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Both aldehydes and ketones contain a carbonyl group. What structurally distinguishes the two?
AIn an aldehyde the carbonyl carbon bonds to a hydrogen; in a ketone, to two carbons
BAldehydes always contain additional nitrogen atoms in their structure
CKetones are always part of an aromatic ring system, aldehydes never are
DAldehydes contain no oxygen atoms anywhere in the molecule
Answer & Solution
Correct answer: A. In an aldehyde the carbonyl carbon bonds to a hydrogen; in a ketone, to two carbons
1. Both functional groups share the same C=O carbonyl core, so the distinction must lie in what else is attached to that carbon.
2. The text states: "In an aldehyde, the carbonyl group is bonded to at least one hydrogen atom. In a ketone, the carbonyl group is bonded to two carbon atoms."
3. Neither group involves nitrogen or aromatic rings by definition, and both clearly contain oxygen (the carbonyl oxygen) -- ruling out the other three options.
4. So the key structural difference is whether the carbonyl carbon is bonded to hydrogen (aldehyde) or to two carbons (ketone).
_Source: OpenStax Chemistry Ch 20 "Organic Chemistry", p.1121 §20.3 Aldehydes, Ketones, Carboxylic Acids, and Esters_
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