Organic compounds made of alkyl/aryl groups bound to oxygen (R?O?R')
In
organic chemistry
,
ethers
are a class of
compounds
that contain an ether
group
?an
oxygen
atom bonded to two
organyl
groups (e.g.,
alkyl
or
aryl
). They have the general formula
R?O?R′
, where R and R′ represent the organyl groups. Ethers can again be classified into two varieties: if the organyl groups are the same on both sides of the oxygen atom, then it is a simple or symmetrical ether, whereas if they are different, the ethers are called mixed or unsymmetrical ethers.
[1]
A typical example of the first group is the
solvent
and
anaesthetic
diethyl ether
, commonly referred to simply as "ether" (
CH
3
?CH
2
?O?CH
2
?CH
3
). Ethers are common in organic chemistry and even more prevalent in
biochemistry
, as they are common linkages in
carbohydrates
and
lignin
.
[2]
Structure and bonding
[
edit
]
Ethers feature bent
C?O?C
linkages. In
dimethyl ether
, the
bond angle
is 111° and C?O distances are 141
pm
.
[3]
The barrier to rotation about the C?O bonds is low. The bonding of oxygen in ethers, alcohols, and water is similar. In the language of
valence bond theory
, the hybridization at oxygen is sp
3
.
Oxygen is more
electronegative
than carbon, thus the alpha hydrogens of ethers are more acidic than those of simple hydrocarbons. They are far less acidic than alpha hydrogens of carbonyl groups (such as in
ketones
or
aldehydes
), however.
Ethers can be symmetrical of the type ROR or unsymmetrical of the type ROR'. Examples of the former are
dimethyl ether
,
diethyl ether
,
dipropyl ether
etc. Illustrative unsymmetrical ethers are
anisole
(methoxybenzene) and
dimethoxyethane
.
Vinyl- and acetylenic ethers
[
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]
Vinyl- and acetylenic ethers are far less common than alkyl or aryl ethers. Vinylethers, often called
enol ethers
, are important intermediates in
organic synthesis
. Acetylenic ethers are especially rare.
Di-tert-butoxyacetylene
is the most common example of this rare class of compounds.
Nomenclature
[
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]
In the
IUPAC Nomenclature
system, ethers are named using the general formula
"alkoxyalkane"
, for example CH
3
?CH
2
?O?CH
3
is
methoxyethane
. If the ether is part of a more-complex molecule, it is described as an alkoxy substituent, so ?OCH
3
would be considered a
"
methoxy
-"
group. The simpler
alkyl
radical is written in front, so CH
3
?O?CH
2
CH
3
would be given as
methoxy
(CH
3
O)
ethane
(CH
2
CH
3
).
Trivial name
[
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]
IUPAC rules are often not followed for simple ethers. The trivial names for simple ethers (i.e., those with none or few other functional groups) are a composite of the two substituents followed by "ether". For example, ethyl methyl ether (CH
3
OC
2
H
5
), diphenylether (C
6
H
5
OC
6
H
5
). As for other organic compounds, very common ethers acquired names before rules for nomenclature were formalized. Diethyl ether is simply called ether, but was once called
sweet oil of vitriol
. Methyl phenyl ether is
anisole
, because it was originally found in
aniseed
. The
aromatic
ethers include
furans
.
Acetals
(α-alkoxy ethers R?CH(?OR)?O?R) are another class of ethers with characteristic properties.
Polyethers
[
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]
Polyethers are generally
polymers
containing ether linkages in their main chain. The term
polyol
generally refers to polyether polyols with one or more functional
end-groups
such as a
hydroxyl
group. The term "oxide" or other terms are used for high molar mass polymer when end-groups no longer affect polymer properties.
Crown ethers
are cyclic polyethers. Some toxins produced by
dinoflagellates
such as
brevetoxin
and
ciguatoxin
are extremely large and are known as
cyclic
or
ladder
polyethers.
The phenyl ether polymers are a class of
aromatic
polyethers containing aromatic cycles in their main chain:
polyphenyl ether
(PPE) and
poly(
p
-phenylene oxide)
(PPO).
Related compounds
[
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]
Many classes of compounds with C?O?C linkages are not considered ethers:
Esters
(R?C(=O)?O?R′),
hemiacetals
(R?CH(?OH)?O?R′),
carboxylic acid anhydrides
(RC(=O)?O?C(=O)R′).
There are compounds which, instead of
C
in the
C?O?C
linkage, contain heavier
group 14
chemical elements
(e.g.,
Si
,
Ge
,
Sn
,
Pb
). Such compounds are considered ethers as well. Examples of such ethers are
silyl enol ethers
R
3
Si?O?CR=CR
2
(containing the
Si?O?C
linkage),
disiloxane
H
3
Si?O?SiH
3
(the other name of this compound is disilyl ether, containing the
Si?O?Si
linkage) and
stannoxanes
R
3
Sn?O?SnR
3
(containing the
Sn?O?Sn
linkage).
