Ethers
Lawrence Karas, W. J. Piel, ARCO Chemical Company
Article Online Posting Date: December 4, 2000.
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Abstract
An overview of ethers is provided that covers the general types of ethers, their synthesis reactions, properties, and some general uses in industry. More detail and expanded discussion is provided on ethyl ether, tetrahydrofuran (THF), and methyl tertiary butyl ether (MTBE), which represent three of the ethers with the highest commercial volumes. One of the fastest growing and largest uses of ethers is for gasoline blending. Though initially developed to provide octane in gasoline, fuel-grade ethers are now used to add oxygen to the gasoline mixture, which helps improve combustion and reduce tailpipe emissions from automobiles. Besides MTBE, other possible fuel ethers such as ethyl tertiary butyl ether (ETBE) and tertiary amyl ether (TAME) are also noted.
Ethers are compounds of the general formula Ar–O–Ar, Ar–O–R, and R–O–R where Ar is an aryl group and R is an alkyl group. If the two R or Ar groups are identical, the compound is a symmetrical ether. Examples of symmetrical ethers are (di)methyl ether, CH3OCH3, and (di)phenyl ether, C6H5OC6H5; examples of unsymmetrical ethers are methyl ethyl ether, CH3OCH2CH3, and methyl tert-butyl ether, CH3OC(CH3)3. Cyclic ethers are variously designated. Three-membered ring cyclic ethers are olefin oxides, eg, ethylene oxide (1) (qv), 1,2-propylene oxide (qv), and 1,2-butylene oxide (see GLYCOLS). This ether group is also known as an epoxy group or oxirane. When the ring containing the ether is larger, the ethers are usually classified as oxygen heterocyclics such as tetrahydrofuran (THF) (2), or p-dioxane (3).
Simple ethers derive their name from the two groups attached to the oxygen followed by the word ether, eg, diethyl ether, CH3CH2OCH2CH3. For symmetrical ethers the “di” prefix is often omitted. If one group has no simple name, the compound may be named as an alkoxy derivative, eg, 2-ethoxyethanol, CH3CH2OCH2CH2OH.
2. Physical Properties
In general, ethers are neutral, pleasant-smelling compounds that have little or no solubility in water, but are easily soluble in organic liquids (1). Their boiling points approximate those of hydrocarbons having comparable molecular weights and geometries. Table 1 gives the basic physical properties for many of the ethers. More detailed physical properties for the ethers most commonly used as solvents are listed in Table 2.
Table 1. Physical Properties of Some Representative Ethers
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Systematic name Common name CAS Registry Number Bp,a °C b
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Saturated
symmetrical
methyl ether [115-10-6] –23.7 1.617c
2-methoxyethyl ether [111-96-6] 162 0.9451 1.4097
ethyl ether [60-29-7] 34.5 0.7146 1.3527
1-chloroethyl ether [986-48-7] 116–117 1.106025 1.418525
n-propyl ether [111-43-3] 90.5 0.7360 1.3809
isopropyl ether [108-20-3] 68.5 0.7257 1.3682
n-butyl ether [142-96-1] 142.0 0.7704 1.3981
sec-butyl ether [6863-58-7] 122.0 0.759025 1.3931
isobutyl ether [628-55-7] 123.0 0.761215
tert-butyl ether [6163-66-2] 108.0 0.762215 1.3946
n-amyl ether [693-65-2] 188.0 0.7849 1.4119
isoamyl ether [544-01-4] 173.0 0.7777 1.4085
sec-amyl ether [56762-00-6] 161.0 0.7830 1.4058
n-hexyl ether [112-58-3] 223102 0.7936 1.4204
n-heptyl ether [629-64-1] 258.5102 0.8008 1.4275
n-octyl ether [629-82-3] 286–287 0.8063 1.4327
unsymmetrical
methyl n-propyl ether [557-17-5] 38.9 0.738 1.3579
methyl isopropyl ether [598-53-8] 32.5104 0.723715 1.3576
methyl n-butyl ether [628-28-4] 70.5 0.7443 1.3736
methyl isobutyl ether [625-44-5] 105–10696 0.7549 1.385225
methyl tert-butyl ether MTBE [1634-04-4] 55.1 0.7406 1.3690
methyl tert-amyl ether TAME [994-05-8] 85–86 0.770 1.3896
ethyl isopropyl ether [625-54-7] 53.5 0.7211 1.3624
ethyl n-butyl ether [628-81-9] 91.5 0.7490 1.3818
