MCAT General Chemistry Question 159: Answer and Explanation

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Question: 159

9. As methanol is converted to methanal, and then methanoic acid, the oxidation number of the carbon:

A. increases.
B. decreases.
C. increases, then decreases.
D. decreases, then increases.

Correct Answer: A

Explanation:

The formula for methanol is H₃COH, for methanal is H₂CHO, and for methanoic acid is HCOOH. If we assign oxidation numbers to carbon in each molecule, it starts at -2, then becomes 0, then becomes +2:

In general, it is often easier to think of oxidation as a gain of bonds to oxygen (or a similarly electronegative element) or loss of bonds to hydrogen for organic compounds. Therefore, because the carbon is oxidized as one converts from an alcohol to an aldehyde to a carboxylic acid, the oxidation number must increase.

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