Organic Chemistry · Ethers and Epoxides; Thiols and Sulfides
Spectroscopy of Ethers
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In 30 seconds
Ethers are the quiet functional group of spectroscopy: strong infrared signals in a busy fingerprint region, ordinary-looking NMR peaks. The key to identifying an ether is a pattern of absences and shifts more than a single "ether peak". In the infrared, every ether shows a strong C–O stretch An IR absorption near 1050–1150 cm⁻¹ from the vibrating C–O bond Full entry → near 1050–1150 cm⁻¹ (aryl alkyl ethers near 1240–1260 cm⁻¹), but the decisive evidence is the absence of a broad O–H stretch A broad IR absorption at 3200–3600 cm⁻¹ from an alcohol/phenol O–H Full entry →. In 1H NMR, protons on carbons attached to oxygen appear at about 3.2–4.0 ppm, and the oxygen-bearing carbons appear at roughly 55–75 ppm in 13C NMR — well downfield of ordinary alkyl carbons. This topic builds the interpretive workflow: IR to rule out alcohols, NMR shifts and integration to locate the O–C bonds, mass spectrometry's α-Cleavage Fragmentation of the C–C bond adjacent to the oxygen-bearing carbon Full entry → fragments to confirm the structure.
Why this matters
Spectroscopy is how chemists identify the products of every reaction in this book. After any reaction meant to make an ether, the first question is "did I make it?" IR answers in minutes (new C–O, no O–H); NMR confirms quantitatively. Ethers are ubiquitous in polymers (PEG), anesthetics (fluorinated ethers like sevoflurane), and solvents (THF, dioxane), so recognizing their signature is a day-one skill. On exams, the most tested contrast is ether vs. alcohol, and the answer usually hangs on the O–H region of the IR spectrum.
The college version
Core Concepts
Infrared spectroscopy: the C–O stretch and the missing O–H
The C–O bond stretches strongly because its dipole changes dramatically during vibration. Aliphatic ethers absorb near 1050–1150 cm⁻¹ (diethyl ether ≈ 1120 cm⁻¹); aryl alkyl ethers such as anisole (C6H5OCH3) shift to ≈ 1240–1260 cm⁻¹ (partial double-bond character in the aryl C–O bond). Two companion observations close it:
- No broad O–H band at 3200–3600 cm⁻¹: this separates an ether from an isomeric alcohol — ethanol and dimethyl ether share the formula C2H6O, but only ethanol shows the broad O–H stretch.
- C–H stretches at 2850–3000 cm⁻¹ plus C–O confirm a saturated oxygen compound with no O–H and no C=O (nothing near 1700 cm⁻¹).
Epoxides are the exception: ring strain changes the C–O vibrations, adding bands near 1250 and 850–950 cm⁻¹.
1H NMR: protons next to oxygen
Oxygen deshields nearby protons: O–CH3 appears at about 3.2–3.4 ppm (methyl tert-butyl ether: 3.2 ppm singlet), O–CH2 at 3.4–3.7 ppm (diethyl ether: 3.4 ppm quartet), and O–CH at 3.5–4.0 ppm. Diethyl ether's full spectrum is a teaching classic: a 6H triplet near 1.2 ppm and a 4H quartet near 3.4 ppm. Epoxide ring protons are the oddity: they appear upfield at 2.5–3.0 ppm because ring strain leaves the C–H bonds with less s-character, counteracting oxygen's Deshielding Oxygen pulls electron density from nearby protons, moving their NMR signals downfield Full entry →. The absence of an exchangeable O–H proton (variable 0.5–5 ppm, gone after a D2O shake) again distinguishes ethers from alcohols.
13C NMR: the oxygen-bearing carbon
The carbon directly bonded to oxygen shifts downfield ~40 ppm versus an alkane: O–CH3 near 55–60 ppm (anisole: 55), O–CH2 near 60–75 ppm (diethyl ether: 66, THF: 68), epoxide carbons near 45–60 ppm. DEPT-135 A 13C NMR experiment distinguishing CH, CH2, CH3 carbons Full entry → confirms proton counts: O–CH2 appears negative while O–CH and O–CH3 stay positive — a quick hydrogen count on the oxygen-bearing carbon.
Mass spectrometry: α-cleavage next to oxygen
The radical cation fragments by breaking the C–C bond α to oxygen, because the resulting cation is stabilized by oxygen's lone pairs — an Oxonium ion A positively charged oxygen cation, e.g., CH3OCH2+ Full entry →. Methyl ethers give CH3OCH2+ at m/z = 45; diethyl ether (M⁺ = 74) loses a methyl radical to give CH3CH2O=CH2+ at m/z = 59.
Interpretive workflow
- Formula first. Compute the degrees of unsaturation: DBE = (2C + 2 + N - H - X)/2. A saturated acyclic ether gives DBE = 0.
- IR. Find C–O (1050–1150 cm⁻¹); confirm no O–H (3200–3600 cm⁻¹), no C=O (~1700 cm⁻¹).
- 1H NMR. Find signals at 3.2–4.0 ppm; use integration and splitting to count O–CHn groups.
- 13C NMR. Confirm carbons at 55–75 ppm.