ethyl tert-butyl ether ETBE [637-92-3] 72–73 0.742 1.3756
ethyl n-amyl ether [17952-11-3] 118.0 0.7622 1.3927
ethyl tert-amyl ether [919-94-8] 101 0.7657 1.3912
isopropyl tert-butyl ether [17348-59-3] 87.6 0.736525 1.3799
2-ethoxyethanol cellosolve [110-80-5] 135 0.93120 1.40625
2-(2-ethoxy)ethoxyethanol ether carbitol [111-90-0] 196 0.985525 1.4273
Unsaturated
vinyl ether [109-93-3] 28–31 0.76725
vinyl methyl ether [107-25-5] 5–6 0.7511
vinyl ethyl ether [109-92-2] 35.0 0.7533 1.373925
vinyl n-butyl ether [111-34-2] 93.5 0.773525 1.399725
allyl ether [557-40-4] 95.0 0.8053 1.4163
bis(2-methallyl) ether [628-56-8] 105.4 0.8627 1.4206
allyl ethyl ether [557-31-3] 67.6 0.765 1.3881
allyl glycidyl ether [106-92-3] 75.06.7 1.431030
ethynyl ethyl ether [927-80-0] 49.0 0.8001 1.3796
ethynyl butyl ether [3329-56-4] 104.0 0.807825 1.403325
Cyclic
ethylene oxide oxirane [75-21-8] 13.599.4 0.882410 1.35977
1,2-propylene oxide methyloxirane [75-56-9] 34.3 0.859010 1.3670
1,3-propylene oxide oxetane [503-30-0] 47.8 0.893025 1.3961
tetrahydrofuran oxolane [109-99-9] 64.5 0.8892 1.4050
furan oxole [110-00-9] 31.36 0.9514 1.42142
tetrahydropyran oxane [142-68-7] 88 0.8810 1.4200
1,4-dioxane [123-91-1] 101100 1.0337 1.4224
Aromatic
methyl phenyl ether [100-66-3] 153.8 0.9954 1.5179
4-methoxytoluene [104-93-8] 176.5 0.9689 1.5124
ethyl phenyl ether [103-73-1] 172 0.9792 1.5076
1-methoxy-4-trans-propenylbenzene trans-anethole [4180-23-8] 253 0.9882 1.5615
21d
1-methoxy-4-allylbenzene phenyl estragole [140-67-0] 215 0.9645 1.5230
[101-84-8] 258 1.0863 1.5780
28d
2-methoxyphenol guaiacol [90-05-1] 205.5 1.1287 1.5385
1,2-dimethoxybenzene veratrole [91-16-7] 206.7 1.084 1.5385
22–23d
1,4-dimethoxybenzene [150-78-7] 212.6
58–60d
2-methoxy-4-allylphenol 4-allylguaiacol [97-53-0] 255 1.0664 1.5410
1,2-dimethoxy-4-allylben-zene 4-allylveratrole [93-15-2] 248 1.055 1.532
1-allyl-3,4-methylenedioxy-benzene safrole [94-59-7] 234.5 1.0950 1.5383
1-propenyl-3,4-methylene-dioxybenzene isosafrole [93-16-3] 252 1.1224 1.5782
2-methoxy-4-cis-propenyl-phenol cis-isoeugenol [5912-86-7] 1341.7 1.0851 1.5700
2-methoxy-4-trans-propenyl-phenol trans-isoeugenol [5932-68-3] 1411.6 1.0852 1.5782
1-benzyloxy-2-methoxy-4-trans-propenylbenzene benzyl isoeugenol [120-11-6] 58d
butyrated hydroxyanisole (BHA), a mixture of: 2-tert-butyl-4-methoxy-phenol and 3-tert-butyl-4-methoxyphenol [25013-16-3, 55949-47-8] 264–27098
48–55d
[121-00-6]
[88-32-4]
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a At 101.3 kPa unless otherwise noted by subscript in kPa; to convert kPa to mm Hg, multiply by 7.5.
b Unless otherwise noted as superscript, °C.
c Specific gravity (air).
d Melting point.
Table 2. Typical Physical and Chemical Properties of Commonly Used Ethersa
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Property Ethyl MTBE THF Isopropyl n-Butyl
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chemical formula C4H10O C5H12O C4H8O C6H14O C8H18O
molecular weight 74.12 88.15 72.10 102.17 139.22
boiling point at 101.3 kPa,b °C 34.5 55.0 66.0 68.4 142.0
vapor pressure at 20°C, kPab 56 27 17 16 0.67
evaporation ratec 11.8 8.5 5.7 8.0 0.66
viscosity at 20°C, mPa · s( = cP) 0.23 0.35 0.48 0.38 0.65
surface tension in air, 25°C, mN/m ( = dyn/cm) 17.0 18.3 26.4 32.0 22.9
dipole moment, 25–50°C, C · md 4.3×10–30 4.7×10–30 5.3×10–30 4.7×10–30 4×10–30
Hilderbrand solubility parameter 7.4 7.4 9.1 7.1 7.7
water solubility, wt %, 20°Ce
ether in water 6.9 4.8 inf. 1.07 0.03
water in ether 1.3 1.4 inf. 0.53 0.19
flash point –40 –30 –17 –13 25
autoignition temp, °C 160 426 321 440 185
flammability limits in air, vol %
lower 1.9 1.6 1.8 1.0 0.9
higher 48.0 8.4 11.8 21.0 8.5
reactivity with hydroxyl radicalf 13.6 3.2 17.8 27.8
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a Ref. 2.
b To convert kPa to mm Hg, multiply by 7.5.
c n-Butyl acetate = 1.
d To convert C C · m to debyes, multiply by 3×1029.
e Ref. 3.
f kOH (298 K), 1012 ; Refs. 4 and 5.
