Dehydration (chemistry) In chemistry, dehydration means removing a molecule of water from a molecule or molecules containing hydrogen and oxygen. It includes both removal of a water molecule from one molecule (intramolecular dehydration), and a condensation reaction between two molecules that has water as a byproduct. Dehydration is a chemical reaction. Removing water from a mixture like a solution is a different process called distillation or desiccation. Hydrolysis, where water is added to break apart a molecule, is the opposite of condensation type dehydration. A desiccant can be used to remove water made by a dehydration reaction and avoid the reverse reactions. Examples Alcohols can be dehydrated using a strong acid (usually sulfuric acid) as a catalyst. Ethanol can go through both types of dehydration: intramolecular dehydration makes ethylene,[1] and condensation makes diethyl ether. References - ↑ Zhang, Minhua; Yu, Yingzhe (2013). "Dehydration of Ethanol to Ethylene". Industrial & Engineering Chemistry Research. 52 (28): 9505–9514. doi:10.1021/ie401157c. In chemistry, a dehydration reaction is a chemical reaction that involves the loss of an H2O from the reacting molecule(s) or ion(s). This reaction results in the release of the H2O as water. When the reaction involves the coupling of two molecules into a single molecule it is referred to as a condensation reaction. Dehydration reactions are common processes in the manufacture of chemical compounds as well as naturally occurring within living organisms. The reverse of a dehydration reaction is called a hydration reaction. The reverse of a condensation reaction yielding water is called hydrolysis. RCO2H + R′OH ⇌ RCO2R′ + H2O Often such reactions require the presence of a dehydrating agent, i.e. a substance that reacts with water. CH3CH2OH → H2C=CH2 + H2O The reaction is accelerated by acid catalysts such as sulfuric acid and certain zeolites. These reactions often proceed via carbocation intermediates as shown for the dehydration of cyclohexanol. Some alcohols are prone to dehydration. 3-Hydroxylcarbonyls, called aldols, release water upon standing at room temperature: RC(O)CH2CH(OH)R' → RC(O)CH=CHR' + H2O The reaction is induced by dehydrating reagents. For example, 2-methyl-cyclohexan-1-ol dehydrates to 1-methylcyclohexene in the presence of Martin's sulfurane, which reacts irreversibly with water. Double dehydration is illustrated by the conversion of glycerol to acrolein: {\displaystyle {\ce {CaSO4.2H2O +{}}}} heat ⟶ {\displaystyle {\ce {->}}} CaSO 4 ⋅ 1 2 H 2 O + 1 1 2 H 2 O {\displaystyle {\ce {CaSO4.1/2H2O + 1 1/2H2O}}} (released as steam). The resulting dry powder is ready to be mixed with water to form a stiff but workable paste that hardens.

sulphuric acid is extensively employed. Its powerful affinity for the elements of water makes it a valuable dehydrating and condensation agent . It extracts It boils at 338°, and at about 400° the vapour dissociates into sulphur trioxide and water; at a red heat further decomposition ensues, the sulphur trioxide dissociating into the dioxide and water. It freezes to a colourless crystalline mass, melting at 10·5°. The acid is extremely hygroscopic, absorbing moisture from the atmosphere with great rapidity; hence it finds considerable application as a desiccating agent. The behaviour of aqueous solutions of sulphuric acid is very interesting. The pure acid, however, may be obtained by strongly cooling this hydrate.  ​ when it separates in the form of white crystals, which melt at lo-5°, and on gentle heating evolve sulphur trioxide and again form the same hydrate. When strong sulphuric acid is mixed with water there is a great development of heat; the heat evolved when four parts of acid are mixed with one of water being sufficient to raise the temperature from 0° to 100° C. (Hence the laboratory precaution of always adding the acid to the Water and not the water to the acid.) In addition to the heat evolution there is also a diminution in volume, the maximum occurring when the components are present in the ratio H 2 SO 4 :2H 2 O, thus pointing to the existence of a hydrate H 2 SO 4 ,2H 2 O. A second hydrate, H 2 SO 4 , H 2 O, may be obtained as rhombic crystals, which melt at 7° and boil at 205°, by diluting the strong acid until it has a specific gravity of 1·78, and cooling