-Alder reaction, typically favored due to favorable secondary orbital interactions between developing π bonds and electron-withdrawing groups in the transition state.

Retro-Diels-Alder

The reverse of the Diels-Alder reaction, where a cyclohexene derivative cleaves to reform a conjugated diene and a dienophile, often predominating at temperatures above roughly 200°C.

HOMO-LUMO Interaction (Cycloadditions)

The concept that a cycloaddition reaction proceeds when the HOMO of one reactant interacts with the LUMO of the other in a phase-matched way, determining if a pericyclic reaction is thermally allowed or forbidden.

Symmetry-Forbidden Thermal Process

A pericyclic reaction that is not allowed to proceed under thermal conditions because the HOMO-LUMO orbital interactions between reactants are out of phase in the ground state.

Photochemical Excitation

The use of light energy to promote an electron to a higher energy level (e.g., from HOMO to a new excited state HOMO), which can change orbital symmetry and allow pericyclic reactions that are otherwise symmetry-forbidden thermally.

Orbital Symmetry Conservation

A fundamental principle, often discussed using Woodward-Hoffmann rules, that governs whether a pericyclic reaction is allowed under given thermal or photochemical conditions based on the symmetry of the interacting molecular orbitals.

Benzene Structure

A six-membered ring with alternating double and single bonds, but best described as a resonance hybrid where all six C-C bonds are equivalent with a bond order of 1.5 due to electron delocalization.

Aromaticity

A special stability exhibited by cyclic, planar, fully conjugated molecules that contain 4n+2 (Hückel's Rule) π electrons, arising from the delocalization of these electrons in molecular orbitals.

Hückel’s Rule

A criterion for aromaticity stating that a cyclic, planar, and fully conjugated molecule must contain 4n+2 total π electrons (where n = 0, 1, 2, 3…) to be considered aromatic.

Antiaromatic

Describes a cyclic, planar, and fully conjugated molecule containing 4n (where n = 1, 2, 3…) π electrons, which results in significant destabilization and makes the molecule highly unstable.

Nonaromatic

Describes a molecule that fails to meet one or more of the criteria for aromaticity (cyclic, planar, fully conjugated, or 4n+2 π electrons), thus lacking the special stability or instability associated with aromatic or antiaromatic compounds.

Frost Circle

A pictorial method used to estimate the relative energies of molecular orbitals for cyclic, conjugated π systems by inscribing a polygon (representing the ring) into a circle with a vertex at the bottom, where each vertex represents an MO energy level.

Cyclooctatetraene (COT)

An eight-membered cyclic polyene that contains 8 π electrons, which would be antiaromatic if planar but adopts a nonplanar tub shape to avoid antiaromaticity, making it nonaromatic.

Tropylium Cation

A cyclohepta-1,3,5-triene cation (C7H7+) that is aromatic, due to its cyclic, planar, fully conjugated structure with 6 π electrons delocalized over seven carbon atoms.

Cyclopentadienyl Anion

A five-membered cyclic anion (C5H5-) that is aromatic, due to its cyclic, planar, fully conjugated structure with 6 π electrons (including a lone pair) delocalized over five carbon atoms.

Annulenes

Monocyclic hydrocarbons containing a fully conjugated π electron system, which can be aromatic, antiaromatic, or nonaromatic depending on their size, geometry, and π electron count.

Polycyclic Aromatic Hydrocarbons (PAHs)

Organic compounds consisting of multiple fused benzene rings, which also exhibit aromaticity, though their stabilization energy per ring can be less than that of benzene.

Aromatic Heterocycles

Cyclic aromatic compounds where one or more ring atoms are a heteroatom (e.g., N, O, S), and where the heteroatom's lone pair may or may not participate in the aromatic π system, influencing basicity.

Aromaticity and Reactivity

A principle stating that reactions which break aromaticity are generally unfavorable, while reactions that generate or restore aromaticity are typically favorable.

Benzylic Carbon

A carbon atom that is directly attached to a benzene ring, known for undergoing characteristic reactions such as radical bromination, nucleophilic substitution, elimination, and oxidation.

Benzylic Oxidation

The oxidation of a benzylic C-H bond by strong oxidizing agents (e.g., chromic acid, KMnO4) to convert the benzylic carbon into a carboxylic acid, forming a benzoic acid derivative, requiring at least one benzylic hydrogen.

Birch Reduction

A partial reduction of an aromatic ring using sodium (Na), liquid ammonia (NH3), and methanol (MeOH), which yields a non-conjugated 1,4-cyclohexadiene product and breaks aromaticity.

Electron-Donating Groups (Birch Reduction)

Substituents that decrease the likelihood of reduction at adjacent carbons in a Birch reduction by destabilizing the intermediate carbanions, leading to reduction at other positions.

Electron-Withdrawing Groups (Birch Reduction)

Substituents that increase the likelihood of reduction at adjacent carbons in a Birch reduction by stabilizing the intermediate carbanions, directing reduction to those carbons.

Infrared (IR) Spectroscopy (Aromatic)

An analytical technique that identifies the presence of aromatic compounds by characteristic absorption bands, such as C-H stretches around 3063 cm⁻¹ for aromatic C-H and specific patterns in the fingerprint region.

1H NMR (Aromatic)

Nuclear Magnetic Resonance spectroscopy for protons, where aromatic protons typically resonate in the deshielded region of δ ≈ 6.5-8.0 ppm due to ring currents, providing information on connectivity and substitution.

13C NMR (Aromatic)

Nuclear Magnetic Resonance spectroscopy for carbon-13, where aromatic carbons typically appear in the δ ≈ 100-150 ppm range, providing information about the number and type of carbon environments and substitution patterns.

