In alcohols, one of the two bonds to the oxygen atom involves hydrogen and one involves carbon. When two or more carbon atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. are present, however, isomeric structures in which oxygen is bonded to two different carbons become possible. Such compounds are called ethers. For example, dimethyl ether is isomeric with ethanol, and methyl ethyl ether is isomeric with propanol:
The general formula for an ether is R—O—R′, where R′ signifies that both R groups need not be the same.
EXAMPLE Draw projection formulas and name all the isomers which correspond to the molecular formulaThe chemical formula of a substance written using the subscripts that reflect the number of each kind of atom present in a molecule of the substance. For example, the simplest formula for ethane is CH3, but the molecular formula is C2H6 because there are 2 C atoms and 6 H atoms in a molecule of ethane. C3H8O.
SolutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. The formula C3H8 would correspond to an alkaneA hydrocarbon containing only single bonds between carbon atoms.. The extra oxygen atom might be added between two carbons, giving ether, or it might be added between a carbon and a hydrogen, giving an alcohol. The alcohol molecules might have the hydroxyl groupThe functional group of an oxygen atom bonded to an hydrogen atom, -OH; found in alcohols. at the end of the three-carbon chain or on the second carbon atom:
Only one ether structure is possible—that in which one methyl and one ethyl group are attached to oxygen:
In an ether there are no hydrogen atoms connected to a highly electronegative neighbor, and so, unlike alcohols, ether molecules cannot hydrogen bondAn attractive force, either intramolecular or intermolecular, between an electronegative atom and a hydrogen atom attached to another electronegative atom. among themselves. Each C—O bond is polarDescribes a molecule that has separated, equal positive and negative charges that consitute a positive and a negative pole; such a molecule tends to assume certain orientations more than others in an electric field., but the bonds are at approximately the tetrahedral angle. The polarity of one partially cancels the polarity of the other. Consequently the forces between two ether molecules are not much greater than the London forces between alkane molecules of comparable size. The boiling pointThe temperature at which the vapor pressure of a liquid equals the pressure of the gas in contact with the liquid; usually this is atmospheric pressure. of dimethyl ether, for example, is –23°C, slightly above that of propane (–42°C), but well below that of ethanol (78.5°C). All three molecules contain 26 electrons and are about the same size. In the table of the boiling points of comparable organic compounds we see this trend again, this time with compounds containing 32 or 34 electrons.
The chemical reactivity of ethers is also closer to that of the alkanes than that of the alcohols. Ethers undergo few characteristic reactions other than combustionVigorous combination of a material with oxygen gas, usually resulting in a flame., and so they are commonly used as solvents. Diethyl ether is also used as an anesthetic, although the flammability of its vaporThe gaseous state of a substance that typically exists as a liquid or solid; a gas at a temperature near or below the boiling point of the corresponding liquid. requires that precautions be taken to prevent fires.