iitdreams.com

ETHERS

Alcohols (ROH) can be thought of as derivatives of water in which one of the hydrogen atoms has been replaced by an alkyl group. If both of the hydrogen atoms are replaced by alkyl groups, we get an ether (ROR). These compounds are named by adding the word ether to the names of the alkyl groups.

Diethyl ether, often known by the generic name “ether,” was once used extensively as an anesthetic. Because mixtures of diethyl ether and air explode in the presence of a spark, ether has been replaced by safer anesthetics.

Alcohols versus Ethers

There are important differences between both the physical and chemical properties of alcohols and ethers. Consider diethyl ether and 1-butanol, for example, which are constitutional isomers with the formula C₄H₁₀O.

The shapes of these molecules are remarkably similar, as shown in the figure below.

The fundamental difference between these compounds is the presence of OH groups in the alcohol that are missing in the ether. Because hydrogen bonds can’t form between the molecules in the ether, the boiling point of this compound is more than 80ºC lower than the corresponding alcohol. Because there are no hydrogen bonds to organize the structure of the liquid, the ether is significantly less dense than the corresponding alcohol.

Ethers can act as a hydrogen-bond acceptor, as shown in the figure below. But, they can’t act as hydrogen-bond donors. As a result, ethers are less likely to be soluble in water than the alcohol with the same molecular weight.

The absence of an OH group in an ether also has important consequences for its chemical properties. Unlike alcohols, ethers are essentially inert to chemical reactions. They don’t react with most oxidizing or reducing agents, and they are stable to most acids and bases, except at high temperatures. They are therefore frequently used as solvents for chemical reactions.

Compounds that are potential sources of an H⁺ ion, or proton, are often described as being protic. Ethanol, for example, is a protic solvent.

CH₃CH₂OH(aq) + H₂O(l) H₃O⁺(aq) + CH₃CH₂O^-(aq)

Substances that can’t act as a source of a proton are said to be aprotic. Because they don’t contain an OH group, ethers are aprotic solvents.

Ethers can be synthesized by splitting out a molecule of water between two alcohols in the presence of heat and concentrated sulfuric acid.

		H+
	2 CH3CH2OH		CH3CH2OCH2CH3

They can also be formed by reacting a primary alkyl halide with an alkoxide ion.