- MS. Check for the α-cleavage oxonium fragment.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Ether (no O–H) | Alcohol (has O–H) | IR: broad 3200–3600 cm⁻¹ band only for the alcohol; NMR: exchangeable OH proton vanishes on D2O shake |
| C–O stretch ≈ 1100 cm⁻¹ | C=O stretch ≈ 1700 cm⁻¹ | Carbonyls absorb much higher (double bond, stronger force constant); "C–O" and "C=O" are not interchangeable exam answers |
| O–CH2 protons at 3.4 ppm | CH2 next to a halogen at 3.4 ppm | Same shift region — use the full data set (MS, integration, 13C), not one peak |
| Epoxide ring protons (2.5–3.0 ppm) | "Ether protons always ≥ 3.2 ppm" | Ring strain shifts epoxide protons upfield — an exception to memorize |
| α-cleavage fragment m/z 45 | Molecular ion of a small molecule | m/z 45 is a fragment, not necessarily the parent peak |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Ethers are like quiet students — you notice them by what they don't do. They never "shout" an O–H band in the infrared, so a missing broad signal tells you they're not alcohols. Their voice is a C–O stretch near 1100 cm⁻¹, and protons next to oxygen speak at 3–4 ppm in NMR. Put the clues together and you can name the molecule without seeing it.
Worked example
Example 1: Two isomers, one formula — ethanol vs dimethyl ether
Problem. A compound has the formula C2H6O. Its IR shows a strong band at 1120 cm⁻¹ and no broad band at 3200–3600 cm⁻¹. Its 1H NMR shows only two singlets: 3H at 3.2 ppm and 9H at 1.2 ppm. Identify it.
Step 1 — formula. Degrees of unsaturation:
DBE = 2(2) + 2 - 62 = 0
No rings or π bonds — consistent with an ether or an alcohol.
Step 2 — IR. A C–O stretch at 1120 cm⁻¹ with no O–H band rules out ethanol — an ether.
Step 3 — NMR. 3H at 3.2 ppm = one O–CH3; 9H at 1.2 ppm = three equivalent CH3 groups on a quaternary carbon. The structure is methyl tert-butyl ether, CH3OC(CH3)3.
Example 2: Assigning a full spectrum — diethyl ether
Problem. A colorless liquid, C4H10O, shows IR bands at 2970, 2870, and 1120 cm⁻¹ (none at 3200–3600 or 1650–1750 cm⁻¹). 1H NMR: 1.2 ppm (t, 6H), 3.4 ppm (q, 4H). 13C NMR: 15 and 66 ppm.
Step 1 — DBE.
DBE = 2(4) + 2 - 102 = 0
Step 2 — IR. C–O at 1120 cm⁻¹, no O–H, no C=O → saturated ether.
Step 3 — NMR. A 6H triplet + 4H quartet is the classic ethyl-on-oxygen pattern: two equivalent CH3 groups coupled to two equivalent CH2 groups. The 13C signal at 66 ppm is the O–CH2 carbon; 15 ppm is CH3. Structure: diethyl ether, CH3CH2OCH2CH3.
Key takeaways
- Ether IR signature: strong C–O stretch 1050–1150 cm⁻¹ (aryl alkyl ≈ 1240–1260 cm⁻¹); key negative evidence: no broad O–H band at 3200–3600 cm⁻¹.
- 1H NMR: O–CH3 ≈ 3.2–3.4 ppm; O–CH2 ≈ 3.4–3.7 ppm; O–CH ≈ 3.5–4.0 ppm; epoxide protons anomalously upfield (≈ 2.5–3.0 ppm).
- 13C NMR: O–C carbons ≈ 55–75 ppm (anisole OCH3 = 55; diethyl ether CH2 = 66).
- MS: α-cleavage gives oxonium ions — m/z 45 (CH3OCH2+) for methyl ethers.
- Isomer test: ethanol vs dimethyl ether differ by IR (O–H band) and by NMR (OH proton, lost on D2O shake, vs two singlets).
- Saturated ethers give DBE = 0.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
Two bottles are labeled only C2H6O: one ethanol, one dimethyl ether. Which single IR observation tells them apart?
Show answer
The O–H stretch: ethanol shows a broad absorption at 3200–3600 cm⁻¹; dimethyl ether none. (Both show C–O near 1100 cm⁻¹.)
Where do O–CH3 protons appear in 1H NMR, and why are they downfield of ordinary CH3 protons?
Show answer
About 3.2–3.4 ppm. Oxygen is electronegative, deshielding the protons and moving their resonance downfield relative to alkyl protons (≈ 0.9–1.5 ppm).
Diethyl ether's 1H NMR shows a triplet at 1.2 ppm and a quartet at 3.4 ppm. What do the splitting patterns tell you about the molecule?
Show answer
The 6H triplet and 4H quartet show two equivalent CH3 groups coupled to two equivalent CH2 groups — the ethyl–O–ethyl pattern.
What fragment at what m/z is diagnostic for methyl ethers, and why is it stable?
Show answer
CH3OCH2+ at m/z = 45. Oxygen's lone pairs delocalize the positive charge (oxonium ion), so α-cleavage is favored.
A compound's 13C NMR shows carbons at 66 and 15 ppm. Which is attached to oxygen, and why?
Show answer
The 66 ppm carbon. Oxygen deshields the directly bonded carbon ~40 ppm relative to an alkane CH2; 15 ppm is an ordinary CH3.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- C–O stretch
- An IR absorption near 1050–1150 cm⁻¹ from the vibrating C–O bond
- O–H stretch
- A broad IR absorption at 3200–3600 cm⁻¹ from an alcohol/phenol O–H
- Deshielding
- Oxygen pulls electron density from nearby protons, moving their NMR signals downfield
- Oxonium ion
- A positively charged oxygen cation, e.g., CH3OCH2+
- α-Cleavage
- Fragmentation of the C–C bond adjacent to the oxygen-bearing carbon
- DEPT-135
- A 13C NMR experiment distinguishing CH, CH2, CH3 carbons
Sources & references
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