Question

The product obtained in the following reaction is \(\frac{\text { (i) Tollen'sreagent }}{\text { (ii) } \mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{2} / \mathrm{H}^{\prime}}{\longrightarrow}\) (A) Tartaric acid (B) Citric acid (C) Succinic acid (D) Malic acid

Short answer

The correct answer is (A) Tartaric acid. The given compound, assumed to be glyoxylic acid, reacts with Tollen's reagent and is oxidized to a carboxylate group. Then, the compound reacts with \(K_2Cr_2O_7 / H^+\) to yield the carboxylic acid version, which is tartaric acid.

Step-by-step solution

Identify the reagents

Tollen's reagent is commonly used as a test for the presence of aldehyde groups in organic compounds. Its structure is \([Ag(NH_3)_2]^+\), which is a complex of silver ion combined with ammonia. \(K_2Cr_2O_7\) is potassium dichromate, which can be used as an oxidizing agent in the presence of an acid like sulfuric acid (\(H^+\)).

Identify the reaction of the given compound with Tollen's reagent

In this case, the given compound is not mentioned. Therefore, we will consider a general aldehyde or ketone group present on it. Tollen's reagent reacts with aldehyde groups, as in the following way: \(RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \longrightarrow RCOO^- + 2Ag + 4NH_3 + 2H_2O\) Where R is the rest of the given compound. If the given compound has an aldehyde group, it will be oxidized to a carboxylate (\(RCOO^-\)) group and silver metal will be deposited as a manifestation of the reaction.

Identify the reaction of the given compound with \(K_2Cr_2O_7 / H^+\)

In the presence of acid, \(H^+\), potassium dichromate (\(K_2Cr_2O_7\)) acts as a powerful oxidizing agent that can further oxidize the \(RCOO^-\) (formed in the previous step) to its corresponding carboxylic acid: \(RCOO^- + H^+ \longrightarrow RCOOH\)

Determine the final product and select the correct answer

After reacting with (i) Tollen's reagent and (ii) \(K_2Cr_2O_7 / H^+\), the final product will be the carboxylic acid version of the compound with an additional carboxyl group at the position where the aldehyde group was present initially. Now, let us examine the given options: (A) Tartaric acid (B) Citric acid (C) Succinic acid (D) Malic acid Out of these options, only tartaric acid has an aldehyde group that can react with Tollen's reagent, forming a carboxylate group, and potassium dichromate further acidifies the carboxylate group. The initial precursor could be glyoxylic acid. Thus, the correct answer is (A) Tartaric acid.

Key concepts

Aldehyde Reaction

In organic chemistry, aldehydes are reactive groups characterized by the carbonyl center bonded to a hydrogen atom. This structure makes aldehydes easy to oxidize.
Aldehydes usually participate in reactions that lead to their transformation into other functional groups. One common reaction is oxidation, which leads to the formation of carboxylic acids.
Tollen's reagent is a classical reagent used to identify aldehyde groups. It involves a silver ammonia complex, \[ [Ag(NH_3)_2]^+ \]which selectively reacts with aldehydes but not ketones. During the reaction, aldehydes are oxidized to carboxylates, \[ RCHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag + 4NH_3 + 2H_2O \]This creates silver metal as a byproduct, recognizable by its formation on the reaction vessel as a silver mirror, thus confirming the presence of an aldehyde.

Oxidizing Agents in Organic Chemistry

Oxidizing agents are crucial in organic synthesis, enabling the conversion of one functional group to another by adding oxygen or removing hydrogen.
Potassium dichromate (\[ K_2Cr_2O_7 \]) is a widely used oxidizing agent, often in acidic solutions where it acts intensively.
In the presence of an acid like sulfuric acid (\[ H^+ \]), potassium dichromate's strength increases, enabling further oxidation of substances beyond what simpler oxidizing conditions might achieve.
In the given exercise, it helps transform the carboxylate intermediate (from the aldehyde) into the corresponding carboxylic acid. This is done by providing the additional \[ H^+ \] needed to convert \[ RCOO^- \] into \[ RCOOH \].
Thus, oxidizing agents help shift the functional groups to the desired end-products efficiently and are integral in various organic synthesis pathways.

Carboxylic Acid Formation

The formation of carboxylic acids often involves the oxidation of aldehydes. Using agents like Tollen's reagent and potassium dichromate, the terminal aldehyde group \[ RCHO \] is first oxidized to a carboxylate ion \[ RCOO^- \].
Carboxylic acids have a distinct feature of a carboxyl group \[ COOH \], and this full transformation requires a proton source.
Involving potassium dichromate under acidic conditions ensures the conversion of carboxylates to the stable acid form, \[ RCOOH \]. This transformation signifies one of the most common pathways in organic chemistry to produce carboxylic acids from aldehyde substrates.
This comprehensive approach using readily available oxidizing agents enables effective design and synthesis of carboxylic acids, important in chemical production and academic studies.

• Versatile in synthesis strategies
• Fulfillment of acidic conditions crucial
• Allows identification through functional groups