Hydration of an Aldehyde or Ketone in Acid Solution

Hydration of an Aldehyde or Ketone in Acid Solution THE OVERALL REACTION  

Aldehyde or ketone Hydronium ion Conjugate acid of carbonyl compound Water 

Step 2 Nucleophilic addition to the protonated aldehyde or ketone 

The product of addition of hydrogen cyanide to an aldehyde or a ketone contains both a hydroxyl group and a cyano group bonded to the same carbon. Compounds of this type are called cyanohydrins. 

Mechanism 17.3 describing cyanohydrin formation is analogous to the mechanism of base-catalyzed hydration. The nucleophile (cyanide ion) bonds to the carbonyl carbon in the first step of the reaction, followed by proton transfer to the carbonyl oxygen in the second step. 

Step 3 Protcm transfer from the cmjugate acid of the geminal diol to a water molecule 

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FIGURE 17.6 The mechanism of hydration of an aldehyde or ketone in acidic solution. Hydronium ion is a catalyst it is consumed in the first step, and regenerated in the third. 

In a similar fashion to the formation of hydrate with water, aldehyde and ketone react with alcohol to form acetal and ketal, respectively. In the formation of an acetal, two molecules of alcohol add to the aldehyde, and one mole of water is eliminated. An alcohol, like water, is a poor nucleophile. Therefore, the acetal formation only occurs in the presence of anhydrous acid catalyst. Acetal or ketal formation is a reversible reaction, and the formation follows the same mechanism. The equilibrium lies towards the formation of acetal when an excess of alcohol is used. In hot aqueous acidic solution, acetals or ketals are hydrolysed back to the carbonyl compounds and alcohols. 

The oxidation of propylene has been chosen as a probe reaction to study the catalytic activity of Cu Pd -TSM. The olefin oxidation in an acidic solution of Cu(II) and Pd(U) chlorides, well known as the Wacker reaction, is achieved when olefins are selectively oxidized to ketones or aldehydes by hydrated Pd, leaving Pd . The Pd is oxidized back to Pd by 2Cu, and the resulting Cu is reoxidized by dissolved oxygen. Because the corrosive nature of the catalyst solution is a serious disadvantage for practical use, supported copper-palladium catalysts have been proposed to operate the reaction in a gas flow reactor (40). 

In addition to hydrogenation, alkynes can be hydrated into either a ketone or an aldehyde form. A (Markovnikov) ketone can be created from an alkyne using a solution of aqueous sulfuric acid (H2/H2SO4) and HgSO4, whereas the anti-Markovnikov aldehyde product requires different reagents and is a multi-step process. 

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