I have a question regarding professional chemical engineering; experts, please help!
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In chemistry, aldehydes can undergo the Pechmann reaction and the Cannizzaro reaction with aldehydes that lack α-hydrogens reactive toward oxidation. What is the essence of these reactions? Do they occur simultaneously in most cases? Could someone with expertise help answer this?
Reply to 1# Xiao Gong 1985: The Zhishenko reaction and the Cannizzaro reaction. 1. The Zhishenko reaction is an organic reaction in which an aldehyde lacking α-active hydrogen undergoes disproportionation to form esters in the presence of an alkoxide. Reaction mechanism: Two successive nucleophilic addition reactions of the alkoxy anion to the aldehyde. 2. Cannizzaro reaction: The Cannizzaro reaction is an organic dismutation or cross-dismutation reaction in which aldehydes lacking α-hydrogen atoms undergo an intermolecular redox reaction under the action of a strong base, resulting in the formation of one molecule of carboxylic acid and one molecule of alcohol. Aldehydes commonly used in the Cannizzaro reaction include aromatic aldehydes (such as benzaldehyde) and formaldehyde. For aldehydes with reactive hydrogens, a base will remove these reactive hydrogens, thereby triggering an aldol reaction and reducing the yield of the Cannizzaro reaction. This reaction is commonly used to prepare aromatic carboxylic acids. An intramolecular Cannizzaro reaction can also occur; sometimes the resulting hydroxy acid can further lose water and cyclize to form a lactone. The reaction mechanism begins with a nucleophilic addition of the base to the carbonyl group; the resulting tetrahedral intermediate then reacts with a strong base, losing a proton to become a dianion (the Cannizzaro intermediate). 3. Comparison: The two are quite similar—both involve nucleophilic addition reactions; the only difference is that the product of the former is an ester, while that of the latter is a carboxylic acid and an alcohol.