Question

Propose a likely reaction mechanism and name the end-product (s) for the reaction of a solution containing phenylacetaldehyde and dilute sodium hydroxide.

Short answer

The reaction between phenylacetaldehyde and dilute sodium hydroxide proceeds through a nucleophilic addition mechanism, leading to the formation of a 2-hydroxy-tetrahydrofuran as the end product. The key steps in this reaction are the nucleophilic attack by the hydroxide ion on the carbonyl carbon, formation of a hydrate intermediate, intramolecular cyclization, and isoconversion to the final product.

Step-by-step solution

1. Identify phenylacetaldehyde structure

Phenylacetaldehyde (C8H8O) is an aromatic aldehyde with a phenyl group (C6H5) attached to the aldehyde group (CHO). Its structure allows it to undergo nucleophilic addition reactions.

2. Reacting with sodium hydroxide

In the presence of a base like sodium hydroxide (NaOH), the oxygen atom on the aldehyde group becomes more negatively charged and more susceptible to nucleophilic attack. This is due to the lone pair of electrons on the oxygen being able to accept the proton from NaOH, leaving behind an electron pair which can then be utilized for nucleophilic attack.

3. Nucleophilic attack on the carbonyl carbon

The negatively charged oxygen atom on the aldehyde group undergoes nucleophilic attack by the hydroxide ion (OH-) acting as a nucleophile. The carbonyl carbon on the aldehyde group undergoes nucleophilic addition, forming a new C-O bond with the hydroxide ion.

4. Formation of a hydrate intermediate

The nucleophilic addition of a hydroxide ion to the carbonyl carbon results in a hydrate intermediate. This intermediate species has an extra hydroxyl group (OH) attached to the carbonyl carbon, resulting in a geminal diol with the formula C6H5CH(OH)2.

5. Intramolecular cyclization

The hydroxyl group located at the alpha position relative to the carbonyl carbon performs an intramolecular nucleophilic attack on the carbonyl carbon. This cyclization step forms a new C-C bond and generates a cyclic intermediate with a 5-membered ring. The intermediate is known as a hydroxy-tetrahydrofuran, and it also bears a negative charge on the oxygen atom of the remaining hydroxyl group.

6. Isoconversion to key intermediate

In order to complete the reaction, the negatively charged oxygen species donates its electron pair to a nearby proton, forming a molecule of water (H2O) and yielding the key intermediate, which is a 2-hydroxy-tetrahydrofuran.

7. Identify end product

The final end product of the reaction between phenylacetaldehyde and dilute sodium hydroxide is a 2-hydroxy-tetrahydrofuran. The reaction mechanism proceeds in the following manner: 1. Nucleophilic attack by the hydroxide ion on the carbonyl carbon 2. Formation of a hydrate intermediate 3. Intramolecular cyclization to form a hydroxy-tetrahydrofuran intermediate 4. Isoconversion to the key intermediate 5. Formation of the end product, 2-hydroxy-tetrahydrofuran.

Key concepts

Phenylacetaldehyde

Phenylacetaldehyde is an aromatic compound that plays a significant role in various chemical reactions. It has the molecular formula \( C_8H_8O \) and an interesting structure that combines a phenyl group \( C_6H_5 \) with an aldehyde group \( CHO \).
This combination allows phenylacetaldehyde to readily participate in several types of chemical reactions, most notably the nucleophilic addition.

• A phenyl group is a benzene ring minus one hydrogen, which gives it an aromatic property.
• An aldehyde group is composed of a carbonyl center (a carbon double-bonded to oxygen) connected to at least one hydrogen.

This particular structure of phenylacetaldehyde increases its reactivity, especially when subjected to bases like sodium hydroxide \( NaOH \).

Nucleophilic Addition

Nucleophilic addition is a common reaction mechanism especially in organic chemistry for compounds containing carbonyl groups. When phenylacetaldehyde is in the presence of a base like sodium hydroxide, it undergoes nucleophilic addition. The carbonyl carbon in the aldehyde becomes a focal point for nucleophilic attack.

• The nucleophile, often a negatively charged ion like hydroxide \( OH^- \) here, attacks the electrophilic carbon of the carbonyl group.
• This leads to the formation of a new C-O bond, expanding the molecular structure.

During this reaction, the base enhances the reactivity of the aldehyde group by increasing the negativity of the oxygen atom, making the carbonyl carbon even more susceptible to attack.

Hydrate Intermediate

The hydrate intermediate is a transitional form that emerges as a result of nucleophilic addition to the carbonyl group of phenylacetaldehyde.Upon attack, the combination of the carbonyl carbon with the hydroxide ion results in a geminal diol structure,
which is essentially a carbon atom bonded to two hydroxyl groups (\( OH \)) through single bonds.
This intermediate, often unstable and short-lived, is significant because:

• It acts as the precursor to subsequent chemical transformations in the mechanism.
• This step aids in setting up the structure necessary for the following intramolecular reactions.

The formation of this intermediate involves the conversion of the original aldehyde group into a diol, appearing as \( C_6H_5CH(OH)_2 \).

Intramolecular Cyclization

Intramolecular cyclization is an intriguing step in the reaction mechanism of phenylacetaldehyde with sodium hydroxide.After forming the hydrate intermediate, an internal rearrangement occurs. Here, an internal hydroxyl \( OH \) group attacks the originally electrophilic carbon.
This is facilitated within the same molecule, allowing the formation of a new carbon-carbon bond.

• This rearrangement leads to the creation of a cyclic intermediate, particularly a five-membered ring called a hydroxy-tetrahydrofuran.
• Cyclization is a critical strategy in organic synthesis for building complex ring structures from simpler molecular forms.

The completion of this transformation results in a stable, cyclic compound, which is identified as the main product in the mechanism.