See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: The Cannizaro reaction involves the disproportionation of an aldehyde lacking alpha-hydrogen in the presence of a base (OH-). For glyoxal (OHC-CHO), the intramolecular Cannizaro reaction (also called the benzilic acid-type or intramolecular hydride shift) is being considered here at normal (not high) base concentration. Step 1 - Identify the reaction type: The shown reaction is the first step of the Cannizaro mechanism for glyoxal - nucleophilic addition of OH- to one aldehyde carbon to form a tetrahedral intermediate (gem-diol anion), followed by intramolecular hydride transfer. Step 2 - Determine the rate-determining step and kinetics: In the Cannizaro reaction at normal (low/moderate) base concentration, the rate-determining step is the attack of OH- on the aldehyde. The rate law is: rate = k[aldehyde][OH-]. This gives an overall second-order reaction - first order in aldehyde and first order in hydroxide. Step 3 - Order determination: Since rate = k[OHC-CHO][OH-], the overall order = 1 + 1 = 2. Step 4 - Why not other options: At high concentration of base, the Cannizaro reaction can become third order (rate = k[aldehyde][OH-]^2) because a second OH- assists in the hydride transfer step. But the question specifies normal concentration of base, where only one OH- is involved in the rate-determining step, giving second-order kinetics. Option (a) first order is too simple. Options (c) and (d) apply to high base concentration conditions or different mechanisms. Therefore, the correct answer is B.