4.10: Spectroscopy of Ethers and Epoxides
After completing this section, you should be able to
- use the 1 H NMR spectrum of an unknown ether or epoxide to determine its identity.
- identify the approximate chemical shift expected for protons attached to the carbon atoms that are bonded to oxygen in an ether or an epoxide.
Infrared Spectroscopy
Ethers are difficult to identify by IR spectroscopy. Although they show an absorption due to C–O single-bond stretching in the range 1050 to 1150 cm –1 , many other kinds of absorptions occur in the same range. Figure \(\PageIndex{1}\) shows the IR spectrum of diethyl ether and identifies the C–O stretch. Phenyl alkyl ethers show two strong absorbances for C–O stretching at 1050 and 1250 cm –1 . Figure \(\PageIndex{2}\) shows the IR spectrum of anisole.
Nuclear Magnetic Resonance Spectroscopy
1 H NMR Spectroscopy
Ethers
- Hydrogens on carbon adjacent to the ether show up in the region of 3.4-4.5 ppm.
The 1 H NMR spectrum of dipropyl ether, Figure \(\PageIndex{3}\), shows three signals with the triplet at 3.37 ppm (B) assigned to the -CH 2 - beside the ether and the other two signals upfield (1.59 and 0.93 ppm). Notice the protons closer to the electron-withdrawing oxygen atom are further downfield, indicating some deshielding. Protons at (A) and (C) are each coupled to two equivalent (B) protons. So, each of these signals appears as a triplet. The (B) protons in turn are coupled to a set of two and three equivalent protons and appears as a sextet.
Dipropyl ether
Epoxides
- Peaks in epoxides are shifted to a slightly higher field than other ethers. Hydrogens on carbons in and epoxide appear in the region of 2.5 to 3.5 ppm.
Epoxides absorb at a slightly higher field than other ethers and show characteristic resonances at 2.5 to 3.5 δ in their 1 H NMR spectra, as indicated for 1,2-epoxypropane ( 2-Methyloxirane ) in Figure \(\PageIndex{4}\). The methylene protons of this epoxide are diastereotopic, and display complex splitting (Review NMR Chapter).
2-Methyloxirane
The methylene protons of this epoxide are diastereotopic and appear as two separate peak. When looking at the 3D structure of 2-methyloxirane it is clear that each methylene hydrogen is distinctly different. Also, hydrogens attached to the carbons in eposxides tend to display complex splitting patterns .
A 3D Interactive Model of 2-Methyloxirane
Thiols
- Hydrogens on carbons adjacent to the sulfur in sulfides and thiols appear in the region of 2.0 to 2.5 ppm.
- The SH hydrogen of a thiol typically appears in the region of 1.3-1.5 ppm.
As Sulfur is not as electronegative as Oxygen, the SH proton is actively involved in splitting in this 1 H NMR spectra. In Ethanethiol, Figure \(\PageIndex{5}\), the SH proton peak at 1.39 ppm is split into a triplet, and the methylene peak at 2.55 ppm is split into a quintet.
The 1 H NMR spectrum shown is that of a cyclic ether with the formula C 4 H 8 O. Propose a structure.
- Answer
-
1,2-Epoxybutane
13 C NMR Spectra
Ethers
Ether carbon atoms also exhibit a downfield shift in the 13 C NMR spectrum, where they usually absorb in the 50 to 80 δ range. For example, the carbon atoms next to oxygen in methyl propyl ether absorb at 58.5 and 74.8 δ . Similarly, the methyl carbon in anisole absorbs at 54.8 δ .
Epoxides
Carbons that are part of the epoxide appear in the 40-60 ppm region. For example, in Figure \(\PageIndex{6}\), the carbon atoms next to oxygen in 2-Methyloxirane absorb at 48.2 δ for CH, and 47.9 δ . for CH 2 .
Thiols
As Sulfur is not as electronegative as Oxygen, carbon adjacent to the sulfur in sulfides and thiols appear in the region of 20-40 ppm. For example, in Figure \(\PageIndex{7}\), the carbon atoms next to sulfur in ethanethiol absorb at 19.71 and 19.20 δ .
The 1 C NMR spectrum shown is that of a linear ether with the formula C 6 H 14 O. Propose a structure.
- Answer
-
Dipropyl ether. Source: SDBSWeb : http://sdbs.db.aist.go.jp
Mass Spectra
Ethers, sulfides, and epoxides all have similar fragmentation patterns in mass spectra with a few subtle variations.
Fragmentation Patterns for Ethers
- The M + is may be weak or absent, but usually stronger than the corresponding alcohol.
- They primarily undergo alpha-cleavage to produce [H 2 C=O-R] +
- They can also undergo an inductive cleavage to produce R +
- Rearrangement with loss of CHR=CHR’. This fragmentation happens after an alpha-cleavage.
alpha-cleavage
inductive cleavage
rearrangement
For Example, the primary fragmentation pattern for ethyl propyl ether (M + = 88) in mass spectrometry involves:
- Alpha-cleavage at the carbon-carbon bond adjacent to the oxygen atom , leading to the formation of prominent fragment ion m/z = 59, corresponding to the loss of an ethyl radical (m = 29). The fragment m/z = 73, corresponding to the loss of a methyl radical (m= 43), is not as prominent as the previous one because the methyl radical is not as stable as larger ones.
- Inductive cleavage, the fragment ion m/z = 29 corresponds to the ethyl cation, and the fragment m/z = 43 corresponds to the propyl cation.
- Rearrangement of fragment ion m/z = 59 leads to the formation of prominent fragment ion m/z = 31, corresponding to the loss of the alkene ethene (m = 28).
Epoxides
- The M + is typically weak or absent
- They primarily undergo alpha-cleavage to produce an alkyl radical
Sulfides
- The M + is typically stronger than the corresponding ether
- They primarily undergo alpha-cleavage to produce an alkyl radical
- They can also undergo an inductive cleavage to produce R +