Which of these is an intermediate structure for an aldol condensation? (select all that apply)

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To tackle this question, we first need to recall the typical intermediates encountered in an aldol condensation mechanism.
Option: alkyl oxonium ion. This species is not part of the aldol mechanism; aldol reactions proceed via carbon–carbon bond formation between an enolate and an aldehyde or ketone, not through oxonium intermediates of this type.
Option: enolate ion. This is a classic intermediate formed when the alpha proton is abstracted to give a resonance-stabilized enolate, which then attacks a carbonyl compound to form the new C–C bond characteristic of aldol reactions.
Option: iminium ion. Iminium ions arise in other reaction contexts (like certain organocatalytic aldol variants or enamine catalysis) but are not the standard intermediates in the classic aldol condensation mechanism, which centers on enolates/enolization rather than iminium pathways.
Option: alpha-hydroxycarbonyl. While related to carbonyl chemistry, an alpha-hydroxycarbonyl is not the typical intermediate in the standard aldol sequence; it does not represent the bond-forming step or the dehydration step’s immediate intermediate in this process.
Option: beta-hydroxycarbonyl. This species directly results from the aldol addition step: after the enolate attacks, a newly formed beta-hydroxy carbonyl is generated and represents the classic intermediate before dehydration to yield the enone (alpha,beta-unsaturated carbonyl).
Option: gamma-hydroxycarbonyl. A gamma-hydroxycarbonyl would imply a different positional relationship than what arises in a canonical aldol reaction; it is not the standard intermediate here.
Option: alpha,beta-unsaturated carbonyl. This product form arises after dehydration of the beta-hydroxycarbonyl, not as an intermediate in the initial aldol addition step itself.
Option: beta,gamma-unsaturated carbonyl. This species is not part of the typical aldol mechanism; it does not correspond to the usual dehydration outcome or the bond-forming step in the standard pathway.
In summary, the two items that align with the established intermediates in an aldol condensation are enolate ion (the nucleophilic starting species) and beta-hydroxycarbonyl (the intermediate after nucleophilic addition but before dehydration). The remaining choices do not fit the canonical sequence or roles of intermediates in this reaction.

CHE 008B A01 SS1 2025 Lecture 9 - Reactions of Enolate Ions

Which of these is an intermediate structure for an aldol condensation? (select all that apply)

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