Organic Chemistry · Ethers and Epoxides; Thiols and Sulfides
Names and Properties of Ethers
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An ether R–O–R′: oxygen bonded to two carbon groups Full entry → is a compound with an oxygen atom bonded to two carbon groups: the general formula is R–O–R′ (or Ar–O–R for aromatic ethers). Because the oxygen has no hydrogen attached, ethers cannot hydrogen-bond to each other — but they can accept hydrogen bonds from molecules that have O–H or N–H groups. That single structural fact explains nearly every property: low boiling points, water solubility for small ethers, and their famous chemical inertness, which makes them the solvents of choice for organometallic chemistry.
The classic examples tell the story: dimethyl ether (CH₃OCH₃, a gas at room temperature, bp −24 °C) versus its isomer ethanol (bp 78 °C); diethyl ether (CH₃CH₂OCH₂CH₃, bp 34.6 °C) — the first general anesthetic — versus 1-butanol (bp 117.7 °C). Same molecular formulas, completely different boiling points, all because the alcohols can hydrogen-bond and the ethers cannot.
Why this matters
Ethers are the quiet workhorses of the lab:
- Solvents: THF Tetrahydrofuran, a five-membered cyclic ether Full entry →, diethyl ether, 1,4-dioxane Six-membered ring with two opposite oxygens Full entry →, and DME dissolve both polar and nonpolar compounds and are inert to most reagents — Grignard reagents, organolithiums, and hydride reductions are all run in ethers.
- Medicine: diethyl ether was the first surgical anesthetic (1846); modern inhaled anesthetics (sevoflurane, isoflurane, desflurane) are fluorinated ethers; many drugs contain ether linkages.
- Industry: MTBE Methyl tert-butyl ether, a fuel oxygenate Full entry → and related ethers are fuel additives; crown ethers (Topic 6) enable phase-transfer catalysis; PEG (polyethylene glycol) is a polyether used in drug delivery.
- Safety: ethers slowly form explosive peroxides on exposure to air and light — a real laboratory hazard. Knowing the properties of ethers is knowing how to handle them (general principles: store properly, test for peroxides, never distill to dryness).
The college version
Core Concepts
Naming ethers: two systems
Common (functional class) names — name each alkyl group attached to oxygen, then add the word "ether," listing groups alphabetically (or with "di-" for identical groups):
- CH₃OCH₃ = dimethyl ether
- CH₃CH₂OCH₂CH₃ = diethyl ether
- CH₃OCH₂CH₃ = ethyl methyl ether
- C₆H₅OCH₃ = methyl phenyl ether (anisole Methyl phenyl ether, C₆H₅OCH₃ Full entry →)
IUPAC (substitutive) names — treat the smaller/simpler group as an alkoxy substituent on the longer carbon chain:
- CH₃OCH₂CH₃ = methoxyethane
- CH₃CH₂OCH₂CH₂CH₃ = 1-ethoxypropane
- C₆H₅OCH₃ = methoxybenzene (anisole)
Cyclic ethers get their own names: tetrahydrofuran (THF, oxolane), 1,4-dioxane, ethylene oxide (oxirane, the epoxide — Topic 4). Crown ethers are named as x-crown-y (Topic 6).
Structure and polarity
The C–O–C bond angle in an ether is about 110° (bent, like water's). Each C–O bond is polar, but the two bond dipoles partially cancel; ethers are weakly polar (diethyl ether's dipole moment Net charge separation in a molecule Full entry → is ~1.2 D, much smaller than water's 1.85 D). The oxygen has two lone pairs, making ethers weak Lewis bases and good hydrogen-bond acceptors.
Boiling points: no O–H, no hydrogen bonding between molecules
Ethers cannot donate hydrogen bonds (no O–H), so their intermolecular forces are dispersion plus weak dipole–dipole. Consequences:
- An ether boils far below the isomeric alcohol (diethyl ether 34.6 °C vs. 1-butanol 117.7 °C).
- An ether boils close to an alkane of similar size and shape (diethyl ether 34.6 °C vs. pentane 36.1 °C).
Solubility: hydrogen-bond acceptance
Although ethers can't donate H-bonds, their oxygen lone pairs accept hydrogen bonds from water. Small ethers are therefore water-soluble: dimethyl ether and THF are fully miscible with water; diethyl ether dissolves to about 7 g per 100 mL of water. Solubility drops as the hydrocarbon portion grows.
Chemical properties: inertness and one big exception
Ethers are remarkably unreactive: stable to strong bases, nucleophiles, mild acids, and reducing agents (NaBH₄, LiAlH₄) — which is precisely why they're used as solvents. Two important exceptions:
- Strong acid cleavage: HBr or HI (strong acids with good nucleophiles) cleave ethers to give an alcohol plus an alkyl halide — the reaction of Topic 3.