3. Chemical Properties
Most ethers, particularly dialkyl ethers, are comparatively unreactive compounds because the carbon-oxygen bond is not readily cleaved. For this reason, ethers are frequently employed as inert solvents in organic synthesis. However, within the ether family, the cyclic ethers are more reactive, the most reactive being the olefin oxides or epoxides. Epoxides are generally used as intermediates for producing other small molecules or polymers (see EPOXY RESINS; POLYETHERS). Ethers do react with exceptionally powerful basic reagents, particularly certain alkali metal alkyls, to give cleavage products (6). Ethers react with less powerful bases to give the same cleavage products, but only under the forcing conditions of high temperature and pressure.
The ether linkage can also be cleaved by strong acids, generally at high temperatures (6):
Other acids that have been used for this cleavage are phosphoric acid, pyridine hydrochloride, boron tribromide, trifluoroacetic acid, and nitric acid. Acid cleavage of aryl alkyl ethers always gives phenol because the aromatic carbon–oxygen bond is much stronger than the aliphatic carbon–oxygen bond. Unsymmetrical aliphatic ethers usually yield a mixture of alkyl halides and alcohols when cleaved by halogen containing acids.
Most ethers are potentially hazardous chemicals because, in the presence of atmospheric oxygen, a radical-chain process can occur, resulting in the formation of peroxides that are unstable, explosion-prone compounds (7). The reaction may be generalized in terms of the following steps involving initiation, propagation, and termination.
The nature of the initiation step, which may occur in a variety of ways, is not known in all cases. Commonly used ethers such as ethyl ether, isopropyl ether, tetrahydrofuran, and p-dioxane are particularly prone to form explosive peroxides on prolonged storage and exposure to air and light (see PEROXIDES AND PEROXY COMPOUNDS, ORGANIC), and should contain antioxidants (qv) to prevent their build-up. One of the exceptions to the peroxide forming tendency of ethers is methyl tert-alkyl ethers such as methyl tert-butyl ether [1634-04-4] (MTBE) and tert-amyl methyl ether [994-05-8] (TAME). Both have shown little tendency if any to form peroxides (2,
.
Ethers are weakly basic and are converted to unstable oxonium salts by strong acids such as sulfuric acid, perchloric acid, and hydrobromic acid; relatively stable complexes are formed between ethers and Lewis acids such as boron trifluoride, aluminum chloride, and Grignard reagents (qv) (9):
Like other aromatic compounds, aromatic ethers can undergo substitution in the aromatic ring with electrophilic reagents, eg, nitration, halogenation, and sulfonation. They also undergo Friedel-Crafts (qv) alkylation and acylation.
Allyl phenyl ethers rearrange cleanly at high temperatures, producing o-allyl phenols or p-allylphenols if both ortho positions are blocked. This reaction is called the Claisen rearrangement (10).
4. Fuel Properties
In addition to MTBE, two other ethers commonly used as fuel additives are tert-amyl methyl ether (TAME) and ethyl tert-butyl ether [637-92-3] (ETBE). There are a number of properties that are important in gasoline blending (see GASOLINE AND OTHER MOTOR FUELS) (Table 3).
Table 3. Typical Gasoline-Related Properties for Ethers Used in Gasolinea
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Property MTBE ETBE TAME
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research octane number (RON) 118 119 112
motor octane number (MON) 102 104 99
(RON + MON)/2 110 111 105.5
boiling point
°F 131 161 187
°C 55 72 86
Reid vapor pressure, kPab 55 28 7
oxygen, wt % 18.2 15.7 15.7
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a Ref. 11.
b To convert kPa to psi, multiply by 0.145.
. Preparation
The most versatile method of preparing ethers is the Williamson ether synthesis, particularly in the preparation of unsymmetrical alkyl ethers (12, 13). The reaction of sodium alcoholates with halogen derivatives of hydrocarbons gives the ethers:
Dialkyl sulfates can replace the halogen derivatives, and this modification is especially useful for the preparation of phenolic ethers:
Aromatic halides do not react easily with phenoxide ions to produce diaryl ethers unless the aromatic halide is substituted with one or more electron-withdrawing groups, eg, nitro or carboxyl groups. The Ullmann reaction uses finely divided copper or copper salts to catalyze the reaction of phenoxides with aromatic halides to give diaryl ethers.
Alcohols can be dehydrated with strong acid catalysts and high reaction temperatures to produce ethers. This method is particularly useful for the preparation of symmetrical lower alkyl ethers, such as ethyl ether.
The reaction gives poor yields of ethers with secondary and tertiary alcohols; dehydration to form the corresponding olefin is a more favorable reaction. The reaction fails for the production of diaryl ethers from phenols.
Alkyl tertiary alkyl ethers can be prepared by the addition of an alcohol or phenol to a tertiary olefin under acid catalysis (Reycler reaction); sulfuric acid, phosphoric acid, hydrochloric acid, and boron trifluoride have all been used as catalysts:
Commercially, sulfonic acid ion-exchange resins are used in fixed-bed reactors to make these tertiary alkyl ethers (14). Since the reaction is very selective to tertiary olefins and also reversible, a two-step procedure is also used to recover commercially pure tertiary olefins from mixed olefin process streams. The corresponding tertiary alkyl ether is produced in the olefin mixture and then easily separated from the unreacted olefins by simple fractionation. The reaction is then reversed in a second step to make a commercially pure tertiary olefin, usually isobutylene or isoamylene.
