Addition of water. Hydration

If an acid catalyst is present, water adds to alkenes. It adds as H—OH, and the products are alcohols. 

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Bromine solution (red-brown) is added to a saturated hydrocarbon (left) and an unsaturated hydrocarbon (right). 

An acid catalyst is required in this case because the neutral water molecule is not acidic enough to provide protons to start the reaction. The stepwise mechanism for this reaction is given later in eq. 3.20. Hydration is used industrially and occasionally in the laboratory to synthesize alcohols from alkenes. 

Aldehydes and ketones undergo a reversible reaction with water to yield 1,1-diols (otherwise known as geminal (gem) diols or hydrates). 

The addition of water is slow but can be catalysed by bases or acids. Base catalysis 

Electron-donating and bulky substituents attached to the carbonyl group decrease the percentage of 1,1-diol present at equilibrium, whereas 

Treatment of an alkene with a strong acid, such as sulfuric acid, that has a relatively nonnucleophilic conjugate base results in the addition of the elements of water (H and OH) to the double bond. This reaction has many similarities to the addition of the halogen acids described in Section 11.2. First H+ adds to produce a carbocation and then water acts as the nucleophile. The reaction follows Markovnikov s rule and the stereochemistry is that expected for a reaction that involves a carbocation—loss of stereochemistry. Some examples are provided in the following equations. Note that the mechanism is the exact reverse of the El mechanism for acid-catalyzed dehydration of alcohols described in Section 10.13. 

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Values of Kadd for the addition of water (hydration) of alkenes to give the corresponding alcohols. These equilibrium constants were obtained directly by determining the relative concentrations of the alcohol and alkene at chemical equilibrium. The acidity constants pATaik for deprotonation of the carbocations by solvent are not reported in Table 1. However, these may be calculated from data in Table 1 using the relationship pA ik = pATR + logA dd (Scheme 7). 

Acetylcholinesterase is the primary target of these drugs, but butyrylcholinesterase is also inhibited. Acetylcholinesterase is an extremely active enzyme. In the initial catalytic step, acetylcholine binds to the enzyme s active site and is hydrolyzed, yielding free choline and the acetylated enzyme. In the second step, the covalent acetyl-enzyme bond is split, with the addition of water (hydration). The entire process occurs in approximately 150 microseconds. 

Mechanism 8-2 Ionic Addition of HX to an Alkene 332 Mechanism 8-3 Free-Radical Addition of HBr to Alkenes 334 8-4 Addition of Water Hydration of Alkenes 337... 

S Addition of water (hydration) anti-Markovnikov addition... 

Nucleophilic acyl addition of water (hydration) to a carbonyl group of an aldehyde or a ketone forms a geminal diol, commonly abbreviated gem-diol. 

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