15.6 Polycyclic Aromatic Compounds
3 min read•may 7, 2024
are fascinating molecules with fused benzene rings. These structures give them unique properties, like increased stability and reactivity compared to single-ring aromatics. They're everywhere, from coal tar to DNA-damaging carcinogens.
Understanding polycyclics is crucial for grasping aromatic chemistry. Their extended π-electron systems affect everything from their physical properties to their chemical behavior. Plus, their heterocyclic cousins play vital roles in biology, forming the backbone of many important biomolecules.
Polycyclic Aromatic Compounds
Structure of polycyclic aromatics
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- Fused aromatic rings form polycyclic aromatic compounds (, )
- Naphthalene consists of two fused benzene rings while anthracene has three fused benzene rings in a linear arrangement
- Planar molecules result from of carbon atoms allowing for delocalized across the fused ring system
- Increased stability compared to isolated double bonds due to the delocalized π-electron system
- Higher melting and boiling points than their non-aromatic counterparts caused by the increased intermolecular forces
- Low solubility in water due to their nonpolar nature making them hydrophobic
- Characteristic arise from the extended of the π-electron system
- Undergo reactions as the primary mode of reactivity
- Increased reactivity compared to benzene due to the additional structures stabilizing the intermediate carbocation
Aromaticity and Electronic Properties
- is a key feature of polycyclic aromatic compounds, contributing to their stability and unique properties
- Resonance stabilization occurs through of π-electrons across the fused ring system
- Conjugation of π-bonds throughout the molecule results in extended electron delocalization
- of the molecule is essential for maximum overlap of p-orbitals and efficient electron delocalization
- These electronic properties contribute to the compounds' reactivity and spectroscopic characteristics
Reactivity of polycyclics vs benzene
- Polycyclic aromatic hydrocarbons exhibit higher reactivity than benzene in electrophilic substitution reactions
- Additional resonance structures stabilize the intermediate carbocation lowering the activation energy for the reaction
- Naphthalene undergoes electrophilic substitution preferentially at the 1-position ()
- Resonance structures of the intermediate carbocation show increased stability when the positive charge is at the 1-position
- Minor product forms from substitution at the 2-position ()
- Anthracene undergoes electrophilic substitution preferentially at the 9-position
- Resonance structures of the intermediate carbocation show increased stability when the positive charge is at the 9-position
- Minor products result from substitution at the 1-position and 2-position
- Reactivity trend follows: Anthracene > Naphthalene > Benzene due to increasing resonance stabilization of the carbocation intermediate
- is a concern with some polycyclic aromatic compounds due to their ability to form DNA adducts
Heterocyclic analogs in biology
- acts as a naphthalene analog with a nitrogen atom replacing one of the CH groups
- Bicyclic structure composed of a benzene ring fused to a pyridine ring
- Found in some alkaloids such as (antimalarial drug)
- exists as an isomer of quinoline with the nitrogen atom in the other ring
- Found in some alkaloids such as (analgesic drug)
- contains a bicyclic structure composed of a benzene ring fused to a pyrrole ring
- Found in the amino acid and neurotransmitter
- Indole ring system is a key component of many bioactive compounds and drugs (, )
- consists of a bicyclic structure composed of a ring fused to an ring
- Forms the basis for important biological molecules such as and (nucleic acid bases in DNA and RNA)
Key Terms to Review (31)
Adenine: Adenine is a purine-based nitrogenous base that is one of the four fundamental building blocks of nucleic acids, including DNA and RNA. It plays a crucial role in various biochemical processes and is closely associated with the topics of polycyclic aromatic compounds, carbohydrates, and nucleic acids.
Anthracene: Anthracene is a polycyclic aromatic hydrocarbon (PAH) consisting of three fused benzene rings. It is a crystalline solid with a characteristic blue fluorescence and is found in coal tar and certain fossil fuels. Anthracene is an important precursor in the synthesis of various organic compounds and has applications in the production of dyes, pharmaceuticals, and other materials.
Aromaticity: Aromaticity is a fundamental concept in organic chemistry that describes the unique stability and reactivity of certain cyclic compounds with delocalized pi electron systems. This term is central to understanding the structure, stability, and reactivity of a wide range of organic compounds, including benzene and other aromatic heterocycles.
Carcinogenicity: Carcinogenicity refers to the ability of a substance or agent to induce the formation of cancer or to promote the growth of cancerous cells. It is a critical concept in the context of understanding the potential health risks associated with various chemical compounds, particularly in the field of organic chemistry.
Conjugation: Conjugation refers to the overlap or sharing of atomic orbitals, resulting in the delocalization of electrons across a system of connected atoms. This concept is central to understanding resonance, the stability of certain molecules and ions, and the interpretation of various spectroscopic techniques in organic chemistry.
Delocalization: Delocalization refers to the dispersal or spreading out of electrons within a molecule, resulting in the stabilization of the overall structure. This concept is particularly important in understanding the behavior and properties of various organic compounds, including those involving resonance, aromatic systems, and conjugated pi systems.
Electrophilic aromatic substitution: Electrophilic aromatic substitution is a chemical reaction in which an atom, typically hydrogen, attached to an aromatic system, such as benzene, is replaced by an electrophile. This process preserves the aromaticity of the compound while introducing a functional group.
Electrophilic Aromatic Substitution: Electrophilic aromatic substitution is a fundamental organic reaction in which an electrophile (a species that is attracted to electrons) replaces a hydrogen atom on an aromatic ring, resulting in the formation of a new carbon-electrophile bond. This reaction is crucial in understanding the behavior and reactivity of aromatic compounds, which are prevalent in many organic molecules and have widespread applications.
