Cross-aldol reaction between benzaldehyde and acetaldehyde under base yields which α,β-unsaturated aldehyde after dehydration?

Master NCEA Level 3 Organic Chemistry Reaction Schemes. Prepare with multiple choice questions, complete with hints and detailed explanations. Ace your exam with our comprehensive tools!

Multiple Choice

Cross-aldol reaction between benzaldehyde and acetaldehyde under base yields which α,β-unsaturated aldehyde after dehydration?

In an aldol condensation under base, the donor aldehyde forms an enolate, which then attacks the carbonyl of the other aldehyde to make a β-hydroxy aldehyde. Dehydration of that intermediate gives an α,β-unsaturated aldehyde. Here, acetaldehyde provides the enolate because benzaldehyde has no α-hydrogens to form its own enolate, so the reaction is a cross-aldol with acetaldehyde as the donor and benzaldehyde as the electrophile. After dehydration, the conjugated product is formed: cinnamaldehyde (Ph–CH=CH–CHO), an α,β-unsaturated aldehyde. This compound is also known as 3-phenylprop-2-enal, which is why that name appears among the options. The dehydration step is driven by the stability of the conjugated system with the phenyl ring and the aldehyde.

In an aldol condensation under base, the donor aldehyde forms an enolate, which then attacks the carbonyl of the other aldehyde to make a β-hydroxy aldehyde. Dehydration of that intermediate gives an α,β-unsaturated aldehyde. Here, acetaldehyde provides the enolate because benzaldehyde has no α-hydrogens to form its own enolate, so the reaction is a cross-aldol with acetaldehyde as the donor and benzaldehyde as the electrophile. After dehydration, the conjugated product is formed: cinnamaldehyde (Ph–CH=CH–CHO), an α,β-unsaturated aldehyde. This compound is also known as 3-phenylprop-2-enal, which is why that name appears among the options. The dehydration step is driven by the stability of the conjugated system with the phenyl ring and the aldehyde.

More practice questions