Editing Hydration reaction (section)

=== Alkenes===
The general [[chemical equation]] for the hydration of alkenes is the following:
:RRC=CH<sub>2</sub>  + H<sub>2</sub>O → RRC(OH)-CH<sub>3</sub>
A [[hydroxyl]] group (OH<sup>&minus;</sup>) attaches to one carbon of the double bond, and a [[proton]] (H<sup>+</sup>) 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 H<sub>2</sub>O [[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 ion|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 [[Sulfate ester|sulphate esters]]. In the case of ethanol production, this step can be written:
:H<sub>2</sub>SO<sub>4</sub> + C<sub>2</sub>H<sub>4</sub> → C<sub>2</sub>H<sub>5</sub>-O-SO<sub>3</sub>H
Subsequently, this sulphate ester is hydrolyzed to regenerate sulphuric acid and release ethanol:
:C<sub>2</sub>H<sub>5</sub>-O-SO<sub>3</sub>H + H<sub>2</sub>O → H<sub>2</sub>SO<sub>4</sub> + C<sub>2</sub>H<sub>5</sub>OH 
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 acid]]s.<ref name="Ullmann1"/>
Here an example reaction mechanism of the hydration of 1-methylcyclohexene to 1-methylcyclohexanol:
[[File:Alkene hydration reaction.svg|center|450px|Hydration reaction mechanism from 1-methylcyclohexene to 1-methylcyclohexanol.]]
Many alternative routes are available for producing alcohols, including the [[hydroboration–oxidation reaction]], the [[oxymercuration reaction#Oxymercuration–reduction|oxymercuration–reduction reaction]], the [[Mukaiyama hydration]], the reduction of ketones and aldehydes and as a biological method [[fermentation]].