Guanine: Guanine is a purine-based nitrogenous base that is one of the four primary components of nucleic acids, including DNA and RNA. It is a crucial element in the genetic makeup of all living organisms, playing a vital role in various biological processes.
Imidazole: Imidazole is a five-membered aromatic heterocyclic organic compound containing two nitrogen atoms. It is an important structural motif found in various biomolecules and is closely related to the topics of organic acids and bases, aromatic heterocycles, polycyclic aromatic compounds, protection of alcohols, basicity of amines, and heterocyclic amines.
Indole: Indole is a heterocyclic aromatic organic compound composed of a benzene ring fused to a five-membered nitrogen-containing pyrrole ring. It is a key structural component found in various biomolecules and plays a crucial role in the context of polycyclic aromatic compounds.
Isoquinoline: Isoquinoline is a heterocyclic aromatic organic compound that consists of a benzene ring fused to a pyridine ring. It is an important structural motif found in many natural and synthetic compounds, particularly in the context of polycyclic aromatic compounds.
LSD: LSD, or lysergic acid diethylamide, is a potent psychedelic drug that alters perception, thoughts, and feelings. It is classified as a polycyclic aromatic compound, a category of organic compounds with unique chemical structures and properties.
Morphine: Morphine is a naturally occurring opioid analgesic derived from the opium poppy plant. It is a powerful pain-relieving drug that has been widely used in medical and recreational contexts, with significant implications in the fields of chirality and polycyclic aromatic compounds.
Naphthalene: Naphthalene is a polycyclic aromatic hydrocarbon composed of two fused benzene rings. It is a white, crystalline solid with a distinctive mothball-like odor and is commonly used in the production of various chemicals and materials.
Planarity: Planarity refers to the flat or planar arrangement of atoms or molecules, where all the atoms lie in the same plane. This geometric property is particularly important in the context of aromatic compounds, as it contributes to their stability and unique electronic properties.
Polycyclic aromatic compound: Polycyclic aromatic compounds are organic molecules composed of two or more fused aromatic rings made of carbon and hydrogen atoms. These rings share a set of conjugated π-electrons that contribute to the compound's stability and unique chemical properties.
Polycyclic Aromatic Compounds: Polycyclic aromatic compounds (PACs) are a class of organic molecules characterized by the presence of two or more fused aromatic rings. These planar, conjugated structures exhibit unique chemical and physical properties that make them important in various scientific and industrial applications.
Purine: Purines are a class of heterocyclic aromatic organic compounds that consist of a pyrimidine ring fused to an imidazole ring. They are essential components of nucleic acids, such as DNA and RNA, and play crucial roles in various biological processes.
Pyrimidine: Pyrimidine is a heterocyclic aromatic organic compound containing two nitrogen atoms in the six-membered ring structure. It is a fundamental component of important biomolecules such as nucleic acids and certain vitamins, and is also found in various heterocyclic compounds.
Quinine: Quinine is a natural compound derived from the bark of the cinchona tree, which has been used for centuries to treat malaria. It is a polycyclic aromatic compound that possesses unique pharmacological properties, making it an important molecule in the context of 15.6 Polycyclic Aromatic Compounds.
Quinoline: Quinoline is a heterocyclic aromatic compound consisting of a benzene ring fused to a pyridine ring. It is an important structural motif found in various natural and synthetic compounds, with applications in organic chemistry, medicinal chemistry, and materials science.
Resonance: Resonance is a fundamental concept in organic chemistry that describes the ability of certain molecules to exist in multiple equivalent structures or resonance forms. This phenomenon arises from the delocalization of electrons within the molecule, leading to the stabilization of the overall structure and the distribution of electron density across multiple atoms.
Serotonin: Serotonin is a neurotransmitter that plays a crucial role in regulating various physiological and psychological processes in the body, including mood, sleep, appetite, and cognition. It is closely associated with the topics of polycyclic aromatic compounds and biological chemistry.
Sp2 Hybridization: sp2 Hybridization is a type of atomic orbital hybridization that occurs when an atom has three equivalent bonding partners, resulting in the formation of three $\sigma$ bonds and one $\pi$ bond. This hybridization pattern is commonly observed in molecules such as ethylene, benzene, and other planar organic compounds.
Sumatriptan: Sumatriptan is a medication primarily used to treat acute migraine headaches. It is a selective serotonin receptor agonist that acts on specific receptors in the brain to constrict blood vessels and block the transmission of pain signals, providing relief for migraine symptoms.
Tryptophan: Tryptophan is an essential amino acid that plays a crucial role in various biological processes, including the synthesis of proteins, the production of the neurotransmitter serotonin, and the regulation of the sleep-wake cycle. This versatile compound is particularly relevant in the context of polycyclic aromatic compounds, amino acid structures, and peptide sequencing.
UV-Vis Absorption Spectra: UV-Vis absorption spectra refer to the pattern of light absorption exhibited by a molecule or compound when exposed to ultraviolet (UV) and visible (Vis) wavelengths of the electromagnetic spectrum. This absorption profile provides valuable information about the electronic structure and transitions within the molecule, which is crucial for understanding the properties and behavior of polycyclic aromatic compounds.
α-position: The α-position refers to the carbon atom that is directly bonded to a functional group or other substituent in an organic compound. It is a crucial concept in the context of polycyclic aromatic compounds, as it helps to describe the spatial arrangement and reactivity of these complex molecular structures.
β-position: The β-position refers to the carbon atom that is two positions away from a functional group or point of attachment in a molecule. It is a crucial concept in the context of polycyclic aromatic compounds, as the β-position can significantly influence the reactivity and properties of these molecules.
π-Electrons: π-Electrons are a type of delocalized electrons that are found in planar, conjugated organic compounds. They are involved in the stabilization of aromatic structures and contribute to the unique chemical and physical properties of these compounds.