Which aldol addition product is formed when two molecules of ${\mathrm{CH}}_3\mathrm{CHO}$ (acetaldehyde) undergo base-catalyzed aldol condensation?

Many of the condensations we have studied are reversible. The reverse reactions are often given the prefix retro-, the Latin word meaning “backward.” Propose mechanisms to account for the following reactions.

(b)

Biochemists studying the structure of collagen (a fibrous protein in connective tissue) found cross-links containing α,β-unsaturated aldehydes between protein chains. Show the structures of the side chains that react to form these cross-links, and propose a mechanism for their formation in a weakly acidic solution.

Propose mechanisms for the following reactions.

(a)

Show how the following compounds can be synthesized. The only carbon-containing compounds available to you for each synthesis are shown.

b.

In an aldol addition reaction, what is the typical product formed before any dehydration occurs?

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

Which compound is formed as the major product when two molecules of ${\mathrm{CH}}_3\mathrm{CH}$₂CHO (butanal) undergo aldol condensation in the presence of base?

What is the primary purpose of adding base in the aldol condensation reaction? Choose the best answer.