Question

Benzaledehyde is oxidized to benzoic acid by potassium permanganate in an acid medium. The rate of the reaction is found to be directly proportional to the concentration of H+, benzaledehyde, and KMnO₄. When the oxidation is carried out with C₆H₅CDO, the rate is much slower than with C₆H₅CHO. The oxidation of C₆H₅CHO in H₂O¹⁸ and MnO₄^- gives the product C₆H₅COOH, whereas the same reaction in the presence of Mn¹⁸O₄^- gave benzoic acid containing ¹⁸O. What important mechanistic conclusions can you draw from the above experimental observations and results?

Answer 1

C₆H₅-CHO + H₂O* + MnO₄^- C₆H₅-COOH

C₆H₅-CHO + H₂O + MnO*₄^- C₆H₅-COO*H

When benzyldehyde is reacted with MnO₄^- , then one oxygen atom from MnO₄^- is transfered to benzyldehyde to form benzoic acid, where as oxygen atom of H₂O molecule donot get transfered to the product.

Concentration of MnO*₄^- determines the rate of reaction.

The rate of reaction for the above reaction depends on the mass of atom attacted to the carbonyl carbon (-CHO or -CDO. In present case, when D (atomic mass = 2) and H (atomic mass = 1 ) are there, the lower mass that is H attached to carbonyl carbon, the rate of reaction is faster.