James Richard Fromm
Two alcohol molecules will react under the proper conditions to produce compounds with the general formula R-O-R'. These compounds are called ETHERS. The other product of the reaction is water which is taken up by a dehydration agent.
2ROH ROR + H2O
Simple ethers are prepared commercially by dehydration of the corresponding alcohols by sulfuric acid:
A typical reaction is:
2CH3-CH2-OH --H2SO4 --> CH3-CH2-O-CH2-CH3 + H2O
2C2H5OH --H2SO4--> C2H5OC2H5 + H2O
The ethers are named by naming the radical on either side of the oxygen atom, and adding the word ether. The two side chains can be the same, of course, in which case the name is mentioned just once, but preceded by the prefix di-. Thus, in the preceding reaction, 2 moles of ethanol will react to form 1 mole of diethyl ether.
Ethers may also be named as OXY DERIVATIVES of the hydrocarbons. In the IUPAC nomenclature system, ethers are named using the general formula "alkoxyalkane", for example
If the ether is part of a more complex molecule, it is described as an alkoxy substituent, so -OCH3 would be considered a "methoxy-" group. The nomenclature of describing the two alkyl groups and appending "ether", e.g. "ethylmethyl ether" is a trivial usage.
The ether radicals for the first four alkanes are:
Another name for diethyl ether is ethoxyethane. Dimethyl ether is methoxymethane, and methylheptyl ether is 1-methoxyheptane.
|dimethyl ether||methylheptyl ether|
The structural formula for 1,4-dimethoxy-2-pentene is
Many ethers are used in industry as solvents. Diethyl ether has found widespread use as an anesthetic since 1846, in which application it is referred to simply as ether. It and other ethers are valuable solvents for gums, fats, waxes, and resins.
Ethers are highly flammable and have relatively high vapor pressures, so they constitute a significant fire hazard. Moreover, on standing in air many simple ethers react slowly with atmospheric oxygen to form unstable and highly explosive organic peroxides, a significant hazard in distillations which use ethers.
It would be uncommon (but correct) to name diethyl ether as ethoxyethane, but the name paraethoxybenzoic acid would be appropriate for
The two simplest aromatic ethers carry the nonsystematic names of anisole (methoxybenzene) and phenetole (ethoxybenzene).
More complex ether structures are prepared in multistep synthesis.
Ether molecules cannot form hydrogen bonds among each other, resulting in a relatively low boiling point comparable to that of the analogous alcohols. However, the differences in the boiling points of the ethers and their isometric alcohols become smaller as the carbon chains become longer, as the hydrophobic nature of the carbon chain becomes more predominant over the presence of hydrogen bonding.
Ethers are slightly polar as the C - O - C bond angle in the functional group is about 110 degrees, and the C - O dipole does not cancel out. Ethers are more polar than alkenes but not as polar as alcohols, esters or amides of comparable structure. However, the presence of two lone pairs of electrons on the oxygen atoms makes hydrogen bonding with water molecules possible, causing the solubility of alcohols (for instance, 1-butanol) and ethers (ethoxyethane) to be quite dissimilar.
Cyclic ethers such as tetrahydrofuran and 1,4-dioxane are totally miscible in water because of the more exposed oxygen atom for hydrogen bonding as compared to aliphatic ethers.
Ethers can act as Lewis bases. For instance, diethyl ether forms a complex with boron compounds, such as boron trifluoride diethyl etherate (BF3.OEt2). Ethers also coordinate to magnesium in Grignard reagents (RMgBr).
Not all compounds of the formula R-O-R are ethers
Ethers are not to be confused with the following classes of compounds with the same general structure R-O-R.
Primary, Secondary & Tertiary Ethers
The terms "primary ether", "secondary ether", and "tertiary ether" are occasionally used and refer to the carbon atom next to the ether oxygen. In a primary ether this carbon is connected to only one other carbon as in diethyl ether CH3-CH2-O-CH2-CH3. An example of a secondary ether is diisopropyl ether (CH3)2CH-O-CH(CH3)2 and that of a tertiary ether is di-tert-butyl ether (CH3)3C-O-C(CH3)3.
a primary ether, diethyl ether
a secondary ether, diisopropyl ether
a tertiary ether, di-tert-butyl ether
Polyethers are compounds with more than one ether group. While the term generally refers to polymers like polyethylene glycol and polypropylene glycol, low molecular compounds such as the crown ethers may sometimes be included.
|Polyethylene Glycol||Crown Ether|
Ethers can be prepared in the laboratory in several different ways.
R-OH + R'-OH R-O-R' + H2O
3R-OH + 3R'-OH R-O-R + R'-O-R + R'-O-R' + 3H2O
R-CH2-CH2(OH) R-CH=CH2 + H2O
R-O- + R-X R-O-R + X-
HO-C6H5 + OH- O--C6H5
O--C6H5 + R-X R-O-C6H5
R2C=CR2 + R-OH R2CH-C(-O-R)-R2
Cyclic ethers which are also known as epoxides can be prepared:
Structure of the polymeric diethyl ether peroxide
Ethers in general are of very low chemical reactivity. Organic reactions are:
|Ethylene Oxide||The smallest cyclic ether.|
|Dimethyl Ether||An aerosol spray propellant.|
|Diethyl Ether||A common low boiling solvent.|
|Dimethoxyethane (DME)||A high boiling solvent:|
|1,4-Dioxane||A cyclic ether and high boiling solvent.|
|Tetrahydrofuran (THF)||A cyclic ether, one of the most polar simple ethers that is used as a solvent.|
|Anisole (methoxybenzene)||An aryl ether and a major constituent of the essential oil of anise seed.|
|Crown Ethers||Cyclic polyethers that are used as phase transfer catalysts.|
|Polyethylene Glycol (PEG)||A linear polyether, e.g. used in cosmetics.|
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