Question

Methanol is converted to formaldehyde by alcohol dehydrogenase. Would the alcohol dehydrogenase reaction be useful in determining whether wine had been adulterated with methanol? Explain. Please and thank you!

Answer 1

Alcohol dehydrogenase enzymes facilitate the conversion of alcohols to aldehydes or ketones with the reduction of nicotinamide adenine dinucleotide (NAD⁺-----> NADH). If the wine sample contains methanol then this enzyme will convert methanol to formaldehyde but also convert ethanol (the main component of alcohol) to acetaldehyde. Unfortunately, there is no test to determine the presence of formaldehyde in presence of acetaldehyde while converse is true. Iodoform test identifies alpha hydrogen atoms or the CH₃ groups directly attached to the carbonyl groups. The alpha hydrogen atoms are acidic and will react with a base to form anion. This anion will react with iodine to form alpha iodo product. The alpha iodo product is attacked by the base to give carboxylic acid and yellow coloured iodoform. Please find image for detailed mechanism of iodoform test. Thus acetaldehyde will show positive results while formaldehyde will show negative results with this test.

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A way to test the presence of methanol through the enzyme route will be to further oxidise the aldehyde to corresponding acids using aldehyde hydrogenase enzyme. Since formic acid contains H-C=O group similar to aldehydes, it will give positive results for most aldehyde tests such as Fehling’s test, Tollen’s test and Benedict’s test.

Tollen’s test: The Tollen's reagent contains diamminesilver complex which oxidises aldehyde to carboxylate ion. The carboxylate ion reduces Ag(I) to Ag(0) thus coating the inside of the test tube with a mirror-like layer of elemental silver. Hence this test is also called silver mirror test. Acetic acid, on a contrary, does not have H-C=O group and hence it cannot reduce Tollen’s reagent and will show negative results with this tests.

2[Ag(NH₃)₂]⁺ + HOCHO + H₂O ----> 2 Ag_(s) + 4 NH₃ +CO₂ + 2H⁺ + H₂O