NCERT · Class 12 · Chemistry · Aldehydes, Ketones and Carboxylic AcidsExplain the mechanism of nucleophilic addition reactions in aldehydes and ketones. Why do aldehydes generally react more readily than ketones in nucleophilic addition reactions? Discuss any two chemical tests to distinguish between aldehydes and ketones.

Mechanism of Nucleophilic Addition Reactions\nAldehydes and ketones contain a carbonyl group ($>C=O$) where the carbon-oxygen double bond is polarized due to the higher electronegativity of oxygen. The carbonyl carbon is electrophilic (electron-deficient) and the oxygen is nucleophilic (electron-rich).

• Step 1 (Attack of Nucleophile): A nucleophile ($Nu^-$) attacks the electrophilic carbonyl carbon from a direction perpendicular to the plane of the carbonyl group, forming a tetrahedral alkoxide intermediate ($>C(O^-)(Nu)$).
• Step 2 (Protonation): The alkoxide intermediate rapidly takes up a proton ($H^+$) from the reaction medium to form the neutral addition product, known as a cyanohydrin, bisulfite addition compound, etc.

Reactivity of Aldehydes vs. Ketones\nAldehydes are generally more reactive than ketones toward nucleophilic addition due to two main factors:

• Electronic Factors: Aldehydes have one alkyl group and one hydrogen atom, whereas ketones have two alkyl groups. Alkyl groups have an electron-donating inductive effect ($+I$ effect), which decreases the positive charge on the carbonyl carbon, making it less susceptible to nucleophilic attack. Since aldehydes have only one such group, their carbonyl carbon is more electrophilic.
• Steric Factors: Ketones have two bulky alkyl groups attached to the carbonyl carbon, which cause steric hindrance to the approaching nucleophile. Aldehydes have only one alkyl group and a smaller hydrogen atom, resulting in lesser steric hindrance and easier nucleophilic attack.

Distinguishing Tests

• 1. Tollens' Test: When an aldehyde is warmed with Tollens' reagent (ammoniacal silver nitrate solution), a bright silver mirror is produced on the inner walls of the test tube due to the formation of elemental silver. Ketones do not reduce Tollens' reagent.
• 2. Fehling's Test: When an aldehyde is heated with Fehling's solution (a mixture of Fehling A and Fehling B), a red-brown precipitate of cuprous oxide ($Cu_2O$) is formed. Ketones generally do not respond to this test.