Organic Chemistry · Ethers and Epoxides; Thiols and Sulfides
Crown Ethers
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In 30 seconds
Crown ethers are large macrocyclic polyethers — rings of repeating –CH2CH2O– units — whose oxygen lone pairs point inward, forming a cavity that can bind metal cations by ion–dipole interactions. The name comes from the shape: the ring crowns the cation like a jewel in a setting. The classic member is 18-crown-6 (18 atoms in the ring, 6 of them oxygen), which binds potassium ion (K+) so well that it can pull K+ out of aqueous solution into an organic solvent. Because a Crown ether Macrocyclic polyether ring that binds cations Full entry → wraps around a cation and hides its charge in a hydrophobic shell, it can drag otherwise insoluble inorganic salts into nonpolar solvents — the basis of Phase-transfer catalysis Carrying a reagent between phases using a carrier Full entry →, one of the most practical ideas in modern organic synthesis.
The chemistry is fundamentally about size matching and supramolecular recognition: the Cavity diameter Size of the hole in the crown ring Full entry → must fit the cation's ionic radius for the strongest binding. 12-crown-4 fits lithium, 15-crown-5 fits sodium, and 18-crown-6 fits potassium. Crown ethers launched the field of host–guest (supramolecular) chemistry; Charles Pedersen's 1967 discovery earned a share of the 1987 Nobel Prize in Chemistry.
Why this matters
Crown ethers matter in four arenas. Synthesis: they enable "naked anions" — by complexing the cation, the crown leaves the anion unsolvated and hyper-reactive, so KMnO4 dissolves in benzene ("purple benzene") and oxidizes alkenes there, and KF, KCN, and other salts become soluble, reactive reagents in organic media. Industry: phase-transfer conditions are used to make polymers, pharmaceuticals, and fine chemicals without expensive polar solvents. Biology: the principle of cation recognition by a ring of oxygen atoms is exactly how the antibiotic Valinomycin Natural cyclic ionophore that transports K⁺ Full entry → (a cyclic depsipeptide, structurally a crown-like ionophore) transports K+ across lipid membranes — and it is why crown ether toxicity exists (they can disturb ion balance in cells). Analytical chemistry and medicine: crown ethers and their relatives are used in ion-selective electrodes, in radiopharmaceutical delivery research, and in separations of radioactive cesium and strontium. For exams, the essential facts are the naming convention, the size-matching rule, and the phase-transfer mechanism.
The college version
Core Concepts
Structure and naming: n-crown-m
A crown ether is written as n-crown-m Naming: n ring atoms, m oxygens Full entry →, where:
- n = total number of atoms in the ring,
- m = number of oxygen atoms in the ring.
So 18-crown-6 has 18 ring atoms, of which 6 are oxygens (alternating –CH2CH2O– units); 15-crown-5 has 15 atoms, 5 oxygens; 12-crown-4 has 12 atoms, 4 oxygens. The ring carbons are CH2 groups, and every oxygen sits between two CH2 groups, so the repeat unit is –OCH2CH2–.
How crown ethers bind cations: ion–dipole recognition
The oxygen lone pairs point into the cavity, and a cation inside is stabilized by multiple ion–dipole interactions — up to six oxygens donating electron density to a K+ in 18-crown-6. Binding strength is greatest when the cavity diameter matches the cation diameter:
- 12-crown-4 cavity ≈ 1.2–1.5 Å → best for Li+ (ionic diameter ~1.5 Å).
- 15-crown-5 cavity ≈ 1.7–2.2 Å → best for Na+ (~2.0 Å).
- 18-crown-6 cavity ≈ 2.6–3.2 Å → best for K+ (~2.7 Å).
The match is not exclusive — smaller cations can sit above or below the plane of a larger ring — but the size match maximizes the binding constant. This is the same "lock and key" logic used to explain enzyme specificity, and it is the core idea of supramolecular chemistry.
Phase-transfer catalysis: the naked anion
A crown ether's exterior is hydrocarbon-like (the CH2 groups), so the whole cation–crown complex is soluble in nonpolar solvents. When 18-crown-6 is added to a two-phase mixture of aqueous KMnO4 and benzene, the crown extracts K+ into the benzene, carrying a permanganate anion with it as an ion pair — the benzene turns purple, and the permanganate (now unsolvated by water) becomes a much more reactive oxidant. The same trick dissolves KF (enabling aromatic fluorination, S_NAr), KCN (enabling nitrile formation), and NaOH in organic solvents. Industrially, quaternary ammonium salts ("phase-transfer catalysts") are often preferred over crowns because they are cheaper and less toxic, but the principle is the same: carry the reactive ion into the organic phase.