the mixture; this compound is sometimes known as glacial sulphuric arid. Both the mono- and di-hydrates form freeing mixtures with snow. Other hydrates have also been described. The power which sulphuric acid exhibits for expelling other acids from their combinations, a power occasioned by its comparative in volatility and high degree of avidityjforms the basis of a considerable number of commercial processes. Hydrochloric, hydro bro mic, hydriodic, hydrofluoric. nitric, phosphoric and many other acids are manufactured by the action of sulphuric acid on their salts; the alkali and chlorine industries, and also the manufacture of bromine and iodine, employ immense quantities of this acid. In organic chemistry sulphuric acid is extensively employed. Its powerful affinity for the elements of water makes it a valuable dehydrating and condensation agent. As a eneral class, the sulphates are soluble in water, and exhibit well crystallized forms. Of the most insoluble we may notice the salts of the metals of the alkaline earths, barium, strontium and calcium, barium sulphate being practically insoluble, and calcium sulphate sparingly but quite appreciably soluble. Lead sulphate is very slightly soluble in water, soluble in strong' sulphuric acid, and almost insoluble in alcohol. Until recently the only agent practically used for this purpose was furnished by the oxides of nitrogen; more recently other oxygen carriers, acting by “ contact processes, " have also come into use (see below). The production of sulphuric acid by the assistance of the oxides of the nitrogen is carried out in the “ vitriol chambers.” These are immense receptacles, mostly from 100 to 200 ft. long, 20 to 30 ft. wide, and 15 to 25 ft. high, constructed of sheet-lead, the joints of the sheets being made by “burning ” or autogenous soldering, Le. fusing them together by a blow-pipe without the aid of solder (which would be quickly destroyed by the acid). For similar reasons it is necessary to employ much more water than is required to form H2S04; and this is all the more necessary as strong sulphuric acid dissolves the nitrous compounds in the shape of nitroso-sulphuric acid, and thus withdraws these oxygen carriers from the gas-space of the chambers where the necessary reactions take place. It follows from this that the acid collecting at the bottom of the chambers must never exceed a certain concentration, say 70 %, H2502 having a specific gravity of I-615, but it is preferable to make it only 66 to 67 /, , having a specific gravity of 1-57 to I-58. by decomposing ordinary sulphuric acid by a high temperature into SOQ, 0, and H20 (the last of course being in the shape of steam), absorbing the water by sulphuric acid, and causing the S02 and 0 to combine to S05 by means of moderately heated platinum in a fine state of division. Winkler showed that this division was best obtained by soaking asbestos with a solution of platinum chloride and reducing the platinum to the metallic state, and he described later a specially active kind of “ contact substance, " prepared from platinum chloride at a low temperature. Therapeutics-For external use, sulphuric acid is a powerful irritant and caustic, acting by its powerful affinity for water and therefore dehydrating the tissues and causing them to turn black. It coagulates the albumen. Strong sulphuric acid is occasionally used as a caustic to venereal sores, warts and malignant growths. It is difficult, however, to limit its action, and glacial acetic and nitric acids are preferable for this purpose. Considerable burns on the face or body may result from the application of sulphuric acid in the practice known as “vitriol-throwing, " a brownish black eschar serving to distin uisl, the burns produced by this acid from those of other corrosive guids. Toxicology-Given in toxic doses or in strong solution, sulphuric acid is a severe gastro-intestinal irritant, causing intense burning pain, extending from the mouth to the stomach, and vomiting of mucous and coffee-coloured material. The effects of the ingestion of large quantities may be so rapid that death may take place in a couple of hours, owing to collapse, consequent on perforation of the walls of the oesophagus or stomach, or from asphyxia due to swelling of the glottis consequent on some of the acid having entered the larynx.