A student oxidises propan-1-ol under reflux to produce propanoic acid. They wish to confirm that the product is indeed propanoic acid and that no propanal (an intermediate aldehyde) remains as a contaminant in the final mixture. Which of the following analytical or chemical methods would most reliably identify the presence of the propanoic acid product and confirm the absence of any propanal?
Infrared (IR) spectroscopy: checking for a broad O-H\text{O-H}O-H absorption band at 2500−3000 cm−12500-3000\text{ cm}^{-1}2500−3000 cm−1 to confirm the carboxylic acid, and the absence of the aldehyde C-H\text{C-H}C-H stretching doublet at 2720−2820 cm−12720-2820\text{ cm}^{-1}2720−2820 cm−1 to confirm the absence of propanal.
Traditional chemical testing with acidified potassium dichromate(VI): an orange-to-green colour change confirms that the reaction has gone to completion and that only propanoic acid is present.
Traditional Tollens' reagent test: the formation of a silver mirror confirms that the aldehyde has been successfully oxidised to the carboxylic acid.
Chemical testing with solid sodium hydrogencarbonate: the evolution of carbon dioxide gas confirms the presence of propanoic acid and proves that no unreacted propanal remains.