Ether production from the reaction of monohydric alcohols with linear olefins has also been demonstrated under more severe conditions using a zeolite-type catalyst system (15). However, it has not been commercialized as yet.
. Economic Aspects
Table 4 compares the prices and availability of several commercially available ethers as of January 1992.
Table 4. Price and Availability of Some Commercial Ethersa
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Ether Price, $/kg Availability
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methyl (aerosol-grade) 0.95 tank car
ethyl 1.17 tank car
n-butyl 3.08 208-Lb drum
tetrahydrofuran 2.77 tank car
isopropyl 1.01 tank car
methyl tert-butyl 0.99c tank truck
butylated hydroxy anisole (BHA)d 19.36 208-Lb drum
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a Ref. 16.
b 55-gal.
c Refined-grade (99.9% purity). Courtesy of ARCO Chemical Co.
d A mixture of 2-tert-butyl-4-methoxyphenol and 3-tert-butyl-4-methoxyphenol.
Fuel-grade ethers are generally produced in crude oil refineries for internal use in the manufacture of gasoline or in petrochemical plants for resale to the refiners. The value of these ethers fluctuates with the price of gasoline and octane in the industry. It is not uncommon for the market price of MTBE to change weekly and sometimes daily. The two largest MTBE markets are in the U.S. Gulf Coast region and in Northern Europe. MTBE is typically sold to refiners in large barges in quantities of ten to one hundred thousand U.S. barrels (1 to 12 103 t). It is also sometimes shipped in batches on gasoline product pipelines. Spot market prices quoted in January 1992 ranged from 92 to 96 U.S. cents per gallon ($244–$254/m3) for the U.S. Gulf Coast region (17).
7. Health and Safety Factors
Although ethers are not particularly hazardous, their use involves risks of fire, toxic effects, and several unexpected reactions.
7.1. Flammability
Since almost all ethers burn in air, an assessment of their potential hazards depends on flash points and ignition temperatures. The flash point of a liquid is the lowest temperature at which vapors are given off in sufficient quantities for the vapor–air mixture above the surface of the liquid to propagate a flame away from the source of ignition. In other words, an explosive vapor–air mixture can form whenever a liquid is used or stored in an open container at a temperature above its flash point. Table 2 includes the flash points of several common ethers.
The ignition temperature is the minimum temperature required to initiate or cause self-sustained combustion. Table 2 also lists ignition temperatures of several common ethers. Attention is directed to the particularly low ignition temperature of ethyl ether, especially with reference to some common ignition sources such as a lighted cigarette (732°C) or a pressurized (0.7 MPa or 100 psi) steam line (180°C).
7.2. Toxicity
The effect of ethers owing to ingestion, skin contact, or inhalation may range from drowsiness and lack of coordination to serious injury or death. Ethers are reported to have a low order of toxicity when ingested, although 30–60 mL may be fatal when swallowed (18), and a case of fatal poisoning owing to ingestion of a large quantity of 2-methoxyethanol has been recorded (19).
Prolonged or repeated contacts of ethers with skin cause tissue defatting and dehydration leading to dermatitis. Some compounds penetrate the skin in harmful amounts. 1,4-Dioxane and 1-allyl-3,4-methylenedioxybenzene (safrole) have been listed as Category I carcinogens by OSHA. Category I carcinogens are confirmed cancer-causing agents based on human data, or based on tests in two mammalian species, or in one species if the tests have been replicated.
Inhalation is the most common means by which ethers enter the body. The effects of various ethers may include narcosis, irritation of the nose, throat, and mucous membranes, and chronic or acute poisoning. In general, ethers are central nervous system depressants, eg, ethyl ether and vinyl ether are used as general anesthetics.
7.3. Peroxide Formation
Except for the methyl tert-alkyl ethers, most ethers tend to absorb and react with oxygen from the air to form unstable peroxides that may detonate with extreme violence when concentrated by evaporation or distillation, when combined with other compounds that give a detonable mixture, or when disturbed by heat, shock, or friction. Appreciable quantities of crystalline solids have been observed as gross evidence for the formation of peroxides, and peroxides may form a viscous liquid in the bottom of ether-filled containers. If viscous liquids or crystalline solids are observed in ethers, no further tests for the detection of peroxides are recommended. Several chemical and physical methods for detecting and estimating peroxide concentrations have been described. Most of the qualitative tests for peroxides are readily performed and strongly recommended when any doubt is present (20).
Applications
Alkyl ethers are used for organic reactions and extractions, as plasticizers, as vehicles for other products, as anesthetics (qv), and octane (and oxygen) enhancers in gasoline. Most ethers have very low solubility in water, but dissolve most organic compounds, and therefore have found wide application in paint and varnish removers; as high boiling solvents for gums, resins, and waxes; in lubricating oils (see LUBRICATION); as an extraction solvent in the fragrance industry; and as an inert reaction medium in the pharmaceutical industry. The vapors of certain ethers are toxic to insects and are useful as agricultural insecticides (see INSECT CONTROL TECHNOLOGY) and industrial fumigants. Except for those used in gasoline, the lower mol wt technical-grade ethers are the least expensive and most commonly used in industry.