Chemistry and limitations of the ethers themselves
Crown ethers are ethers, so they share ether chemistry: inert to base and most nucleophiles, cleaved by hot concentrated HI/HBr (topic 3), and capable of forming peroxides on long exposure to air (the polyether chains make this a real concern). Practically, they are used in catalytic amounts when possible because they are expensive and toxic.
Biological relatives and toxicity
Valinomycin — a cyclic molecule of alternating amino acids and hydroxy acids — is the biological "crown ether": its six carbonyl oxygens bind K+ selectively and ferry it across lipid membranes, collapsing the ion gradients that mitochondria need. That mechanism is both the antibiotic's power and the source of crown-ether toxicity: synthetic crowns disturb cellular ion homeostasis. The general safety principle: handle crown ethers (especially 18-crown-6) with gloves, avoid ingestion/inhalation, and treat them as hazardous — they are far from benign solvents.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| n-crown-m meaning | 18-crown-6 vs 6-crown-18 | First number = total ring atoms (18); second = oxygens (6). The names are not interchangeable |
| Cavity size vs ring size | More atoms always = bigger cavity | Not always linear: 18-crown-6 has a bigger cavity than 15-crown-5, but flexibility means a cation can sit off-center; size match is empirical |
| Crown ethers vs cryptands | Flat ring vs 3-D bicyclic cage | Cryptands (Lehn) wrap cations in 3-D and bind much more strongly; crowns are 2-D rings |
| Phase-transfer catalyst vs surfactant | Crown vs soap-like molecule | Crowns carry cations (and thus anions) by complexation; surfactants emulsify phases without complexing ions |
| Crown ether vs acyclic polyether (PEG) | Ring vs open chain | Open-chain PEGs wrap cations weakly ("pseudocrown"); only the cyclic crown has a preorganized cavity |
| K⁺ selectivity vs Na⁺ selectivity | 18-crown-6 vs 15-crown-5 | 18-crown-6 prefers K⁺ (fit); 15-crown-5 prefers Na⁺. Using the wrong crown gives poor selectivity |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a ring of people holding hands, with their free hands pointing into the middle — that's a crown ether, and the hands are oxygen atoms that love positive charges. If a positively charged metal ball (like potassium) is exactly the right size for the hole in the middle, the ring grabs it tight and won't let go. The crown also wears a slippery, greasy coat on the outside, so it can smuggle the salty metal ball from water into oil — which is how you can get salt to dissolve in gasoline-like solvents and do chemistry there.
Worked example
Example 1: Choose the crown ether for a given cation
Problem. You need to extract sodium ion into dichloromethane. Which crown ether gives the strongest binding: 12-crown-4, 15-crown-5, or 18-crown-6?
Step 1 — cavity sizes. 12-crown-4 ≈ 1.2–1.5 Å, 15-crown-5 ≈ 1.7–2.2 Å, 18-crown-6 ≈ 2.6–3.2 Å.
Step 2 — cation size. Na+ has an ionic diameter of ~2.0 Å.
Step 3 — match. The 15-crown-5 cavity (1.7–2.2 Å) best matches Na+ (~2.0 Å).
Answer. 15-crown-5. (18-Crown-6 would still bind Na⁺ but with lower selectivity; 12-crown-4 is too small and prefers Li⁺.)
Example 2: Explain "purple benzene" and its use
Problem. A student adds 18-crown-6 to a flask of benzene over solid KMnO4 and stirs. The benzene turns purple. Explain what happened and how this enables alkene oxidation.
Step 1 — extraction. 18-Crown-6 complexes K+; the K⁺–crown complex (with a permanganate counterion) dissolves in the benzene, coloring it purple.
Step 2 — naked anion. In benzene the permanganate is not hydrogen-bonded by water, so it is far more electrophilic/nucleophilic — more reactive.
Step 3 — application. Alkenes in the benzene are oxidized (dihydroxylation or cleavage, depending on conditions) by the dissolved KMnO4, a reaction that otherwise would not happen in a nonpolar solvent with solid oxidant.
