- The acid-catalyzed formation of a diethyl acetal is studied for three ketones: acetone, cyclohexanone, and di-tert-butyl ketone. Which option correctly ranks these ketones from the fastest rate of acetal formation to the slowest?
Answer: acetone > cyclohexanone > di-tert-butyl ketone
The rate of nucleophilic addition to a carbonyl is highly sensitive to steric hindrance. Acetone has two relatively small methyl groups. Cyclohexanone is a cyclic ketone, and its carbonyl is more sterically accessible than many acyclic ketones but less so than acetone. Di-tert-butyl ketone is extremely hindered by two bulky tert-butyl groups, which effectively block the approach of the nucleophile. Therefore, the rate decreases as steric bulk increases: acetone > cyclohexanone > di-tert-butyl ketone.
- In the acid-catalyzed mechanism for the formation of an acetal from acetone and two equivalents of methanol, which of the following species is a key, resonance-stabilized intermediate formed immediately after the loss of a water molecule?
Answer: An oxonium ion
The mechanism proceeds through a hemiacetal. The hydroxyl group of the hemiacetal is protonated by the acid catalyst, converting it into a good leaving group (H₂O). Loss of water generates an oxonium ion, where the carbon has a positive charge but is stabilized by resonance with the adjacent oxygen atom. This oxonium ion is the electrophile that is attacked by the second molecule of methanol.
- A reaction is performed by treating 2-butanone with excess methanol and a catalytic amount of H₂SO₄. The IR spectrum of the purified product shows the disappearance of a strong absorption at ~1715 cm⁻¹ and the appearance of a new strong absorption at ~1100 cm⁻¹. What do these spectral changes indicate?
Answer: The reaction successfully formed 2,2-dimethoxybutane.
The strong absorption at ~1715 cm⁻¹ is characteristic of a ketone C=O stretch. Its disappearance indicates that the carbonyl group has reacted. The appearance of a new strong band in the 1050-1150 cm⁻¹ region is characteristic of C-O single bond stretching, typical for ethers and acetals. Together, these data strongly support the conclusion that the ketone was converted to the corresponding dimethyl acetal, 2,2-dimethoxybutane.
- An acetal is prepared from unlabeled benzaldehyde and methanol. This acetal is then placed in a solution of H₂¹⁸O (water enriched with the ¹⁸O isotope) and a catalytic amount of acid. After equilibrium is reached, where will the ¹⁸O label be predominantly found?
Answer: Incorporated as the oxygen atom of the regenerated benzaldehyde carbonyl group.
Acetal hydrolysis is the microscopic reverse of acetal formation. The mechanism involves nucleophilic attack by a water molecule on a resonance-stabilized oxonium ion intermediate. The oxygen atom from the attacking H₂¹⁸O molecule becomes the new carbonyl oxygen. The original methoxy groups are protonated and leave as methanol, containing the original unlabeled oxygen atoms.
- A student treats benzaldehyde with exactly one equivalent of methanol and a catalytic amount of TsOH in a sealed container. The reaction is allowed to reach equilibrium. Besides the starting materials, which organic compound is expected to be present in the highest concentration?
Answer: The hemiacetal
When you encounter a carbonyl compound reacting with an alcohol under acidic conditions, you're dealing with acetal/hemiacetal formation equilibria. This is a fundamental reaction where aldehydes and ketones form reversible bonds with alcohols.
With exactly one equivalent of methanol and benzaldehyde under acidic catalysis, the primary equilibrium involves adding one molecule of methanol across the carbonyl group. This forms a hemiacetal - a carbon bonded to both an OH group and an OR group. The equilibrium heavily favors hemiacetal formation when you have equimolar amounts of reactants, making this the predominant product.
Option A (dimethyl acetal) is incorrect because forming the full acetal requires two equivalents of methanol. With only one equivalent present, you can't generate significant amounts of the acetal, which needs a second methanol molecule to replace the remaining OH group.
Option B (benzyl methyl ether) is wrong because this would require reduction of the carbonyl and substitution chemistry that doesn't occur under these mild acidic conditions. The carbonyl carbon remains sp³ hybridized in the actual product.
Option C (benzoic acid) is incorrect because oxidation of the aldehyde doesn't happen under these conditions. You'd need an oxidizing agent, not just methanol and acid.
Remember this key principle: with equimolar aldehyde and alcohol under acidic conditions, hemiacetal formation dominates the equilibrium. The hemiacetal represents the first, more thermodynamically favorable step before full acetal formation, which requires excess alcohol to drive the equilibrium forward.
- Which statement best explains why acetal formation requires an acid catalyst and cannot be effectively catalyzed by a base like NaOH?
Answer: The hydroxyl group of the hemiacetal intermediate is a poor leaving group and requires protonation to be eliminated as water.
The mechanism involves two stages: formation of a hemiacetal, then conversion to an acetal. While the initial addition can occur under basic conditions (forming a hemiacetal alkoxide), the second stage requires the elimination of the original carbonyl oxygen (now a hydroxyl group). Hydroxide (OH⁻) is a very poor leaving group. Acid catalysis overcomes this barrier by protonating the -OH group to form -OH₂⁺, which is an excellent leaving group (H₂O).
- In aqueous solution, glucose exists predominantly as a stable six-membered cyclic hemiacetal (a pyranose). What is the primary factor that drives the equilibrium so strongly in favor of the cyclic form over the open-chain aldehyde form?
Answer: The high favorability of an intramolecular reaction forming a low-strain six-membered ring.
The formation of the cyclic hemiacetal is an intramolecular reaction. Such reactions, especially those forming stable, low-strain five- or six-membered rings, are kinetically fast and thermodynamically favorable compared to their intermolecular counterparts. This high effective concentration of the nucleophile (the C5-hydroxyl group) drives the equilibrium almost completely to the cyclic form.
