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Organic Chemistry
Table of Contents
Unit 18 Overview: Ethers, Epoxides, Thiols, and Sulfides
18.1 Names and Properties of Ethers
18.2 Preparing Ethers
18.3 Reactions of Ethers: Acidic Cleavage
18.4 Cyclic Ethers: Epoxides
18.5 Reactions of Epoxides: Ring-Opening
18.6 Crown Ethers
18.7 Thiols and Sulfides
18.8 Spectroscopy of Ethers

Crown ethers are cyclic polyethers that can bind metal cations in their central cavity. These molecules have a polar interior that attracts positively charged ions, while their non-polar exterior allows the complex to dissolve in organic solvents.

Crown ethers enhance reactivity in SN2 reactions by sequestering metal cations, leaving anions more "naked" and reactive. They're similar to polar aprotic solvents in this regard, but work through a different mechanism of cation binding.

Crown Ethers

Cation sequestration by crown ethers

  • Crown ethers are cyclic polyethers that can bind metal cations within their central cavity
    • Oxygen atoms in the ring orient inward, creating a polar interior that attracts positively charged ions (Na+, K+)
    • Exterior of the ring is non-polar, allowing the complex to dissolve in organic solvents (benzene, chloroform)
  • Size of the central cavity determines which metal cations can be sequestered
    • For optimal binding, cavity size should match ionic radius of the metal cation
    • 18-crown-6 has a cavity size that matches the ionic radius of potassium ($K^+$)
    • 15-crown-5 has a cavity size that matches the ionic radius of sodium ($Na^+$)
  • Crown ethers can increase solubility of salts in organic solvents by forming complexes with metal cations
    • Crown ether-metal complex is soluble in organic solvents due to non-polar exterior of the ring
    • Allows salt to dissolve in organic solvent, which would otherwise be insoluble
    • Potassium fluoride (KF) becomes soluble in benzene when 18-crown-6 is added
  • The binding of metal cations by crown ethers is an example of chelation, where multiple donor atoms coordinate to a central metal ion

Crown ethers vs polar aprotic solvents

  • Both crown ethers and polar aprotic solvents can increase reactivity of anions in SN2 reactions
  • Crown ethers enhance SN2 reactivity by sequestering metal cation, leaving anion more "naked" and reactive
    • Crown ether binds metal cation, preventing ion pairing and increasing anion's nucleophilicity
    • Leads to faster SN2 reaction rates
    • 18-crown-6 can increase reactivity of potassium acetate ($CH_3CO_2K$) in SN2 reactions
  • Polar aprotic solvents enhance SN2 reactivity by solvating cation and leaving anion more reactive
    • Solvents (DMSO, DMF) have high dielectric constants and can stabilize charged species
    • Solvate cation more effectively than anion, increasing anion's nucleophilicity
    • DMSO can increase reactivity of sodium azide ($NaN_3$) in SN2 reactions
  • Crown ethers and polar aprotic solvents can be used together for even greater increase in SN2 reactivity
    • Crown ether sequesters metal cation while polar aprotic solvent solvates remaining ions

Structure and nomenclature of crown ethers

  • Crown ethers are cyclic molecules composed of repeating ethylene oxide ($-CH_2CH_2O-$) units
    • Oxygen atoms are linked by two-carbon bridges, forming a ring structure
    • Number of oxygen atoms in the ring can vary, typically ranging from 4 to 20
  • Naming convention for crown ethers follows format: x-crown-y
    1. "x" represents total number of atoms in the ring (including carbon and oxygen)
    2. "y" represents number of oxygen atoms in the ring
    • 18-crown-6 has 18 atoms in the ring, with 6 of them being oxygen atoms
    • 15-crown-5 has 15 atoms in the ring, with 5 of them being oxygen atoms
  • Crown ethers are synthetic analogs of naturally occurring ionophores, such as valinomycin
    • Ionophores are molecules that can transport ions across cell membranes
    • Like crown ethers, ionophores have cyclic structure with oxygen atoms that can bind metal cations
    • Valinomycin, a naturally occurring ionophore, has similar structure and function to 18-crown-6, selectively binding potassium ions
  • Crown ethers are examples of macrocycles, large ring-shaped molecules with at least 12 atoms in the ring
  • Crown ethers participate in host-guest chemistry, where the crown ether (host) selectively binds a metal cation (guest)
  • The process of a crown ether binding a metal cation is called complexation
  • Cryptands are related compounds to crown ethers, featuring a three-dimensional cage-like structure for even stronger metal cation binding

Key Terms to Review (26)