Aryl ethers have distinctive, pleasant odors and flavors which make them valuable to the perfume (qv) and flavor industries (see FLAVORS AND SPICES). Because of their heat stability, they are useful as heat-transfer fluids (see HEAT-EXCHANGE TECHNOLOGY). Other aryl ethers are useful as food preservatives and antioxidants(see FOOD ADDITIVES).
. Commercially Important Ethers
9.1. ETHYL ETHER
Diethyl ether [60-29-7] is one of the more important members of the ether family. It is a colorless, very volatile, highly flammable liquid with a sweet, pungent odor and burning taste. As a commercial product it is available in several grades; it is used in chemical manufacture, as a solvent, extractant, or reaction medium, and as a general anesthetic.
Manufacture Much of the diethyl ether manufactured is obtained as a by-product when ethanol (qv) is produced by the vapor-phase hydration of ethylene (qv) over a supported phosphoric acid catalyst. Such a process has the flexibility to adjust to some extent the relative amounts of ethanol and diethyl ether produced in order to meet existing market demands. Diethyl ether can be prepared directly to greater than 95% yield by the vapor-phase dehydration of ethanol in a fixed-bed reactor using an alumina catalyst (21).
Handling and Shipment The handling of ethyl ether is hazardous because of its highly flammable properties. Not only is it highly volatile, but it also has a low autoignition temperature and, as a nonconductor, can generate static electrical charges that may result in ignition or vapor explosion. The area in which ethyl ether is handled should be considered a Class I hazardous location as defined by the National Electrical Code (22). All tools used in making connections or repairs should be of the nonsparking type. All possible care should be taken in loading and unloading tank cars. The tank cars should be properly spotted, and the usual caution signs and derails placed in position. Before any connection is made, the tank car should be grounded and bonded. Tank cars should always be unloaded through dome connections rather than through bottom outlets, eg, a pressure-type LPG tank car. A positive suction-type pump or natural gas can be used to remove the ether from the tank car. In no event should air pressure be used.
Special containers have been developed for anesthetic ether to prevent deterioration before use. Their effectiveness as stabilizers usually depends on the presence of a lower oxide of a metal having more than one oxidation state. Thus the sides and the bottoms of tin-plate containers are electroplated with copper, which contains a small amount of cuprous oxide. Stannous oxide is also used in the linings for tin containers. Instead of using special containers, iron wire or certain other metals and alloys or organic compounds have been added to ether to stabilize it.
Economic Aspects The production of ethyl ether from 1956 through 1973 ranged from 29.5 to 48.6×106 kg as reported by Synthetic Organic Chemicals, U.S. Production and Sales. Production was estimated at 13.6 – 18×106 kg in 1986, 12.7×106 kg in 1989. Though 1990 U.S. production capacity was estimated at 25.5×106 kg, production was estimated as only 12×106 kg in 1991 (21). Much of the decrease has been the result of a decline in arsenal demand (smokeless gun powder). List prices for ether have been steadily increasing, and reached $1.12/kg by 1989, refined, tanks (fob).
Analytical Methods Most of the analytical and testing methods used for ethyl ether are conventional laboratory methods. Ethyl ether that is to be used for anesthetic purposes or in processes that involve heating or distillation must be peroxide-free, and should pass the USP standard test with potassium iodide. This test detects approximately 0.001% peroxide as hydrogen peroxide.
Specifications Ethyl ether is commercially available in the following grades: USP anesthesia, absolute (ACS), industrial, solvent (conc), and synthetic. Specifications vary, depending on the consumer and use. In many instances, the ether has to meet a specific test written into the specification, eg, it may be important that the ether is completely anhydrous or free from alcohol and aldehyde.
The technical concentrated ether contains very small amounts of alcohol, water, aldehydes, peroxides, and other impurities (Table 5). The more refined grades, such as anesthetic ether, are obtained from technical ether by redistillation and dehydration followed by alkali or charcoal treatment.
Table 5. Specifications for Various Grades of Ethyl Ether
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Specification Technical refined ACS, absolute USP XIXa
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color, max water-white water-white colorless
0.710–0.713 0.7079 0.713–0.716
acidity as acetic acid, wt % 0.0025 0.0010 passes testb
peroxides, max wt % passes textc 0.0001 passes testc
aldehydes, max wt % passes testd 0.0005 passes testd
alcohol, max % 0.5 vol % 0.05 wt %
nonvolatile matter, max wt % 0.002 0.0010 0.003
water, max wt % 0.3 0.0100
odor nonresidual passes teste passes teste
net container contents, kgf
19-L (5-gal) drum 13.6 13.6 13.6
208-L (55-gal) drum 147 147 147
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a For use in anesthesia; the USP (22) also recognizes slightly less pure grades: ethyl oxide (solvent ether), ether abs, and reagent-grade ether.
b Free acid, requiring no more than 0.4 mL of 0.02 N sodium hydroxide for 25 mL of ethyl ether.
c No color with potassium iodide reagent; USP test.
d No turbidity with alkaline mercuric chloride–potassium iodide reagent; USP test.
e No foreign odor when last traces of ether evaporate from odorless absorbent paper; USP test.
f The tech-grade can also be shipped in 103 W insulated tank cars of 15.1-m3 (4000-gal), 22.7-m3 (6000-gal), 30.3-m3 (8000-gal), and 37.9-m3 (10,000-gal) capacity.
