The term "dehydration" is used in medicine (means dehydration of the body), in aviation (this is the loss of water by an airplane during a flight), in chemistry (a reaction as a result of which water is split off from an organic molecule). The dehydration reaction of alcohols is characteristic of compounds previously called alcohols. The process proceeds when heated in the presence of water-removing agents, which are sulfuric acid, zinc chloride, aluminum oxide or phosphoric acid. Unsaturated hydrocarbons or ethers may form depending on the reaction conditions, and it is divided into intramolecular and intermolecular dehydration, respectively.
Ethers are formed when the reaction is carried out at a lower temperature and an excess of alcohol. At the first stage, a sulfuric acid ester is obtained: C2H5OH + HO — SO2OH ↔ C2H5O — SO2OH + H2O. If the reaction mixture is then heated, the resulting ethyl sulfuric acid reacts with an excess of alcohol: C2H5O – SO2OH + C2H5OH → C2H5 – O – C2H5 + H2SO4. The result is diethyl (or sulfur) ether. According to this scheme, intermolecular dehydration of alcohols proceeds. As a result, due to the separation of two hydrogen atoms and one oxygen atom from two alcohol molecules (they make up one water molecule), ethers are formed. In industry, diethyl ether is produced by passing alcohol vapor over alumina at a temperature of 250 ° C. The reaction scheme is as follows: 2C2H5 — OH → C2H5 — O — C2H5 + H2O.
Intramolecular dehydration of alcohols can occur only at higher temperatures. In this case, a water molecule is separated from one alcohol molecule (and not from two, as in the previous case), and one double bond is formed between two carbon atoms. Such a method is used to produce unsaturated hydrocarbons. Only monohydric alcohols are used for this purpose. For example, the removal of water from ethyl alcohol proceeds at a temperature of 500 ° C over a water-taking catalyst, for example, aluminum oxide Al2O3 or zinc chloride ZnCl2.
Dehydration of alcohols is an easy way to produce gaseous alkenes in the laboratory. It is used to produce ethylene from ethanol: CH3 — CH2 — OH → CH2 = CH2 + H2O. Dehydration can occur in the presence of a catalyst, which is alumina. Vapors of ethyl alcohol are passed over heated alumina powder. Separated water in the form of steam is absorbed, while pure ethylene is released. As a catalyst for this process, concentrated acids, for example sulfuric or phosphoric, can be used. They, like aluminum oxide, have water-taking properties. But since sulfuric acid is also a strong oxidizing agent, many by-products are formed (for example, alcohol is oxidized to carbon dioxide, and the acid is reduced to sulfur dioxide), so the resulting gas requires additional purification.
Dehydration of cyclic alcohols, for example, dehydration of cyclohexanol, can occur in the presence of phosphoric acid. It is often preferred sulfuric, as it, having good water-taking properties, to a lesser extent contributes to the formation of by-products, and is also safer. As a result of this reaction, cyclohexene is formed. The fact that carbon atoms are combined into a ring does not change the chemistry of the reaction: C6H11 – OH → C6H10 + H2O. Cyclohexanol is heated with concentrated phosphoric (V) acid. The obtained cyclic hydrocarbon with one double bond in the ring is a liquid substance; therefore, it is distilled to remove impurities.
Dehydration of alcohols containing several hydroxyl groups in the molecule is also possible. As an example for polyhydric alcohols, one can consider the reaction, as a result of which two water molecules are split off from two ethylene glycol molecules, and a cyclic ether dioxane is formed: 2OH – CH2 – CH2 – OH → (C2H4O) 2. Dehydration occurs during the distillation of ethylene glycol in the presence of sulfuric acid. This is one of the industrial technologies for the production of dioxane.
Thus, we can say that the dehydration of alcohols has both industrial and laboratory applications. As a result, chemical compounds are formed, which are used for research, as well as raw materials or auxiliary reagents for chemical production.