- peroxide ROOR′ / ROOH formed by autoxidation Full entry → formation: on prolonged exposure to air and light, ethers (especially diethyl ether and THF) autoxidize to hydroperoxides, which are explosive when concentrated. General safety principles: buy ethers with inhibitors, store in tightly sealed containers away from light, test old ether for peroxides before use, and never distill an ether to dryness.
Everyday and industrial ethers
- Anesthetic ethers: diethyl ether (historic); sevoflurane, isoflurane, desflurane (modern, fluorinated).
- Solvents: THF, 1,4-dioxane, DME (1,2-dimethoxyethane, "glyme"), diglyme.
- Fuel additives: MTBE (methyl tert-butyl ether) and ETBE.
- Crown ethers & PEG: phase-transfer catalysts; polyether drug-delivery polymers.
How It Works / Step-by-Step Process
Naming an ether (IUPAC, step by step):
- Find the longest carbon chain — this is the parent alkane.
- Number the chain from the end nearest the –OR carbon.
- Name the –OR as an alkoxy substituent (methoxy, ethoxy, propoxy…), giving its position number.
- Assemble: position + alkoxy + parent name (e.g., 1-ethoxypropane for CH₃CH₂OCH₂CH₂CH₃).
- Cross-check with the common name (ethyl propyl ether) when asked.
Handling old ethers (general safety principle, not a lab procedure):
- Recognize the hazard: ethers stored long-term or exposed to air/light may contain hydroperoxides.
- Never distill an old ether to dryness — peroxides concentrate in the pot and can detonate.
- Use a peroxide test (e.g., potassium iodide/starch paper) before any distillation or evaporation; if positive, treat or discard appropriately.
- Store ethers in sealed containers, away from light, and use inhibitors where supplied.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Ether (R–O–R′) | Alcohol (R–O–H) | No O–H in an ether → no H-bond donation, far lower boiling point, no D₂O-exchangeable proton, no IR O–H stretch; same formula, totally different properties |
| Ether | Ester (R–CO–O–R′) | Ether is C–O–C; ester contains a carbonyl (C=O). Esters show a strong IR C=O stretch (~1735 cm⁻¹) and react with nucleophiles; ethers do not |
| "Ether" (colloquial) | Diethyl ether specifically | "Ether" as a class includes THF, dioxane, anisole, crown ethers…; the solvent "ether" usually means diethyl ether |
| Ethers are inert | Ethers never react | Strong acids (HBr/HI) cleave them, and air/light oxidize them to explosive peroxides — two very real reactions |
| Alkoxy naming | Alkyl naming | In IUPAC, the smaller group is the alkoxy substituent (methoxyethane), not a separate word; common names put both groups before "ether" |
| Anisole | Phenol | Anisole is an ether (C₆H₅OCH₃, no O–H): it is not acidic, gives no FeCl₃ test, and cannot hydrogen-bond donate — phenol does all three |
Eli explains
The same idea, in plain words
Explain it like I’m 10
An ether is like two kids holding an oxygen ball between them — the oxygen holds their hands but has no spare hydrogen of its own. Because ether molecules can't grab each other strongly, they're light and evaporate easily (low boiling point), but the oxygen can still hold hands with water, so small ethers mix with water. Ethers are calm and don't react with almost anything, which makes them perfect "neutral playgrounds" — solvents — where other reactions can happen safely. Just don't let old ether sit in sunlight: it slowly turns into an explosive.
Worked example
Example 1: Name these ethers both ways
(a) CH₃OCH₂CH₂CH₃. IUPAC: longest chain = propane; methoxy at C1 → 1-methoxypropane. Common: methyl propyl ether.
(b) CH₃CH₂OCH₂CH₃. IUPAC: ethoxy on ethane → ethoxyethane. Common: diethyl ether.
(c) C₆H₅OCH₂CH₃. IUPAC: ethoxybenzene. Common: ethyl phenyl ether. (Anisole is the methyl version.)
(d) The five-membered ring –CH₂CH₂OCH₂CH₂– (THF): tetrahydrofuran (oxolane).
Example 2: Boiling points — why is diethyl ether so low?
Problem: Rank these near-isomers by boiling point and explain: CH₃CH₂OCH₂CH₃ (74 g/mol), CH₃CH₂CH₂CH₂OH (74 g/mol), CH₃CH₂CH₂CH₂CH₃ (72 g/mol).
Data (standard values): pentane 36.1 °C; diethyl ether 34.6 °C; 1-butanol 117.7 °C.
Reasoning: The ether and the alkane have similar molar masses and similar dispersion forces; the ether adds only weak dipole–dipole forces, so its bp (34.6 °C) is close to pentane's (36.1 °C). The alcohol, with the same molar mass as the ether, can donate and accept hydrogen bonds, so its molecules stick together far more strongly — 1-butanol boils 83 °C higher. Rule of thumb: for similar molar mass, ROH ≫ ROR′ ≈ alkane.