Ethyl ether is classified by the ICC as a flammable liquid. As such it must be packed in ICC specification containers when shipped by rail, water, or highway; all ICC regulations regarding loading, handling, and labeling must be followed. Each container of ethyl ether must carry an identifying label or stencil. Tank cars and boxcars, either carload or less than carload, must bear the ICC dangerous placard. Each drum or each box with inside containers must bear the ICC red label for flammable liquids.
Toxicology The toxicity of ethyl ether is low and its greatest hazards in industry are fire and explosion. The vapor is absorbed almost instantly from the lungs and very promptly from the intestinal tract. It undergoes no chemical change in the body. Prevention and control of health hazards associated with the handling of ethyl ether depend primarily on prevention of exposure to toxic atmospheric concentrations and scrupulous precautions to prevent explosion and fire.
A concentration of 35,000 ppm in air produces unconsciousness in 30–40 minutes. This concentration also constitutes a serious fire and explosion hazard, and should not be permitted to exist under any circumstance. Any person exposed to ethyl ether vapor of any appreciable concentration should be promptly removed from the area. Recovery from exposure to sublethal concentrations is rapid and generally complete. Except in emergencies, and then only with appropriate protective equipment, no one should enter an area containing ether vapor until the concentration has been found safe by measurement with a combustible-gas indicator.
If an ethyl ether fire occurs, carbon dioxide, carbon tetrachloride, and dry chemical fire extinguishers meeting National Fire Prevention Association Code 1 and 2 requirements may be used successfully (23). Water may also be effectively applied (see PLANT SAFETY). Hose streams played into open tanks of burning ethyl ether serve only to scatter the liquid and spread the fire. However, ether fires may be extinguished by a high pressure water spray that cools the burning surface and smothers the fire. Automatic sprinklers and deluge systems are also effective.
Uses Ethyl ether [60-29-7] has a wide range of uses in the chemical industry. It is a good solvent or extractant for fats, waxes, oils, perfumes, resins, dyes, gums, and alkaloids. When mixed with ethanol, ethyl ether becomes an excellent solvent for cellulose nitrate in the manufacture of guncotton (see EXPLOSIVES AND PROPELLANTS), collodion solutions (see MEMBRANE TECHNOLOGY), and pyroxylin plastics (see CELLULOSE ESTERS). Another important use is an extractant for acetic acid as well as other organic acids, eg, in the cellulose acetate and plastic industries to recover acetic acid from dilute aqueous systems. Ethyl ether is also used as a denaturant in several denaturant alcohol formulas. It has been used as a starting fuel for diesel engines and as an entrainer for dehydration of ethanol and isopropyl alcohol. It may be used as an anhydrous, inert reaction medium for the Grignard and Wurtz-Fittig reactions. Ethyl ether is used as a general anesthetic in surgery.
The estimated use pattern of ethyl ether during 1990 was as follows: solvents and military production of smokeless powder, 35%; chemical synthesis and solvent extraction, 35%; diesel starting fluid, 30% (21).
9.2. METHYL TERTIARY BUTYL ETHER
MTBE was first commercially produced in Italy in 1973 for use as an octane enhancer in gasoline. U.S. production of MTBE started in 1979 after Atlantic Richfield Co. (ARCO) was granted a waiver by the U.S. Environmental Protection Agency (EPA) that allowed MTBE to be blended up to 7 vol % in U.S. unleaded gasoline. The use of other aliphatic ethers was allowed when the U.S. EPA issued its “substantially similar” definition for unleaded gasoline specifications in 1981. Under this definition, any aliphatic ether or ether mixture could be blended in unleaded gasoline as long as the total oxygen contribution from the ethers does not exceed 2.0% oxygen by weight in the gasoline. This allowed MTBE to be blended up to approximately 11 vol % in gasoline.
MTBE production grew during the 1980s as lead-based octane enhancers were being phased out of gasoline. In August 1988, U.S. EPA granted a waiver requested by Sun Refining and Marketing Co. that allowed the use of MTBE up to 15 vol %, which is approximately equal to 2.7 wt % oxygen in gasoline. This oxygen limit was extended to all aliphatic ethers in gasoline in 1990 when the U.S. EPA raised the oxygen in the “substantially similar” definition to 2.7 wt % oxygen.
In November 1990, the U.S. government again revised the Clean Air Act (CAA). One of the new requirements was for gasolines sold in ambient air quality nonattainment areas of the United States to contain a minimum amount of oxygen. For the 39 metropolitan areas that exceed the federal standard for carbon monoxide, the gasoline would have to contain at least 2.7 wt % oxygen for at least four winter months beginning November 1992. This represents nearly 30% of the U.S. gasoline market.
Starting in January 1995, the nine metropolitan areas with the most severe ozone problem must sell a “reformulated” gasoline all year long. These nine cities represent approximately 20% of the U.S. gasoline market. In addition to many other very restrictive specifications, this gasoline must also contain at least 2.0 wt % oxygen. Other metropolitan areas exceeding the federal ozone standard are also allowed to choose the program when sufficient capacity exists to make the reformulated gasoline.
