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Concept
Hydration reaction
Summary
In chemistry, a hydration reaction is a chemical reaction in which a substance combines with water. In organic chemistry, water is added to an unsaturated substrate, which is usually an alkene or an alkyne. This type of reaction is employed industrially to produce ethanol, isopropanol, and butan-2-ol.
Several million tons of ethylene glycol are produced annually by the hydration of oxirane, a cyclic compound also known as ethylene oxide:
C2H4O + H2O → HO–CH2CH2–OH
Acid catalysts are typically used.
For the hydration of alkenes, the general chemical equation of the reaction is the following:
RRC=CH2 + H2O → RRC(OH)-CH3
A hydroxyl group (OH−) attaches to one carbon of the double bond, and a proton (H+) adds to the other. The reaction is highly exothermic. In the first step, the alkene acts as a nucleophile and attacks the proton, following Markovnikov's rule. In the second step an H2O molecule bonds to the other, more highly substituted carbon. The oxygen atom at this point has three bonds and carries a positive charge (i.e., the molecule is an oxonium). Another water molecule comes along and takes up the extra proton. This reaction tends to yield many undesirable side products, (for example diethyl ether in the process of creating ethanol) and in its simple form described here is not considered very useful for the production of alcohol.
Two approaches are taken. Traditionally the alkene is treated with sulfuric acid to give alkyl sulphate esters. In the case of ethanol production, this step can be written:
H2SO4 + C2H4 → C2H5-O-SO3H
Subsequently, this sulphate ester is hydrolyzed to regenerate sulphuric acid and release ethanol:
C2H5-O-SO3H + H2O → H2SO4 + C2H5OH
This two step route is called the "indirect process".
In the "direct process," the acid protonates the alkene, and water reacts with this incipient carbocation to give the alcohol. The direct process is more popular because it is simpler. The acid catalysts include phosphoric acid and several solid acids.
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Ontological neighbourhood
Related concepts (16)
Properties of water
Water () is a polar inorganic compound that is at room temperature a tasteless and odorless liquid, which is nearly colorless apart from an inherent hint of blue. It is by far the most studied chemical compound and is described as the "universal solvent" and the "solvent of life". It is the most abundant substance on the surface of Earth and the only common substance to exist as a solid, liquid, and gas on Earth's surface. It is also the third most abundant molecule in the universe (behind molecular hydrogen and carbon monoxide).
Markovnikov's rule
In organic chemistry, Markovnikov's rule or Markownikoff's rule describes the outcome of some addition reactions. The rule was formulated by Russian chemist Vladimir Markovnikov in 1870. The rule states that with the addition of a protic acid HX or other polar reagent to an asymmetric alkene, the acid hydrogen (H) or electropositive part gets attached to the carbon with more hydrogen substituents, and the halide (X) group or electronegative part gets attached to the carbon with more alkyl substituents.
Addition reaction
In organic chemistry, an addition reaction is an organic reaction where two or more molecules combine to form a larger one (the adduct). Addition reactions are limited to chemical compounds that have multiple bonds, such as molecules with carbon–carbon double bonds (alkenes), or with triple bonds (alkynes), and compounds that have rings, which are also considered points of unsaturation. Molecules containing carbon—hetero double bonds like carbonyl () groups, or imine () groups, can undergo addition, as they too have double-bond character.