Two separate reactions are conducted in which a compound containing a ketone, an ester, and a carboxylic acid is reacted with borane (BH₃) in THF in one reaction and with NaBH₄ in methanol in the other. Which of the following explains why different products are observed?

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11ecba2d_127c_6761_a3bf_8b48679666f1_MD0008_00 Reduction of a functional group results in a gain of electrons from the transfer of a hydride, a decreased oxidation state of carbon, and a decreased number of C-O bonds. Several reagents can be used to reduce a functional group, including borane (BH₃) and NaBH₄. These reducing agents transfer one or more hydrides to the carbon atom being reduced. BH₃ reacts most readily with carboxylic acid carbonyl groups and therefore will selectively reduce carboxylic acids. As a result, the intermediate aldehyde will be further reduced to a primary alcohol. NaBH₄ reduces the reactive carbonyls-ketones and aldehydes-and will not reduce the less reactive esters and carboxylic acids. If a compound that contains a ketone, an ester, and a carboxylic acid is reacted with BH₃, the carboxylic acid will be selectively reduced to a primary alcohol, leaving the ketone and ester intact. When the same compound is reacted with NaBH₄, only the ketone will be reduced, generating a secondary alcohol. Therefore, the products of the two reactions will be different. (Choice A) BH₃ and NaBH₄ are both reducing agents, but BH₃ selectively reduces carboxylic acids rather than ketones, and NaBH₄ is not strong enough to reduce carboxylic acids. NaBH₄ selectively reduces ketones and aldehydes. (Choice B) Although it is true that BH₃ does not reduce ketones, NaBH₄ does not reduce esters. (Choice C) Esters cannot be reduced by BH₃ or NaBH₄; this functional group can only be reduced to a primary alcohol by LiAlH₄. Educational objective: Carboxylic acids can be selectively reduced to primary alcohols by the reducing agent BH₃. They cannot be reduced by NaBH₄, which is selective for the reactive carbonyls ketones and aldehydes.

Two Separate Reactions Are Conducted in Which a Compound Containing