Manufacture MTBE is easily made by the selective reaction of isobutylene and methanol over an acidic ion-exchange resin catalyst, in the liquid phase and at temperatures below 100°C. To be economically competitive, MTBE's use as an octane enhancer in gasoline has been dependent on low cost isobutylene. There are a number of possible isobutylene sources for making MTBE (see BUTYLENES). During the 1980s, much of the MTBE was made with isobutylene contained in mixed butanes/butylenes process streams that was produced from petrochemical olefin plants or refinery fluid catalytic crackers. MTBE plant sizes from these feedstock sources are limited by the amount of by-product isobutylene produced in these operations, and are usually in the range of one to six thousand barrels per day (40 to 250 t/yr). Larger amounts of isobutylene for MTBE can be made from butanes by first isomerizing the normal butanes to isobutane, and then dehydrogenating to isobutylene. This is a much more capital intensive process, and therefore the plant sizes in the United States are usually at least 500,000 t/yr to be economically competitive. Even at this size, it is still a relatively higher cost source of MTBE than that made from by-product isobutylene in ethylene plants or refineries. However, because of the growing need to meet the gasoline oxygen requirements for the U.S. Clean Air Act, many of these world-scale MTBE plants will need to be built (24).
Production MTBE production capacity has grown steadily, usually at an annual rate of 10 to 20% per year. In 1980, world capacity was 30 thousand barrels per day (1.5×106 t/yr). By 1990, capacity was up to 180 thousand barrels per day (7×106 t/yr). Because of the requirements of the U.S. CAA, production capacity is expected to more than double from 1990 to 1995 (25). By 2000, MTBE may be the second largest organic chemical produced in the United States, second only to ethylene (26).
Economic Aspects Prior to the 1990 revision of the CAA, MTBE sold as an octane enhancer for gasoline. Its market value has therefore been closely tied to the industry's cost of gasoline and octane. To reflect its higher-than-gasoline octane, it sold at a price higher than gasoline, usually in the range of 120 to 160% of gasoline. This price was not usually high enough to economically justify new world-scale merchant capacity based on converting butanes and methanol to MTBE.
Because there does not appear to be sufficient MTBE capacity to satisfy the oxygen requirements for the 1990 CAA revision, there has been a general upward movement in MTBE market prices that should provide the incentive for additional capacity to be built (27).
Specifications For fuel-grade MTBE, the typical specification of merchant product is 95 wt % minimum purity in the United States and 98% minimum in Europe. The maximums for impurities are usually 0.5 wt % for methanol, and 0.15 wt % for water. To maintain quality, fuel-grade MTBE is normally stored in fixed-roof tanks, to keep rain water out, with internal floaters, to minimize vapor losses.
Besides its use in gasoline, a high purity refined grade (99.9% min) of MTBE is produced for other industrial uses.
Uses MTBE and related ethers are used to add octane to gasoline. It also adds oxygen to the gasoline, which allows for more efficient combustion, and therefore less carbon monoxide and unburned hydrocarbon in the exhaust emissions (11, 24).
A refined grade of MTBE is used in the solvents and pharmaceutical industries. The main advantage over other ethers is its uniquely stable structural framework that contains no secondary or tertiary hydrogen atoms, which makes it very resistive to oxidation and peroxide formation. In addition, its higher autoignition temperature and narrower flammability range also make it relatively safer to use compared to other ethers (see Table 3).
The refined grade's fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene using lithium alkyls as catalyst. Other laboratory applications include use as a solvent for Grignard reagents, and also for phase-transfer catalysts.
One other unique use of MTBE is a medical procedure for the removal of gallstones. This alternative to gallbladder surgery was developed at the Mayo Clinic, and takes advantage of MTBE's capability to quickly dissolve cholesterol. A small incision is used to inject a small amount of MTBE directly into the gallbladder, and the gallstone can then be removed in solution form (28).
9.3. TETRAHYDROFURAN
Except for the special case of the epoxides, THF represents the largest use of a heterocyclic ether. Its consumption is also much larger than that of ethyl ether. Unlike the dialkyl ethers, THF is totally miscible in water at ambient conditions. Its cyclic structure also allows it to be more reactive than the dialkyl ethers. More than half of the THF produced is used as an intermediate in making other chemicals or elastomers.
Manufacturing Almost all the THF in the United States is currently produced by the acid-catalyzed dehydration of 1,4-butanediol [10-63-4]. Only one plant in the United States still makes THF by the hydrogenation of furfural (29). Du Pont recently claimed a new low cost process for producing THF from n-butane that they plan to commercialize in 1995 (30–32). The new process transport-bed oxidizes n-butane to crude maleic anhydride, then follows with a hydrogen reduction of aqueous maleic acid to THF (30).
Production and Specification In 1990, approximately 105×106 kg of THF were produced from a U.S. annual capacity of about 123×106 kg. Consumption is expected to grow by 5 to 6% per year through 1995. THF is generally shipped in tank cars, tank trucks, or drums (29, 31).
The general industry specifications are a minimum purity of 99.9 wt %. Maximum impurities are 300 ppm water and 150 ppm of the hydroperoxide of THF. Like most ethers, it is prone to forming peroxides on storage, and therefore is usually sold containing an antioxidant such as BHT.