Physical properties
[
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]
Ethers have
boiling points
similar to those of the analogous
alkanes
. Simple ethers are generally colorless.
Selected data about some alkyl ethers
|
Ether
|
Structure
|
m.p. (°C)
|
b.p. (°C)
|
Solubility in 1 liter of H
2
O
|
Dipole moment (
D
)
|
Dimethyl ether
|
CH
3
?O?CH
3
|
?138.5
|
?23.0
|
70 g
|
1.30
|
Diethyl ether
|
CH
3
CH
2
?O?CH
2
CH
3
|
?116.3
|
34.4
|
69 g
|
1.14
|
Tetrahydrofuran
|
O(CH
2
)
4
|
?108.4
|
66.0
|
Miscible
|
1.74
|
Dioxane
|
O(C
2
H
4
)
2
O
|
11.8
|
101.3
|
Miscible
|
0.45
|
Reactions
[
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]
The C-O bonds that comprise simple ethers are strong. They are unreactive toward all but the strongest bases. Although generally of low chemical
reactivity
, they are more reactive than
alkanes
.
Specialized ethers such as
epoxides
,
ketals
, and
acetals
are unrepresentative classes of ethers and are discussed in separate articles. Important reactions are listed below.
[4]
Cleavage
[
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]
Although ethers resist hydrolysis, they are cleaved by hydrobromic acid and
hydroiodic acid
.
Hydrogen chloride
cleaves ethers only slowly. Methyl ethers typically afford
methyl halides
:
- ROCH
3
+ HBr → CH
3
Br + ROH
These reactions proceed via
onium
intermediates, i.e. [RO(H)CH
3
]
+
Br
?
.
Some ethers undergo rapid cleavage with
boron tribromide
(even
aluminium chloride
is used in some cases) to give the alkyl bromide.
[5]
Depending on the substituents, some ethers can be cleaved with a variety of reagents, e.g. strong base.
Despite these difficulties the chemical
paper pulping
processes are based on cleavage of ether bonds in the
lignin
.
Peroxide formation
[
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]
When stored in the presence of air or oxygen, ethers tend to form
explosive
peroxides
, such as
diethyl ether hydroperoxide
. The reaction is accelerated by light, metal catalysts, and
aldehydes
. In addition to avoiding storage conditions likely to form peroxides, it is recommended, when an ether is used as a solvent, not to distill it to dryness, as any peroxides that may have formed, being less volatile than the original ether, will become concentrated in the last few drops of liquid. The presence of peroxide in old samples of ethers may be detected by shaking them with freshly prepared solution of a ferrous sulfate followed by addition of KSCN. Appearance of blood red color indicates presence of peroxides. The dangerous properties of ether peroxides are the reason that diethyl ether and other peroxide forming ethers like
tetrahydrofuran
(THF) or
ethylene glycol dimethyl ether
(1,2-dimethoxyethane) are avoided in industrial processes.
Lewis bases
[
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]
Ethers serve as
Lewis bases
. For instance,
diethyl ether
forms a
complex
with
boron trifluoride
, i.e. borane diethyl etherate (
BF
3
·O(CH
2
CH
3
)
2
). Ethers also coordinate to the
Mg
center in
Grignard reagents
.
Tetrahydrofuran
is more basic than
acyclic
ethers. It forms with many
complexes
.
Alpha-halogenation
[
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]
This reactivity is similar to the tendency of ethers with
alpha
hydrogen atoms to form peroxides. Reaction with chlorine produces alpha-chloroethers.
Synthesis
[
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]
Dehydration of alcohols
[
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]
The
dehydration
of
alcohols
affords ethers:
[7]
- 2 R?OH → R?O?R +
H
2
O
at high temperature
This direct nucleophilic substitution reaction requires elevated temperatures (about 125 °C). The reaction is catalyzed by acids, usually sulfuric acid. The method is effective for generating symmetrical ethers, but not unsymmetrical ethers, since either OH can be protonated, which would give a mixture of products. Diethyl ether is produced from ethanol by this method. Cyclic ethers are readily generated by this approach. Elimination reactions compete with dehydration of the alcohol:
- R?CH
2
?CH
2
(OH) → R?CH=CH
2
+ H
2
O
The dehydration route often requires conditions incompatible with delicate molecules. Several milder methods exist to produce ethers.