Answer. The crown transfers KMnO4 into the organic phase as a reactive ion pair, making a classic two-phase oxidation possible in one phase.
Example 3: Stoichiometry of crown-mediated fluorination
Problem. 1.00 g of 18-crown-6 (molar mass 264.32 g/mol) is used to complex K+ in a KF-mediated aromatic fluorination. How many moles of K+ can it carry at 1:1 crown:K⁺ stoichiometry, and how many grams of KF (molar mass 58.10 g/mol) correspond?
Step 1 — moles of crown.
1.00 g × 1 mol264.32 g = 3.78 × 10-3 mol crown
Step 2 — moles of K⁺ carried (1:1).
3.78 × 10-3 mol crown × 1 mol K+1 mol crown = 3.78 × 10-3 mol K+
Step 3 — mass of KF.
3.78 × 10-3 mol × 58.10 gmol = 0.220 g KF
Answer. 1.00 g of 18-crown-6 carries 3.78 × 10⁻³ mol of K⁺, enough to solubilize 0.220 g of KF — which is why crowns are used catalytically or in small amounts relative to the substrate.
Key takeaways
- Naming: n-crown-m = n ring atoms, m oxygens; 18-crown-6 = 18 atoms, 6 O.
- Binding is by ion–dipole interactions; size matching rules: 12-crown-4 → Li⁺, 15-crown-5 → Na⁺, 18-crown-6 → K⁺.
- Crowns make inorganic salts soluble in organic solvents (phase-transfer catalysis): KMnO₄ + 18-crown-6 in benzene = "purple benzene."
- The unsolvated ("naked") anion is hyper-reactive: KF → fluorination, KCN → nitriles.
- Valinomycin is the natural ionophore: K⁺ transport across membranes by the same oxygen-ring principle.
- Crown ethers are toxic (disturb ion balance) and can form peroxides — handle with gloves, avoid inhalation/ingestion.
- 18-crown-6 binds K⁺ ~10⁴–10⁶ times more strongly than Na⁺ in typical solvents (selectivity factor depends on solvent and medium — see notes).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What do the two numbers in "18-crown-6" mean?
Show answer
18 = total atoms in the ring; 6 = number of oxygens in the ring (so 6 –CH₂CH₂O– units).
Which crown ether would you choose to bind Li⁺, and why?
Show answer
12-crown-4, because its ~1.2–1.5 Å cavity best matches Li⁺'s small ionic diameter (~1.5 Å) — the size-match principle.
Explain in one sentence how a crown ether makes KCN react with an alkyl halide in toluene.
Show answer
The crown complexes K⁺, making the KCN ion pair soluble in the toluene, where the "naked" cyanide is a strong enough nucleophile to displace the halide (S_N2).
Why is the anion called "naked," and why does that make it more reactive?
Show answer
The cation is wrapped by the crown, so the anion is left without a solvation shell or counterion contact — its reactivity is no longer damped by solvent interactions, making it far more nucleophilic/basic.
How is valinomycin like a crown ether, and what biological process does it disrupt?
Show answer
Valinomycin is a cyclic molecule whose carbonyl oxygens form a K⁺-selective cavity — a biological crown. Transporting K⁺ across mitochondrial membranes collapses the proton/ion gradients used for ATP synthesis, uncoupling oxidative phosphorylation.
2.00 g of 15-crown-5 (molar mass 220.26 g/mol) complexes Na+ 1:1. How many grams of NaCl (58.44 g/mol) could be solubilized stoichiometrically?
Show answer
2.00 g × (1 mol / 220.26 g) = 9.08 × 10⁻³ mol crown = 9.08 × 10⁻³ mol Na⁺ = 9.08 × 10⁻³ × 58.44 g/mol = 0.531 g NaCl.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Crown ether
- Macrocyclic polyether ring that binds cations
- n-crown-m
- Naming: n ring atoms, m oxygens
- Ion–dipole interaction
- Attraction between a charge and a dipole (O lone pair)
- Cavity diameter
- Size of the hole in the crown ring
- Phase-transfer catalysis
- Carrying a reagent between phases using a carrier
- Naked anion
- Anion with no solvation shell (cation complexed away)
- Valinomycin
- Natural cyclic ionophore that transports K⁺
- Host–guest chemistry
- Study of molecules binding other molecules in cavities
Sources & references
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