What is the major organic product formed when two molecules of ${\mathrm{CH}}_3\mathrm{CHO}$ (acetaldehyde) undergo an aldol addition under basic conditions?

• A. ${\mathrm{CH}}_3\mathrm{COCH}{3}_{}$ (acetone)
• B. ${\mathrm{CH}}_3\mathrm{COOH}$ (acetic acid)
• C. ${\mathrm{CH}}_3\mathrm{CH}{2}_{}\mathrm{CHO}$ (propanal)
• D. ${\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{OH}\right)\mathrm{CH}{2}_{}\mathrm{CHO}$ (3-hydroxybutanal)

Answer: (D)

Verified step by step guidance

1. Identify the reactants and conditions: Two molecules of acetaldehyde (CH\_3CHO) are reacting under basic conditions, which is a classic setup for an aldol addition reaction.
2. Recognize that under basic conditions, the alpha hydrogen of acetaldehyde is abstracted by the base, forming an enolate ion. This enolate ion acts as a nucleophile.
3. The enolate ion attacks the carbonyl carbon of a second acetaldehyde molecule, forming a new carbon-carbon bond. This step creates a beta-hydroxy aldehyde intermediate.
4. After the nucleophilic addition, protonation of the alkoxide intermediate occurs, resulting in the formation of 3-hydroxybutanal, which is the aldol addition product.
5. Note that this product is not yet dehydrated; under mild conditions, the aldol addition product (3-hydroxybutanal) is the major product before any dehydration to an α,β-unsaturated aldehyde can occur.