Dehydration of alcohols

The term "dehydration" is used in medicine (means dehydration of the body), in aviation (this is the loss of water by airplane during the flight), in chemistry (a reaction that splits water from a molecule of organic matter). The reaction of dehydration of alcohols is characteristic of compounds formerly called alcohol. The process takes place when heated in the presence of water-removing agents, which are sulfuric acid, zinc chloride, aluminum oxide or phosphoric acid. Depending on the reaction conditions, unsaturated hydrocarbons or ethers may be formed, and it is subdivided accordingly into intramolecular and intermolecular dehydration.

Ethers are formed when the reaction is carried out at a lower temperature and an excess of alcohol. The first stage produces a sulfuric acid ester: C2H5OH + HO-SO2OH ↔ C2H5O-SO2OH + H2O. If the reaction mixture is then heated, the resulting ethylsulfuric acid reacts with an excess of alcohol: C2H5O-SO2OH + C2H5OH → C2H5-O-C2H5 + H2SO4. As a result, diethyl (or sulfur) ether is obtained. According to this scheme, intermolecular dehydration of alcohols proceeds. As a result, due to the separation of two molecules of alcohol, two hydrogen atoms and one oxygen atom (they constitute one molecule of water), ethers are formed. In industry, diethyl ether is produced by passing pairs of alcohol 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, from one molecule of alcohol (rather than from two, as in the previous case), the water molecule separates, and between the two carbon atoms, one double bond is formed. This method is used to produce unsaturated hydrocarbons. Only monohydric alcohols are used for this purpose. For example, the splitting off of water from ethyl alcohol proceeds at a temperature of 500 ° C above the water-removal catalyst, for example, aluminum oxide Al2O3 or zinc chloride ZnCl2.

Dehydration of alcohols is a simple way of obtaining gaseous alkenes in laboratory conditions. 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. The vapors of ethyl alcohol are passed over the heated aluminum oxide powder. The separated water in the form of steam is absorbed, while pure ethylene is released. As the catalyst of this process, concentrated acids, for example sulfuric or phosphoric, can be used. They, like aluminum oxide, have water-removing properties. But since sulfuric acid is also a strong oxidizer, 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 take place in the presence of phosphoric acid. It is often preferred for sulfuric, since it, having good water-removing properties, is less conducive to the formation of by-products, and also safer. As a result of this reaction, cyclohexene is formed. The fact that the carbon atoms are combined into a ring does not change the reaction chemistry: C6H11-OH → C6H10 + H2O. Cyclohexanol is heated with concentrated phosphoric (V) acid. The resulting cyclic hydrocarbon with one double bond in the ring is a liquid substance, so it is distilled to remove impurities.

It is also possible to dehydrate alcohols containing several hydroxyl groups in the molecule. As an example for polyhydric alcohols, one can consider the reaction resulting in the splitting of two water molecules from two molecules of ethylene glycol, with the formation of a cyclic ether of dioxane: 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 producing dioxane.

Thus, it can be said that dehydration of alcohols has both industrial and laboratory applications. As a result, chemical compounds are formed, which are used for research, and also as raw materials or auxiliary chemicals for chemical production.