Nucleophilic Addition of H2O Hydration

Treatment of an aldehyde or ketone with cyanide ion ( C=N), followed by protonation of the tetrahedral alkoxide ion intermediate, gives a cyanohydrin. Show the structure of the cyanohydrin obtained from cyclohexanone. 

Aldehydes and ketones react with water to yield 1,1-diols, or geminal (gem) diols. The hydration reaction is reversible, and a gem diol can eliminate water to regenerate an aldehyde or ketone. 

The position of the equilibrium between a gem diol and an aldehyde or ketone depends on the stmcture of the carbonyl compound. The equilibrium generally favors the carbonyl compound for steric reasons, but the gem diol is favored for a few simple aldehydes. For example, an aqueous solution of formaldehyde consists of 99.9% gem diol and 0.1% aldehyde at equilibrium, whereas an aqueous solution of acetone consists of only about 0.1% gem diol and 99.9% ketone. 

The negatively charged nucleophile OH adds to the electrophilic carbon and pushes tt electrons from the C=0 bond onto oxygen, giving an alkoxide ion. 

The alkoxide ion is protonated by water to give the neutral hydrate as the addition product and regenerating OH . 

Note the key difference between the base-catalyzed and acid-catalyzed reactions. The base-catalyzed reaction takes place rapidly because water is converted into hydroxide ion, a much better fmclcophile. I he acid-catalyzed reaction takes place rapidly because the carbonyl compound is converted by prolonation into a much better electrophile. 

The mechanism of acid-catalyzed hydration of an aldehyde or ketone. Acid protonates the carbonyl group, making it more electrophilic and more reactive. 

ThomsonNOW Click Organic Process to view an animation of the acid catalyzed hydration of a carbonyl. 

O The nucleophilic hydroxide ion adds to the aldehyde or ketone and yields a tetrahedral alkoxide ion intermediate. 

The nucleophilic addition of water to a ketone or aldehyde is slow in pure water but is catalyzed by both acid and base. Like all catalysts, acids and bases don t change the position of the equilibrium they affect only the rate at which the hydration reaction occurs. 

Mechanism of base-catalyzed hydration of a cetone or aldehyde. Hydroxide ion is a more reactive nucleophile than neutral water. 

Hydroxide ion nucleophile adds to the ketone or aldehyde carbonyl group to yield an aikoxide ion intermediate. 

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Nucleophilic addition of H2O to coordinated alkyne is the key step of catalytic alkyne hydration with traditional Hg(II) or Au(I) catalysts (Eq. 8.26) that convert terminal alkynes RC=CH to the methyl ketones RCOMe. As part of a general trend associated with the rise of green chemistry (Section 1.1), toxicity and expense concerns have led to the advent of base metal catalysts, such as a water-soluble Co(III) porphyrin. ... 

Hydration is simply another example of electrophilic addition. The first two steps of the mechanism are similar to those of electrophilic addition of HX—that is, addition of (from HgO" ) to generate a carbocation, followed by nucleophilic attack of H2O. Mechanism 10.2 illustrates the addition of H2O to cyclohexene to form cyclohexanol. 

Insertion of a methyl group at the site where nucleophilic attack (by OH or H2O) occurs during hydration considerably hinders the addition of water, thus lowering the percentage of the hydrated... 

Treatment of an alkene with mercuric acetate in aqueous THF results in the electrophilic addition of mercuric ion to the double bond to form an intermediate mercuri-um ion. Nucleophilic attack by H2O at the more substituted carbon yields a stable organomercury compound, which upon addition of NaBH4 undergoes reduction. Replacement of the caiton-mercury bond by a carbon-hydrogen bond during the reduction step proceeds via a radical process. The overall reaction represents Markovnikov hydration of a double bond, which contrasts with the hydroboration-oxidation process. 

In acidic media, polarized multiple bonds often undergo acid catalyzed addition, and a common mode of addition is the Ad 2. Deprotonation of the nucleophile by solvent gives the neutral compound. Common examples of this easily reversible Adg2 reaction are the formation of hydrates (NuH is H2O) and, if NuH is ROH, hemiacetals (from aldehydes) and hemiketals (from ketones). Usually this reaction favors reactants. 

The reactivity of hydroxide ion (and that of other oxy anions) is interpreted in terms of two unifying principles (1) the redox potential of the YO I YO- (Y = H, R, HO, RO, and O) couple (in a specific reaction) is controlled by the solvation energy of the YO anion and the bond energy of the R-OY product (RX + YO —> R-OY -I- X ), and (2) the nucleophilic displacement and addition reactions of YO occur via an inner-sphere single-electron shift.l The electron is the ultimate base and one-electron reductant, which, upon introduction into a solvent, is transiently solvated before it is "leveled" (reacts) to give the conjugate base (anion reductant) of the solvent. Thus, in water the hydrated electron (e )H2O yields HO via addition to the H-OH bond of water ... 

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