Carbonyl Compounds

(i) Aldol Condensation:
Aldol condensation is a reaction in which aldehydes or ketones having -hydrogen atoms undergo self-condensation in the presence of a dilute base (NaOH, KOH) to form -hydroxy aldehydes (aldols) or -hydroxy ketones. On heating, these dehydrate to form -unsaturated aldehydes or ketones.
(ii) Decarboxylation:
Decarboxylation is the elimination of a carboxyl group ( ) from carboxylic acids as carbon dioxide. This generally takes place in the presence of soda lime (NaOH + CaO).
This reaction is useful for decreasing the carbon chain length by one carbon atom. (iii) Cannizzaro Reaction:
Aldehydes which do not have -hydrogen undergo self-oxidation and reduction (disproportionation) in the presence of concentrated alkali to form a primary alcohol and a carboxylate salt.
Example: Formaldehyde gives methanol and sodium formate. Conclusion:
Aldol condensation forms C–C bonds, decarboxylation removes groups, and Cannizzaro reaction gives alcohols and salts from aldehydes without -hydrogen.

(i) Claisen Reaction:
The Claisen condensation is the reaction of two esters or one ester and another carbonyl compound in the presence of a strong base (like sodium ethoxide) to form a -keto ester or a -diketone.
This reaction is important in C–C bond formation in organic synthesis. (ii) Cannizzaro’s Reaction:
Aldehydes without -hydrogen undergo self-oxidation and reduction (disproportionation) in the presence of concentrated alkali to form a primary alcohol and a carboxylate salt.
Example: Formaldehyde gives methanol and sodium formate. (iii) Rosenmund’s Reaction:
This reaction involves the hydrogenation of acid chlorides in the presence of palladium (Pd) catalyst on barium sulfate (BaSO ) to produce aldehydes.
This method is used for the controlled synthesis of aldehydes from acyl chlorides. Conclusion:
Claisen reaction forms -keto esters, Cannizzaro’s reaction converts aldehydes without -H into alcohols and acids, and Rosenmund’s reaction selectively reduces acyl chlorides to aldehydes.

Step 1: General case of ketones.
Normally, ketones cannot reduce Fehling’s solution or Tollen’s reagent because they cannot be easily oxidized under mild conditions. Only aldehydes undergo such mild oxidation. Step 2: Special case of fructose.
Fructose is a ketohexose, but in alkaline medium it undergoes
tautomerization (Lobry de Bruyn–van Ekenstein transformation) to form glucose and mannose, both of which are aldehydes. Step 3: Reaction with reagents.
Since glucose and mannose contain aldehyde groups, they reduce Fehling’s solution and Tollen’s reagent. Thus, fructose indirectly gives positive tests. Conclusion:
Ordinary ketones do not reduce Fehling’s or Tollen’s reagent, but fructose does due to tautomerization into aldoses in alkaline medium.