Electrophilic addition of alcohols to alkenes
[
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]
Alcohols add to electrophilically activated
alkenes
. The method is atom-economical:
- R
2
C=CR
2
+ R?OH → R
2
CH?C(?O?R)?R
2
Acid
catalysis
is required for this reaction. Commericially important ethers prepared in this way are derived from
isobutene
or
isoamylene
, which protonate to give relatively stable
carbocations
. Using ethanol and methanol with these two alkenes, four fuel-grade ethers are produced:
methyl tert-butyl ether
(MTBE),
methyl tert-amyl ether
(TAME),
ethyl tert-butyl ether
(ETBE), and
ethyl tert-amyl ether
(TAEE).
[4]
Solid acid catalysts
are typically used to promote this reaction.
Epoxides
[
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]
Epoxides
are typically prepared by oxidation of alkenes. The most important epoxide in terms of industrial scale is ethylene oxide, which is produced by oxidation of ethylene with oxygen. Other epoxides are produced by one of two routes:
Many ethers,
ethoxylates
and
crown ethers
, are produced from epoxides.
Williamson and Ullmann ether syntheses
[
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]
Nucleophilic displacement
of
alkyl halides
by
alkoxides
- R?ONa + R′?X → R?O?R′ + Na
X
This reaction, the
Williamson ether synthesis
, involves treatment of a parent
alcohol
with a strong
base
to form the alkoxide, followed by addition of an appropriate
aliphatic compound
bearing a suitable
leaving group
(R?X). Although popular in textbooks, the method is usually impractical on scale because it cogenerates significant waste.
Suitable leaving groups (X) include
iodide
,
bromide
, or
sulfonates
. This method usually does not work well for aryl halides (e.g.
bromobenzene
, see Ullmann condensation below). Likewise, this method only gives the best yields for primary halides. Secondary and tertiary halides are prone to undergo E2 elimination on exposure to the basic alkoxide anion used in the reaction due to steric hindrance from the large alkyl groups.
In a related reaction, alkyl halides undergo nucleophilic displacement by
phenoxides
. The R?X cannot be used to react with the alcohol. However
phenols
can be used to replace the alcohol while maintaining the alkyl halide. Since phenols are acidic, they readily react with a strong
base
like
sodium hydroxide
to form phenoxide ions. The phenoxide ion will then substitute the ?X group in the alkyl halide, forming an ether with an aryl group attached to it in a reaction with an
S
N
2
mechanism.
- C
6
H
5
OH + OH
?
→ C
6
H
5
?O
?
+ H
2
O
- C
6
H
5
?O
?
+ R?X → C
6
H
5
OR
The
Ullmann condensation
is similar to the Williamson method except that the substrate is an aryl halide. Such reactions generally require a catalyst, such as copper.
[8]
Important ethers
[
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]
See also
[
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]
References
[
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]
- ^
IUPAC
,
Compendium of Chemical Terminology
, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "
ethers
".
doi
:
10.1351/goldbook.E02221
- ^
Saul Patai, ed. (1967).
The Ether Linkage
. PATAI'S Chemistry of Functional Groups. John Wiley & Sons.
doi
:
10.1002/9780470771075
.
ISBN
978-0-470-77107-5
.
- ^
Vojinovi?, Krunoslav; Losehand, Udo; Mitzel, Norbert W. (2004). "Dichlorosilane?Dimethyl Ether Aggregation: A New Motif in Halosilane Adduct Formation".
Dalton Trans.
(16): 2578?2581.
doi
:
10.1039/b405684a
.
PMID
15303175
.
- ^
a
b
Wilhelm Heitmann, Gunther Strehlke, Dieter Mayer "Ethers, Aliphatic" in
Ullmann's Encyclopedia of Industrial Chemistry
Wiley-VCH, Weinheim, 2002.
doi
:
10.1002/14356007.a10_023
- ^
J. F. W. McOmie and D. E. West (1973).
"3,3′-Dihydroxylbiphenyl"
.
Organic Syntheses
;
Collected Volumes
, vol. 5, p. 412
.
- ^
F.A.Cotton; S.A.Duraj; G.L.Powell; W.J.Roth (1986). "Comparative Structural Studies of the First Row Early Transition Metal(III) Chloride Tetrahydrofuran Solvates".
Inorg. Chim. Acta
.
113
: 81.
doi
:
10.1016/S0020-1693(00)86863-2
.
- ^
Clayden; Greeves; Warren (2001).
Organic chemistry
. Oxford University Press. p.
129
.
ISBN
978-0-19-850346-0
.
- ^
Frlan, Rok; Kikelj, Danijel (29 June 2006). "Recent Progress in Diaryl Ether Synthesis".
Synthesis
.
2006
(14): 2271?2285.
doi
:
10.1055/s-2006-942440
.
|
---|
Hydrocarbons
(only C and H)
| |
---|
Only
carbon
,
hydrogen
,
and
oxygen
(only C, H and O)
| |
---|
Only one
element,
not being
carbon,
hydrogen,
or oxygen
(one element,
not C, H or O)
| |
---|
Other
| |
---|
|