Stoichiometry of oxidation indicates 2-electron transfer per molecule of X

Steps:

• Dichromate reduction accepts 6 electrons per mole of Cr₂O₇²⁻.
• One mole of dichromate oxidizes 3 moles of X, so 6 electrons total from 3 X, meaning 2 electrons per X.
• Oxidation half-equation for X involves net loss of 2H⁺ + 2e⁻.
• Propan-1-ol oxidizes to propanal (CH₃CH₂CH₂OH → CH₃CH₂CHO + 2H⁺ + 2e⁻), matching 2-electron change.

Why A is correct:

• Primary alcohol propan-1-ol loses 2 hydrogen atoms to form aldehyde, a 2-electron oxidation consistent with the 3:1 molar ratio and policy for hydrogen loss (H⁺ + e⁻ per H).

Why the others are wrong:

• B. Propan-1,2-diol has adjacent OH groups; oxidation affects both (4e⁻ for primary to acid + 2e⁻ for secondary to ketone = 6e⁻ total).
• C. Propan-1,3-diol has two primary OH groups, requiring 4e⁻ each for carboxylic acids (8e⁻ total).
• D. Propan-2-ol oxidizes to ketone (2e⁻), but conditions favor primary alcohol partial oxidation to aldehyde for the given ratio.

Final answer: A