Electrophilic Aromatic Substitution (EAS)

A type of reaction where an aromatic proton is replaced by an electrophile while maintaining the aromatic character of the ring, with the aromatic π system acting as the nucleophile.

Halogenation (EAS)

An Electrophilic Aromatic Substitution reaction where a halogen (e.g., Br₂ or Cl₂) adds to an aromatic ring in the presence of a Lewis acid catalyst (e.g., FeBr₃ or AlCl₃) to introduce a halogen substituent.

Sulfonylation (EAS)

An Electrophilic Aromatic Substitution reaction where a sulfonyl group (–SO₃H) is installed onto an aromatic ring using fuming sulfuric acid (containing SO₃ as the electrophile), a process that can be reversible.

σ-complex (Arenium ion)

A resonance-stabilized carbocation intermediate formed in the first step of Electrophilic Aromatic Substitution, where the aromatic ring attacks the electrophile, temporarily breaking aromaticity.

Nitration (EAS)

An Electrophilic Aromatic Substitution reaction that introduces a nitro group (–NO₂) onto an aromatic ring using a mixture of nitric acid (HNO₃) and sulfuric acid (H₂SO₄) to generate the electrophile, nitronium ion (NO₂⁺).

Friedel-Crafts Alkylation

An Electrophilic Aromatic Substitution reaction that introduces an alkyl group onto an aromatic ring using an alkyl halide and a Lewis acid catalyst (e.g., AlCl₃), typically involving a carbocation intermediate.

Limitations of Friedel-Crafts Alkylation

Includes carbocation rearrangements, the requirement for an sp³-hybridized carbon with a leaving group, the potential for polyalkylation, and the unsuitability for moderately or strongly deactivated aromatic rings.

Friedel-Crafts Acylation

An Electrophilic Aromatic Substitution reaction that installs an acyl group (R-CO-) onto an aromatic ring using an acyl halide or anhydride and a Lewis acid catalyst (e.g., AlCl₃), with an acylium ion as the electrophile.

Clemmensen Reduction

A reaction (typically Zn(Hg), HCl) used to reduce the carbonyl group of an acyl benzene product (from Friedel-Crafts Acylation) to a methylene (–CH2–) group, yielding an alkyl benzene.

Substituent Effects on EAS

The influence of groups already present on a benzene ring on the rate and regioselectivity (ortho/meta/para) of subsequent Electrophilic Aromatic Substitution reactions, determined by their electron-donating or withdrawing properties.

Activating Groups (EAS)

Substituents that increase the electron density of an aromatic ring, making it more reactive towards Electrophilic Aromatic Substitution, and generally direct incoming electrophiles to the ortho and para positions.

Ortho/Para Directors

Substituents on an aromatic ring that preferentially steer incoming electrophiles to the ortho (adjacent) and para (opposite) positions relative to themselves during Electrophilic Aromatic Substitution, usually by stabilizing the σ-complex via resonance.

Deactivating Groups (EAS)

Substituents that decrease the electron density of an aromatic ring, making it less reactive towards Electrophilic Aromatic Substitution, and typically direct incoming electrophiles to the meta position.

Meta Directors

Substituents on an aromatic ring that preferentially steer incoming electrophiles to the meta (one carbon removed) position relative to themselves during Electrophilic Aromatic Substitution, usually by destabilizing ortho/para attack.

Halogen Substituents (EAS)

A unique class of substituents on an aromatic ring that are overall deactivating (due to inductive electron withdrawal) yet direct incoming electrophiles to the ortho and para positions (due to resonance stabilization of the σ-complex).

Steric Effects in EAS

The influence of the size and bulkiness of existing substituents on an aromatic ring, which can hinder electrophilic attack at certain positions, leading to a preference for less hindered positions (e.g., para over ortho product).

Blocking Groups

Substituents that are temporarily installed onto an aromatic ring to control the regioselectivity of subsequent Electrophilic Aromatic Substitution reactions by directing other groups, and can later be removed (e.g., sulfonic acid).

Nucleophilic Aromatic Substitution (SNAr)

A reaction where a leaving group on an aromatic ring is replaced by a nucleophile, requiring a strong electron-withdrawing group (EWG) ortho or para to a good leaving group, and where the aromatic ring acts as an electrophile.

Meisenheimer Complex

A resonance-stabilized anion intermediate formed in the first step of Nucleophilic Aromatic Substitution (SNAr), where a nucleophile adds to the aromatic ring, and the negative charge is delocalized onto the ring and the electron-withdrawing group.

Elimination-Addition (Benzyne) Mechanism

An aromatic substitution pathway that occurs under very strong basic and high-temperature conditions involving the elimination of a leaving group and a proton to form a highly reactive benzyne intermediate, followed by nucleophilic addition.

Benzyne Intermediate

A highly reactive, unstable intermediate in aromatic substitution reactions, characterized by a formal triple bond within the benzene ring, formed by elimination of a leaving group and a proton under harsh conditions.

EAS Mechanism (Distinguishing)

Characterized by electrophilic reagents attacking the ring, forming a σ-complex (arenium ion), with a proton as the leaving group to restore aromaticity.

SNAr Mechanism (Distinguishing)

Characterized by nucleophilic attack on a ring with a strong Electron-Withdrawing Group (EWG) ortho or para to a good leaving group, forming a Meisenheimer complex intermediate, then loss of the leaving group.

E-A Mechanism (Distinguishing)

Characterized by high temperatures and strong bases, involving a benzyne intermediate formed via elimination, followed by nucleophilic addition to the benzyne, leading to potential regioisomeric products.