Crown ethers: Crown ethers are cyclic organic compounds that consist of several ether groups (typically -O- units) linked together in a ring. They are known for their ability to selectively bind certain metal ions, such as potassium and sodium, within their central cavity.
Ethylene Oxide: Ethylene oxide is a colorless, flammable gas that is widely used in industrial and medical applications. It is a cyclic ether with the chemical formula C₂H₄O, and it serves as a key intermediate in the production of various chemicals and materials.
Nucleophilicity: Nucleophilicity refers to the ability of a species to donate electrons and form a covalent bond with an electrophilic center. It is a key concept in organic chemistry that governs the reactivity and selectivity of many important reactions, including substitution, addition, and elimination reactions.
DMF: DMF, or dimethylformamide, is a versatile organic solvent that has applications in various chemical reactions and processes. It is a polar aprotic solvent that is widely used in organic chemistry, particularly in the context of nucleophilic substitution reactions, nucleophilic aromatic substitution, and the preparation of ethers and crown ethers.
Polar Aprotic Solvents: Polar aprotic solvents are a class of organic solvents that are polar in nature but do not contain any hydrogen atoms bonded to highly electronegative atoms, such as oxygen or nitrogen. These solvents are widely used in organic chemistry due to their unique properties and ability to facilitate certain chemical reactions.
SN2 Reactions: SN2 reactions, or bimolecular nucleophilic substitution reactions, are a type of organic reaction where a nucleophile attacks the backside of a carbon atom bearing a good leaving group, resulting in the inversion of stereochemistry at that carbon center.
Crown Ethers: Crown ethers are a class of cyclic polyether compounds that have the ability to bind and transport certain metal cations. They are named for their resemblance to a crown when viewed from the side, and their unique structure allows them to act as hosts for guest molecules, making them useful in various applications.
18-crown-6: 18-crown-6 is a macrocyclic polyether compound composed of six oxygen atoms arranged in a ring structure with a total of 18 atoms in the ring. It is a type of crown ether, a class of cyclic compounds that are known for their ability to form stable complexes with metal cations.
Non-Polar Exterior: The non-polar exterior refers to the outer layer or surface of a molecule or compound that lacks significant electrical charge separation, resulting in an overall neutral or balanced distribution of electrons. This characteristic is particularly relevant in the context of crown ethers, which are cyclic polyether compounds.
Cryptands: Cryptands are a class of macrocyclic organic compounds that are capable of encapsulating and complexing with various cations, forming stable host-guest complexes. They are structurally similar to crown ethers but have a more rigid and three-dimensional structure, allowing them to selectively bind and transport specific ions.
Cyclic Polyethers: Cyclic polyethers are a class of organic compounds consisting of a cyclic structure with multiple ether linkages. These compounds are known for their ability to form stable complexes with metal cations, making them useful in various applications such as ion transport, catalysis, and molecular recognition.
Sodium: Sodium is a highly reactive alkali metal that plays a crucial role in various biological processes within the human body. It is an essential mineral that helps maintain fluid balance, nerve function, and muscle contraction.
Phase-Transfer Catalysis: Phase-transfer catalysis is a technique used to facilitate reactions between organic and inorganic reactants that are present in different immiscible phases, such as an organic solvent and an aqueous solution. It involves the use of a catalyst that can transport the inorganic reactant from the aqueous phase to the organic phase, where the reaction can occur more efficiently.
Chelation: Chelation is a process in which a metal ion forms multiple bonds with a ligand, creating a stable, cyclic structure. This phenomenon is particularly relevant in the context of crown ethers, a class of cyclic polyethers known for their ability to selectively bind and transport specific metal cations.
Macrocycles: Macrocycles are cyclic organic compounds containing a large number of atoms, typically 12 or more, in the ring structure. They are an important class of molecules with diverse applications in fields such as supramolecular chemistry, drug design, and molecular recognition.
15-crown-5: 15-crown-5 is a type of cyclic polyether compound, also known as a crown ether, that is characterized by a 15-membered ring structure with 5 oxygen atoms evenly spaced around the ring. These crown ethers have a unique ability to selectively bind and transport certain metal cations.
Polar Interior: The polar interior refers to the central region of a crown ether molecule, which is characterized by a high electron density and the ability to coordinate with cations or other polar species. This unique structural feature allows crown ethers to selectively bind and transport specific ions or molecules.
Cavity Size: Cavity size refers to the internal dimensions of a crown ether molecule, which determines its ability to effectively bind and encapsulate specific cations or molecules. The size of the cavity is a critical factor in the selectivity and binding affinity of crown ethers.
Valinomycin: Valinomycin is a cyclic depsipeptide that functions as a potassium-selective ionophore, allowing the transport of potassium ions across cell membranes. It is known for its ability to disrupt the electrochemical gradient and influence various cellular processes.
Ionophores: Ionophores are organic compounds that can transport specific ions across biological membranes, facilitating the movement of ions down their concentration gradients. They play a crucial role in various biological processes and have applications in the field of chemistry, particularly in the context of crown ethers.
Complexation: Complexation is the process by which a central metal ion or atom forms a complex with one or more surrounding molecules or ions, known as ligands. This interaction creates a stable, coordinated structure that has unique chemical and physical properties.
Cation Sequestration: Cation sequestration refers to the process of selectively binding and removing positively charged ions, or cations, from a solution or environment. This is a crucial concept in the context of crown ethers, which are cyclic polyether compounds capable of encapsulating and trapping specific cations.
Ionic Radius: Ionic radius is the measure of the size of an ion, which is an atom or molecule that has gained or lost one or more valence electrons, resulting in a net positive or negative charge. This property is crucial in understanding the behavior and interactions of ions, particularly in the context of 18.6 Crown Ethers.
Host-Guest Chemistry: Host-guest chemistry refers to the study of the interactions and complexation between a host molecule and a guest molecule. The host molecule, typically a larger and more complex structure, has a binding site or cavity that can accommodate and interact with a smaller guest molecule, forming a stable host-guest complex.
Potassium: Potassium is a chemical element that is essential for the proper functioning of cells, tissues, and organs in the body. It plays a crucial role in maintaining fluid balance, nerve function, and muscle contraction, making it a key component in the context of crown ethers.
DMSO: DMSO, or dimethyl sulfoxide, is a highly polar organic solvent known for its ability to dissolve both polar and nonpolar compounds. Its unique properties make it a valuable reagent in various chemical reactions, particularly in nucleophilic substitution processes, where it enhances the solubility of reactants and facilitates the formation of intermediates.