See image — Aldehydes Ketones and Carboxylic Acids Chemistry Question

💡 Solution & Explanation

Concept: The equilibrium constant Keq for cyanohydrin formation (nucleophilic addition of HCN to a carbonyl compound) depends on the electrophilicity of the carbonyl carbon and the steric accessibility of the carbonyl group. A more electrophilic and less sterically hindered carbonyl carbon gives a higher Keq (reaction favors product more). Step 1 - Identify the carbonyl compounds: - a: PhCHO (benzaldehyde) - aldehyde, one H and one Ph group on carbonyl carbon - b: CH3-CO-C2H5 (methyl ethyl ketone / butан-2-one) - ketone, two alkyl groups (moderate steric hindrance) - c: Ph-CO-CH3 (acetophenone) - ketone, one Ph and one CH3; Ph conjugates with C=O reducing electrophilicity AND provides steric hindrance - d: CH3-CHO (acetaldehyde) - aldehyde, one H and one CH3; least sterically hindered, no conjugation reducing carbonyl electrophilicity Step 2 - Apply electronic and steric effects: - Aldehydes are more reactive than ketones toward nucleophilic addition because: (i) less steric hindrance (one H instead of two alkyl/aryl groups), and (ii) less electron donation to carbonyl carbon. - Among aldehydes: acetaldehyde (d) has only a methyl group; benzaldehyde (a) has a phenyl group that conjugates with C=O (resonance donation reduces electrophilicity). So d > a. - Among ketones: methyl ethyl ketone (b) has two alkyl groups (electron donating but less than aryl, less steric than aryl); acetophenone (c) has a phenyl group conjugated with C=O reducing electrophilicity significantly plus more steric bulk. So b > c. - Overall ranking: d (acetaldehyde, least hindered aldehyde) > a (benzaldehyde, aldehyde but Ph conjugation) > b (dialkyl ketone, more hindered) > c (acetophenone, aryl ketone with conjugation reducing electrophilicity most among ketones). Step 3 - Why other options fail: - Option (a) a > b > c > d: Incorrect because aldehydes are more reactive than ketones, so d and a should rank above b and c. - Option (c) d > b > a > c: Incorrect because benzaldehyde (a) is an aldehyde and should be more reactive than methyl ethyl ketone (b), a ketone. - Option (d) d > a > c > d: Contains a typo/repeat and is clearly incorrect. Therefore, the correct answer is B.