25. Condensation Chemistry

Predict the products of aldol condensation, followed by dehydration, of the following ketones and aldehydes.

(a) butyraldehyde

Identify A–C. (Hint: A shows three singlets in its 1H NMR spectrum with integral ratios 3 : 2 : 3 and gives a positive iodoform test; see Problem 58.)

The following compounds can be synthesized by aldol condensations, followed by further reactions. (In each case, work backward from the target molecule to an aldol product, and show what compounds are needed for the condensation.)

(c)

Using cyclopentanone as the reactant, show the product of

b. an aldol addition. 

c. an aldol condensation.

Which of the following best describes the major product formed when $\mathrm{cyclopentanone}$ undergoes aldol condensation under basic conditions?

In an aldol condensation reaction between an $\mathrm{aldehyde}$ and a $\mathrm{ketone}$, why is an excess of $\mathrm{aldehyde}$ often used?

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)

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?

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

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

Show how you would use an aldol, Claisen, or another type of condensation to make each compound.

(c)

Which of the following lists all four possible $\beta$-hydroxy aldehydes that can be formed by a crossed aldol reaction between ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{CHO}$ and $C_6{H}_5{\mathrm{CH}}_2\mathrm{CHO}$?

Which of the following best describes the major product formed when $3-\mathrm{methylbutanal}$ undergoes aldol condensation under basic conditions?

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