Uses Approximately 70% of the U.S. production is used to make poly(tetramethylene ether glycol) [25190-06-1] (PTMEG), also known as poly-THF, which is used in the production of urethane elastomers, polyurethane fibers (ether-based spandex), and copolyester–ether elastomers. PTMEG is also the fastest growing use (see POLYETHERS, TETRAHYDROFURAN). The remaining production is used as a solvent for the manufacture of poly(vinyl chloride) cements and coating, precision magnetic tape, a reaction solvent in the production of pharmaceuticals, and other miscellaneous uses.
9.4. OTHER ETHERS
n-Butyl Ether n-Butyl ether is prepared by dehydration of n-butyl alcohol by sulfuric acid or by catalytic dehydration over ferric chloride, copper sulfate, silica, or alumina at high temperatures. It is an important solvent for Grignard reagents and other reactions that require an anhydrous, inert medium. n-Butyl ether is also an excellent extracting agent for use with aqueous systems owing to its very low water-solubility.
Isopropyl Ether Isopropyl ether is manufactured by the dehydration of isopropyl alcohol with sulfuric acid. It is obtained in large quantities as a by-product in the manufacture of isopropyl alcohol from propylene by the sulfuric acid process, very similar to the production of ethyl ether from ethylene. Isopropyl ether is of moderate importance as an industrial solvent, since its boiling point lies between that of ethyl ether and acetone. Isopropyl ether very readily forms hazardous peroxides and hydroperoxides, much more so than other ethers. However, this tendency can be controlled with commercial antioxidant additives. Therefore, it is also being promoted as another possible ether to be used in gasoline (33).
Tertiary-Amyl Methyl Ether Like MTBE, TAME is produced by the simple reaction of methanol and isoamylenes (2-methyl-1-butene and 2-methyl-2-butene). It is used for gasoline blending, and also as a feedstock to make high purity isoamylene. Its use in gasoline octane blending has not grown nearly as fast as MTBE because of less favorable economics (TAME's lower octane number). However, with the future need for gasolines that have lower vapor pressure and contain oxygen, TAME capacity is expected to grow using isoamylene produced by refinery catalytic crackers (24).
Ethyl Tertiary-Butyl Ether Similar to methanol in the MTBE reaction, ethanol can react with isobutylene to produce ETBE. Which alcohol is used to make the ether is highly dependent on the relative cost of the alcohols. To make ethanol more economically competitive with methanol, the federal tax credit for biomass-based ethanol used in fuel also applies to ethanol used to make ETBE in the United States (24).
Vinyl Ether Vinyl ether is manufactured by the pyrolytic dehydrochlorination of 1,1-dichloroethyl ether. Vinyl ether is used as a general inhalation anesthetic for procedures of short duration. Approximately 4% ethanol is added to the vinyl ether used as an anesthetic to reduce ice formation in the masks used for administration (see also VINYL POLYMERS).
2-Methoxyphenol This ether is prepared by methylating 1,2-dihydroxybenzene. It is useful as an antioxidant for fats, oils, and vitamins, and as a polymerization inhibitor. 2-Methoxyphenol is effective as an antioxidant and inhibitor at levels of 10–100 ppm.
Butylated Hydroxyanisole 2- and 3-tert-Butyl-4-methoxyphenol (butylated hydroxyanisole (BHA)) is prepared from 4-methoxyphenol and tert-butyl alcohol over silica or alumina at 150°C or from hydroquinone and tert-butyl alcohol or isobutene, using an acid catalyst and then methylating. It is widely used in all types of foods such as butter, lard, and other fats, meats, cereals, baked goods, candies, and beer as an antioxidant (see ANTIOXIDANTS; FOOD ADDITIVES). Its antioxidant properties are not lost during cooking so that flour, fats, and other BHA-stabilized ingredients may be used to produce stabilized products.
Bibliography
“Ethers” in ECT 1st ed., Vol. 5, pp. 858–876, by M. A. Magill, Chemical Abstracts, and J. G. Parks and C. M. Beamer, Enjay Co., Inc; in ECT 2nd ed., Vol. 8, pp. 470–, by A. P. Lurie, Eastman Kodak Co.; in ECT 3rd ed., Vol. 9, pp. 381–393, by D. E. Keeley, General Electric Co.
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22. The Pharmacopeia of the United States of America, 19th ed., USP XIX, Mack Publishing Co., Easton, Pa., 1975; USPXX-NFXF, 1980.
23. Chemical Safety Data Sheet SD-29, Manufacturing Chemist Association, Washington, D.C., 1972.
24. W. J. Piel, Fuel Reformulation 2(6) (1992).
25. MTBE/Oxygenates, 1991 annual, DeWitt & Co., Houston, Tex.
26. MTBE/Oxygenates/New Fuels, weekly newsletter, DeWitt & Co., Houston, Tex., Aug. 8, 1991.
27. Chem. Eng., 57 (Oct. 1991).
28. “A Compound To Dissolve Gallstones,” New York Times, July 23, 1988, p. 36.
29. 1,4-Butanediol, Chem. Economics Handbook, SRI International, Menlo Park, Calif., Sept. 1991.
30. Chem. Mark. Rep., 3 and 14 (Sept. 3, 1990).
31. Chem. Mark. Rep., 9 (June 24, 1991).
32. Eur. Chem. News, 218 (Mar. 9, 1992).
33. Oil Gas J., 39–41 (May 25, 1992).
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