Primary alcohol oxidation to aldehyde followed by cyanohydrin formation

Steps:

• Propan-1-ol, CH₃CH₂CH₂OH, is a primary alcohol oxidized by acidified K₂Cr₂O₇.
• Reflux typically gives carboxylic acid, but taking the distillate captures the volatile intermediate aldehyde, propanal (CH₃CH₂CHO), preventing further oxidation.
• The distillate, propanal, contains a carbonyl group reactive toward nucleophiles.
• HCN, generated in situ from KCN, adds across the C=O bond of propanal to form the cyanohydrin.

Why B is correct:

• Cyanohydrin formation follows the general reaction RCHO + HCN → RCH(OH)CN; for R = CH₃CH₂, the product is CH₃CH₂CH(OH)CN.

Why the others are wrong:

• A: Product from acetaldehyde (CH₃CHO) cyanohydrin, not propanal.
• C: Propanenitrile from amide dehydration or ethyl halide + CN⁻, not carbonyl addition.
• D: Butanenitrile from butanal cyanohydrin or propyl halide + CN⁻, one extra carbon.

Final answer: B