Question

Which of the following compounds reacts with concentrated alcohol? (A) Benzaldehyde (B) Phenylacetaldehyde (C) Acetaldehyde (D) All of these

Short answer

Both Phenylacetaldehyde (B) and Acetaldehyde (C) react with concentrated alcohol to form hemiacetals. Benzaldehyde (A) does not react under normal conditions due to steric hindrance. Thus, the correct answer is (B) Phenylacetaldehyde and (C) Acetaldehyde.

Step-by-step solution

Understanding the Reactivity of Aldehydes

Aldehydes are organic compounds containing a carbonyl functional group (C=O) directly bonded to a hydrogen atom. They can react with alcohols (ROH) to form hemiacetals, and in some cases, further react to form acetals. The reactivity of aldehydes towards alcohols depends on the presence of any electron-donating or electron-withdrawing groups that can stabilize the intermediate products formed during the reaction.

Analyzing Each Compound

Let's analyze each compound and its reactivity with concentrated alcohol: (A) Benzaldehyde: Benzaldehyde is an aromatic aldehyde with a structure of C6H5CHO. The benzene ring has a delocalized electron cloud (resonance) that can stabilize the intermediate product formed during the reaction with alcohol. However, due to steric hindrance that makes it difficult for alcohol molecules to approach the carbonyl group closely, benzaldehyde does not react with concentrated alcohol under normal conditions. (B) Phenylacetaldehyde: Phenylacetaldehyde has the structure C6H5CH2CHO. It contains an aliphatic aldehyde group, and its carbonyl carbon is more exposed to nucleophilic attacks compared to benzaldehyde. The phenyl group can also stabilize the intermediate product through resonance. Phenylacetaldehyde is reactive towards alcohol and forms a hemiacetal. (C) Acetaldehyde: Acetaldehyde has the structure CH3CHO, an aliphatic aldehyde with no electron-donating or electron-withdrawing groups. It can react with alcohol to form a hemiacetal.

Identifying the Reactive Compounds

Comparing the reactivity of the three compounds towards concentrated alcohol, we can conclude that: - Benzaldehyde (A) does not react with concentrated alcohol under normal conditions due to steric hindrance. - Phenylacetaldehyde (B) reacts with concentrated alcohol to form a hemiacetal, as its aliphatic aldehyde group is reactive. - Acetaldehyde (C) also reacts with concentrated alcohol to form a hemiacetal. Hence, the correct answer is \( (B) \) Phenylacetaldehyde and \( (C) \) Acetaldehyde, which react with concentrated alcohol.

Key concepts

Hemiacetal Formation

Aldehydes have a penchant for reacting with alcohols, a process that can lead to the creation of intermediate compounds known as hemiacetals. But what initiates this transformation? It starts with the nucleophilic alcohol molecule, which mounts an attack on the electrophilic carbonyl carbon of the aldehyde. During this reaction, a new bond is formed between the oxygen of the alcohol and the carbon of the aldehyde, while the double bond of the carbonyl shifts to form a single bond, creating a hydroxyl group (OH).

Basic Steps of Hemiacetal Formation

• The nucleophilic oxygen of the alcohol approaches the electrophilic carbonyl carbon.
• A covalent bond forms between the oxygen and carbon, with electrons from the carbonyl double bond moving to the oxygen atom.
• The outcome is a hemiacetal structure, featuring both an ether (ROR') and a hydroxyl (OH) group attached to the same carbon.

In the exercise provided, acetaldehyde and phenylacetaldehyde can form hemiacetals due to their exposed and less hindered carbonyl groups, ready to align with and be attacked by the nucleophilic alcohol.

Steric Hindrance in Aromatic Aldehydes

Aromatic aldehydes, such as benzaldehyde, present a unique challenge when it comes to reactivity. The aromatic ring attached directly to the carbonyl carbon does more than just add a note of elegance to the molecular structure—it introduces steric hindrance. This spatial interference makes it challenging for nucleophiles, like alcohol molecules, to cozy up to the carbonyl carbon and perform the attack needed for reactions such as hemiacetal formation.

Impact of Steric Hindrance

• Steric bulk near the reactive site repels potential reactants.
• Reactivity diminishes as collision between the reactant and the electrophilic center becomes less likely.

Despite this hindered landscape, benzaldehyde still vents an aura of stability due to the delocalized electrons within its ring. However, this stability does little to overcome the physical blockades stopping the approaching alcohol molecules, explaining why benzaldehyde is less reactive in forming hemiacetals with concentrated alcohol.

Aldehyde Reactivity Towards Nucleophiles

Aldehydes are distinguished by their reactivity towards nucleophiles, thanks to their carbonyl group which houses an electrophilic carbon—ever ready for attack. In our exercise, acetaldehyde and phenylacetaldehyde become prime targets for nucleophilic alcohol molecules due to their relatively unhindered carbonyl carbons. The less steric bulk around this vital carbon, the easy it is for alcohol molecules to strike, paving the way for hemiacetal formation.

Factors Influencing Aldehyde Reactivity

• The less steric clutter around the carbonyl carbon, the higher the reactivity towards nucleophiles.
• Resonance and electron donating effects can stabilize the intermediate, elevating reactivity.
• Aliphatic aldehydes, like acetaldehyde, are typically more reactive than their aromatic counterparts.

This intrinsic reactivity explains why, in the exercise, compounds (B) Phenylacetaldehyde and (C) Acetaldehyde are reactive with concentrated alcohol, whereas (A) Benzaldehyde, is inhibited by its bulkier structure.