Example 3: Water solubility of diethyl ether
Problem: Diethyl ether's solubility in water is about 7 g per 100 mL at room temperature. Express this as a mass fraction and as parts per thousand.
Formula first: mass fraction = msolutemsolute + msolvent
mass fraction = 7 g7 g + 100 g = 7107 ≈ 0.065
ppt = 0.065 × 1000 = 65 ppt
So roughly 6.5% of a saturated water layer is ether — enough that ether and water are visibly partially miscible (two layers, each saturated with the other), and enough that ether extracts many organic compounds from water.
Example 4: Choosing a solvent for a Grignard reaction
Problem: You need a solvent for phenylmagnesium bromide (PhMgBr) reacting with a ketone. Why are diethyl ether and THF good choices, and why is ethanol a terrible choice?
Answer: Ethers are inert to the strongly basic, strongly nucleophilic Grignard reagent (no acidic O–H to deprotonate it, no electrophilic site to attack), and they dissolve both the reagent and the alkoxide product. Ethanol has an O–H (pKa ≈ 16): the Grignard reagent would instantly deprotonate it, quenching the reagent and destroying the reaction. This is the same protection logic as Chapter 17 — no acidic protons allowed near organometallics.
Key takeaways
- Ether = R–O–R′; no O–H, so no hydrogen-bond donation — but the oxygen accepts H-bonds.
- Boiling points: ether < isomeric alcohol (diethyl ether 34.6 °C vs. 1-butanol 117.7 °C); ether ≈ alkane of similar size (pentane 36.1 °C).
- Water solubility from H-bond acceptance: small ethers (dimethyl ether, THF) fully miscible; diethyl ether ~7 g/100 mL.
- Naming: common (ethyl methyl ether) and IUPAC (methoxyethane) systems; cyclic ethers have special names (THF, dioxane, oxirane).
- Ethers are inert to bases, nucleophiles, mild acids, and reducing agents — the basis of their use as solvents for Grignard and hydride chemistry.
- Exception 1: strong acid (HBr/HI) cleaves ethers to alcohol + alkyl halide.
- Exception 2: air/light form explosive peroxides — handle old ethers with care (test, don't distill to dryness).
- Weakly polar (~1.2 D for diethyl ether), bent C–O–C (~110°).
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Name CH₃CH₂OCH₂CH₂CH₃ two ways (IUPAC and common).
Show answer
IUPAC: 1-ethoxypropane (longest chain = propane, ethoxy at C1). Common: ethyl propyl ether.
Why does diethyl ether (bp 34.6 °C) boil so much lower than its isomer 1-butanol (bp 117.7 °C)?
Show answer
1-Butanol has an O–H and can donate and accept hydrogen bonds with other 1-butanol molecules, giving strong intermolecular forces. Diethyl ether has no O–H, so it relies on dispersion and weak dipole–dipole forces — much weaker, hence a much lower boiling point.
If ethers have no O–H, why are small ethers like THF fully miscible with water?
Show answer
The ether oxygen's lone pairs accept hydrogen bonds from water's O–H. Hydrogen-bond acceptance needs no O–H of its own — water does the donating.
Give two reagents/conditions that ethers are stable toward, and two that they are NOT stable toward.
Show answer
Stable toward: strong bases (NaOH), nucleophiles, mild acids, reducing agents (NaBH₄, LiAlH₄), and organometallics. NOT stable toward: strong acids with good nucleophiles (HBr/HI — acidic cleavage) and air/light (peroxide formation).
What is the main safety hazard of aged ethers, and what general rule protects you?
Show answer
Peroxide formation: ethers autoxidize to hydroperoxides that are explosive when concentrated. General rules: store properly sealed and away from light, test for peroxides before use, and never distill an ether to dryness.
Why is ethanol unsuitable as a solvent for a Grignard reagent while THF is ideal?
Show answer
Ethanol's O–H is acidic enough (pKa ≈ 16) that the strongly basic Grignard reagent deprotonates it instantly, destroying the reagent. THF has no acidic protons and no electrophilic sites, so the Grignard survives and the reaction proceeds.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- ether
- R–O–R′: oxygen bonded to two carbon groups
- alkoxy group
- –OR group (e.g., –OCH₃ = methoxy)
- anisole
- Methyl phenyl ether, C₆H₅OCH₃
- THF
- Tetrahydrofuran, a five-membered cyclic ether
- 1,4-dioxane
- Six-membered ring with two opposite oxygens
- hydrogen-bond acceptor
- Molecule with lone pairs that accepts H-bonds from O–H/N–H
- dipole moment
- Net charge separation in a molecule
- inert solvent
- Solvent that doesn't react under the reaction conditions
- peroxide
- ROOR′ / ROOH formed by autoxidation
- MTBE
- Methyl tert-butyl ether, a fuel oxygenate
- crown ether
- Cyclic polyether that binds metal ions
Sources & references
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