Question

Identify the correct statements among the following. (a) Formaldehyde can react with both \(-\mathrm{NH}_{2}\) groups in semicarbazide. (b) In mixed Cannizzaro reaction involving HCHO, it is always the intermediate formed from HCHO which transfers its hydride to the other aldehyde. (c) Benzaldehyde reduces both Tollens' reagent and Fehling's solution. (d) In Fehling's solution, Cu(II) ions are kept in soluble form of a complex in alkaline medium.

Short answer

a) Formaldehyde can react with both NH₂ groups in Semicarbazide (HN-C(=NH)-NH₂). b) In a mixed Cannizzaro reaction, HCHO always serves as the hydride donor. c) Benzaldehyde reduces both the Tollens' reagent and Fehling's solution. d) In Fehling's solution, Cu(II) ions are kept in soluble form as a complex. Answer: a, b, and d are correct.

Step-by-step solution

Statement (a) Analysis

Formaldehyde, or \(\mathrm{HCHO}\), can react with a variety of molecules containing nucleophilic sites, such as amine groups (\(-\mathrm{NH}_{2}\)). Semicarbazide contains two amine groups that can potentially react with formaldehyde. Let us evaluate if formaldehyde indeed can react with both these groups. Semicarbazide has the following structure: $$\mathrm{HN-C(=NH)-NH_2}$$ A reaction between formaldehyde and an amine group involves the formation of an imine or Schiff base. Analyzing the structure of semicarbazide, we observe that it can indeed form an imine with formaldehyde at both its amine sites. Therefore, statement (a) is correct.

Statement (b) Analysis

The mixed Cannizzaro reaction involving HCHO (formaldehyde) refers to the reaction between formaldehyde and another aldehyde without an alpha hydrogen atom. The reaction occurs via hydride transfer between the two aldehyde molecules. Let's assess if it's true that HCHO is always the intermediate which transfers its hydride to the other aldehyde. Formaldehyde does not have an alpha hydrogen atom, and in the mixed Cannizzaro reaction, it typically serves as a hydride donor. As the reaction progresses, formaldehyde is oxidized into formic acid, transferring a hydride to the other aldehyde, reducing it to an alcohol. Therefore, statement (b) is correct.

Statement (c) Analysis

Benzaldehyde is an aromatic aldehyde that can potentially participate in redox reactions. In this statement, we need to evaluate if benzaldehyde can reduce both Tollens' reagent and Fehling's solution. Tollens' reagent consists of a silver-ammonia complex which can oxidize aldehydes to carboxylic acids. Benzaldehyde, however, cannot be oxidized by Tollens' reagent because of the presence of an aromatic ring that prevents the standard oxidation process from taking place. Fehling's solution is a mixture of copper(II) complex and sodium hydroxide. The Cu(II) ion in the solution can oxidize aldehydes to carboxylic acids, with the Cu(II) ion being reduced to Cu(I) ion. Like with Tollens' reagent, benzaldehyde will not react with Fehling's solution because the aromatic ring prevents oxidation. Since both cases fail to oxidize benzaldehyde, statement (c) is incorrect.

Statement (d) Analysis

Fehling's solution is a mixture of copper(II) ions as a copper(II) complex and an alkaline medium. We need to assess if the Cu(II) ions are kept in a soluble form as a complex. Recalling the composition of Fehling's solution, it consists of copper(II) sulfate, sodium potassium tartrate, and sodium hydroxide. The copper(II) ion forms a complex with the tartrate anion in the alkaline medium, maintaining its solubility. Consequently, statement (d) is correct. To summarize: - Statement (a) is correct. - Statement (b) is correct. - Statement (c) is incorrect. - Statement (d) is correct.

Key concepts

Formaldehyde

Formaldehyde, with the chemical formula \( ext{HCHO}\), is the simplest type of aldehyde and is widely used in chemical industries. It plays a crucial role in multiple reactions due to its electrophilic nature. When dealing with nucleophiles like amine groups, formaldehyde can form imines or Schiff bases by reacting with these groups. This is why formaldehyde can react with both -\( ext{NH}_2\) groups in structures such as semicarbazide.
Although it is a simple molecule, formaldehyde's reactivity allows it to participate widely in organic synthesis, creating more complex molecules as intermediates or endpoints. Thus, understanding its interactions is vital for studying various organic chemistry reactions.

Cannizzaro Reaction

The Cannizzaro reaction refers to a unique reaction where aldehydes without alpha hydrogen atoms undergo disproportionation when treated with a concentrated base. In the context of the mixed Cannizzaro reaction, involving formaldehyde \( ext{HCHO}\), the aldehyde is often used with another aldehyde that also lacks an alpha hydrogen. Formaldehyde acts as a hydride donor in this reaction.
During the process, one aldehyde gets oxidized to an acid, and the other reduces to an alcohol. For example, when \( ext{HCHO}\) is mixed with benzaldehyde in a basic solution, \( ext{HCHO}\) typically donates the hydride ion, becoming formic acid, while benzaldehyde is reduced to benzyl alcohol. This specific interaction demonstrates the exceptional role formaldehyde plays in transferring a hydride, highlighting the particular mechanisms in the Cannizzaro reactions.

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons between two species. In these reactions, one reactant loses electrons (is oxidized), while the other gains electrons (is reduced).
In the context of aldehyde reactions, oxidizing agents like Tollens' reagent or Fehling's solution are commonly tested. Benzaldehyde, a common aromatic aldehyde, fails to reduce these agents due to the stability offered by its aromatic ring, making it resistant to oxidation processes.
Understanding redox chemistry is crucial in comprehending how substances like formaldehyde are involved in reactions like the Cannizzaro reaction where electron transfer (in the form of hydride ions) is a fundamental component. It highlights how even small molecular changes can influence the directional flow of such redox interactions.

Fehling's Solution

Fehling's solution is an important chemical reagent used to differentiate between aldehyde functionalities and distinguish them from ketones. It consists of a copper(II) sulfate complex mixed with sodium potassium tartrate in an alkaline environment. In this solution, copper(II) ions are maintained in a soluble form thanks to their complexation with tartrate.
When tested with an aldehyde, the copper(II) ions are reduced to copper(I), forming a red precipitate of copper(I) oxide, indicating a positive result. However, aromatic aldehydes like benzaldehyde do not give a positive result with Fehling's solution due to the stability of their aromatic rings preventing the necessary redox reaction for the formation of the precipitate.
Fehling's solution is therefore a useful tool for differentiating between various types of aldehyde compounds based on their redox behavior and can help reinforce their understanding of oxidation and reduction processes within organic chemistry.