Spectroscopy of Ethers

Ethers arc difficult to identify by IR spectroscopy. Although tney show an absorption due to C—O single-bond stretching in the range 1050 to 1150 cm , many other kinds of absorptions occur in the same range. I igure 18.3 shows the IR spectrum of diethyl ether and ideiilifies the C—O stretch. 

Hydrogens on carbon next to an ether oxygen are shifted downfield from the normal alkane resonance and show H NMR absorptions in the region 3.4 to 4.5 S. This dowmfield shift is clearly seen in the spectrum of dipropyl ether 

Ether carbon atoms also exhibit a downfield shift in the NMR spectrum, where they usually absorb in the 50 to 80 5 range. 1-or example, the carbon atoms next to oxygen in methyl propyl ether absorb at 58.5 and 74.8 5. Similarly, the methyl carbon in anisolc absorbs at 54.8 8. 

Problem 18.18 The NMR spectrum shown is that of an ether with the tormula C4lTfjO. Propose 

I cw nonchemists know exactly what an epoxide is, but practically everyone has used an epoxy glue for householo repairs or an epoxy resin for a protective coating. I poxy resins and adhesives generally consist of two compone its that are mixed just prior to use. One comjwnent is a liquid prepolymer. and the second is a curing agent that reacts with the prepolymer and causes it to solidify. 

Kayaks are often made of a high-strength polymer coated with epoxy resin. 

2-Butene-l-thiol is one component of skunk spray. How would you synthesize this substance from methyl 2-butenoate From 1,3-butadiene  

The importance of crown ethers derives from their extraordinary ability to solvate metal cations by sequestering the metal in the center of the polyether cavity. For example, 18-crown-6 complexes strongly with potassium ion. 

Complexes between crown ethers and ionic salts are soluble in nonpolar organic solvents, thus allowing many reactions to be carried out under aprotic conditions that would otherwise have to be carried out in aqueous solution. For example, the inorganic compound KMn04 actually dissolves in benzene in the presence of 18-crown-6. The resulting solution of purple benzene is a valuable reagent for oxidizing alkenes. 

Charles John Pedersen (1904-1989) was born in Pusan, Korea, to a Korean mother and Norwegian father. A U.S. citizen, he moved to the United States in the early 1920s and received an M.Sc. at the Massachusetts Institute of Technology in 1927. He spent his entire scientific career at the Du Pont Company (1927-1969) and received the 1987 Nobel Prize in. chemistry. He is among a veiy small handful of Nobel Prize-winning scientists who never received a formal doctorate. 

KMn04 solvated by 18-crown-6 (this solvate is soluble in benzene) 

Many other inorganic salts, including KF, KCN, and NaN , can be dissolved in organic solvents with the help of crown ethers. The effect of using a crown ether to dissolve a salt in a hydrocarbon or ether solvent is similar to the effect of dissolving the salt in a polar aprotic solvent such as DMSO, DMF, or HMPA (Section 11.5). In both cases, the metal cation is strongly solvated, leaving the anion bare. Thus, the S j2 reactivity of an anion is tremendously enhanced in the presence of a crown ether. 

4-Dioxane is made commercially by the acid-catalyzed dehydration of an alcohol. 

Another common cleavage is the loss of either of the two alkyl groups to give another oxonium ion or an alkyl cation. 

Propose a fragmentation to account for each numbered peak in the mass spectrum of n-butyl isopropyl ether. 

The mass spectrum of diethyl ether shows major peaks for the molecular ion, loss of an ethyl group, a cleavage, and a cleavage combined with loss of a molecule of ethylene. 

THIOLS, AND SULFIDES Both alcohols and ethers have an intense C—O stretching vibration of alcohols occurs in the 1050— 

The stretching vibration of the weaker C—S bond appears in the 590—700 cm region. This absorption is very weak because the C—S bond is essentially nonpolar. Therefore, the identification of a thiol by the C— S bond is quite tenuous. For the reasons just stated for alcohols and thiols, it is not possible to identify either an ether by the C—O stretching vibration or a thioether by the C—S vibration. 

The O—H stretching vibration of an alcohol occurs at 3600 cm if the sample is observed at low pressures in the gas phase. However, in the liquid phase or even a relatively dilute solution, there is extensive hydrogen bonding and the absorption occurs in the 3200-3400 cm region. The band is very broad because of the variety of hydrogen-bonded species present, but it is a positive identification of an O—H bond. 

The IR spectrum of diethyl ether has a characteristic C—O bond stretching frequency at about 1100 cm h 

The S—H stretching vibration of a thiol occurs in the 2550—2600 cm region. This lower wave-number position compared to the O—H stretching vibration is the result of the weaker S—H bond. The absorption of the S—H bond is much less intense than that of the O—H bond because the S—H bond is less polar. The S—H absorption is not broad because S—H does not form hydrogen bonds. 

When the epoxide is to be used, a basic curing agent such as a tertiary amine, R3N, is added to cause the individual prepolymer chains to link together. This cross-linking of chains is simply a base-catalyzed epoxide 

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Nomenclature of Ethers 630 14-4 Spectroscopy of Ethers 633 14-5 The Williamson Ether Synthesis 635... 

Infrared Spectroscopy of Ethers Infrared spectra do not show obvious or reliable... 

Spectroscopy of Ethers A Deeper Look—Epoxy Resins and Adhesives... 

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