Introduction
Fig 1 Structure of ether
Ethers are a class of organic compounds which contain an ether group - an oxygen atom connected to two alkyl or aryl groups. They can be derived from alcohols in which H atoms of hydroxyl are replaced by alkyl groups. Ethers have the general formula R–O–R’ and they may exist in straight chain carbons or as part of a carbon ring. Ethers can be classified into two varieties: if the alkyl groups are the same on both sides of the oxygen atom, then it is a simple or symmetrical ether, whereas if the alkyl groups are different, the ethers are called mixed or unsymmetrical ethers. Ethers are common in organic chemistry and even more prevalent in biochemistry, as they are common linkages in carbohydrates and lignin.
Properties
Dimethyl ether and ethyl methyl ether are gases at ordinary temperature and their other homologues are usually colorless, volatile liquids with typical ether smell. The C-O bonds in ether are polar, so ethers are only slightly more polar than alkenes but not as polar as alcohols, esters, or amides of comparable structure. Ether molecules cannot form hydrogen bonds with each other, resulting in relatively low boiling points compared to those of the analogous alcohols. Most ethers are insoluble in water, due to their oxygen atoms are surrounded in the molecules, making it difficult to form hydrogen bonds with water. But cyclic ethers such as tetrahydrofuran and 1,4-dioxane are miscible in water because of the more exposed oxygen atom for hydrogen bonding as compared to linear aliphatic ethers. Ethers are appreciably soluble in organic solvents like alcohol, benzene, acetone etc. Ethers are generally very unreactive in nature. When an excess of hydrogen halide is added to the ether, cleavage of C-O bond takes place leading to the formation of alkyl halides.
R-O-R + HX → RX + R-OH
Applications
Reactants or solvent: Ethers can be used as excellent solvents, reactants or extraction agents applied in many chemistry reactions. They are considered to be nonpolar and thus can dissolve nonpolar substances. In addition, ethers are great solvents for fats, waxes, oils, perfumes, alkaloids and gums.
Dimethyl ether fuel: Dimethyl ether has been tested as a transportation fuel because it is relatively nontoxic and has a higher cetane number than diesel. A higher cetane number means that dimethyl ether will combust more cleanly and more completely than diesel fuel, optimizing energy efficiency for the car.
Application of polyethylene glycol (PEG): Polyethylene glycol (PEG) are a class of long-chained, contained ether bonds compounds with the chemical structure of H−(O−CH2−CH2)n−OH. Low molecule PEG chains can be used as laxatives, skin creams, lubricants, dispersants in toothpastes, thickening agents, and binding agents in tablets and molds. Larger molecule PEG chains are often used as packing materials for foods, binding agents and thickeners for paints, and polar stationary phases for gas chromatography.
Application of crown ethers: Crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups. Crown ethers can strongly bind certain cations, forming complexes. The oxygen atoms are well situated to coordinate with a cation located at the interior of the ring, whereas the exterior of the ring is hydrophobic. The resulting cations often form salts that are soluble in nonpolar solvents, and for this reason crown ethers are useful in phase transfer catalysis.
Fig 2 Polyethylene glycol (PEG)
Fig 3 Structures of common crown ethers
