Question

Choose the incorrect option. (A) Fehling solution ' \(\mathrm{A}\) ' is aqueous copper sulphate and ' \(\mathrm{B}\) ' is Rochelle salt \& \(\mathrm{NaOH}\). (B) Aromatic aldehydes gives positive response to Fehling test (C) Rochelle salt is sodium potassium tartarate (D) Reaction with aldehydes is \(\mathrm{R}-\mathrm{CHO}+2 \mathrm{Cu}^{2+} 5 \mathrm{OH}^{-} \longrightarrow \mathrm{RCOO}^{-}+\mathrm{Cu}_{2} \mathrm{O}+3 \mathrm{H}_{2} \mathrm{O}\)

Short answer

The incorrect option is (B), as aromatic aldehydes do not give a positive response to Fehling's test.

Step-by-step solution

(A) Analyze Fehling solution components

Fehling's A solution contains aqueous copper sulfate with Cu^2+ ions. In Fehling's B, Rochelle salt and NaOH are present, and sodium potassium tartrate is the common name for Rochelle salt. Therefore, option (A) is correct.

(B) Aromatic aldehydes and Fehling test

Fehling's test is used to detect the presence of aldehydes. Aromatic aldehydes, however, don't give a positive response to Fehling's test. This is because they are less reactive due to resonance between the carbonyl group and the aromatic ring. Thus, option (B) is incorrect, as aromatic aldehydes don't give a positive response to Fehling's test.

(C) Rochelle salt composition

Rochelle salt is a double salt of sodium and potassium tartrate with the formula \(\mathrm{NaKC}_{4}\mathrm{H}_{4}\mathrm{O}_{6}\). In Fehling's B solution, Rochelle salt is combined with NaOH. Therefore, option (C) is correct.

(D) Reaction with aldehydes

In the presence of an aldehyde, Fehling's solution undergoes a redox reaction where aldehyde is oxidized to its corresponding carboxylate anion and Cu^2+ is reduced to Cu₂O (copper (I) oxide): \(\mathrm{R}-\mathrm{CHO} + 2\mathrm{Cu}^{2+} + 5\mathrm{OH}^{-} \longrightarrow \mathrm{RCOO}^{-} + \mathrm{Cu}_{2}\mathrm{O} + 3\mathrm{H}_{2}\mathrm{O}\) Hence, option (D) is the correct representation of the reaction with aldehydes in the presence of Fehling's solution. Based on the analysis, we can conclude that option (B) is incorrect, as aromatic aldehydes do not give a positive response to Fehling's test.

Key concepts

Aldehydes Detection

Determining the presence of aldehydes in a substance can be pivotal in organic chemistry, and one classic method for aldehyde detection is the Fehling's test. This test specifically targets aliphatic aldehydes, which are known to readily react with Fehling's solution resulting in a color change. When performing Fehling's test, a clear solution containing copper(II) ions is mixed with the sample. On heating, if an aldehyde is present, the solution changes from blue to a brick red precipitate indicating a positive result.

However, it's crucial to note that not all aliphatic aldehydes respond the same, and as revealed by the incorrect option in our exercise, aromatic aldehydes will not provide a positive response. The rigidity of the aromatic ring and the resonance stabilization of the carbonyl group make them less reactive in this test. Therefore, proper identification of the type of the aldehyde, either aliphatic or aromatic, is essential before one can rely on Fehling's test as a definitive detection method.

Fehling Solution Composition

Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate and ketone functional groups, and is commonly applied in the detection of reducing sugars. The reagent is a mixture of two solutions, commonly referred to as Fehling's A and Fehling's B. Fehling's A contains aqueous copper(II) sulfate which provides the Cu^2+ ions necessary for the reaction. On the other hand, Fehling's B contains Rochelle salt (potassium sodium tartrate) and sodium hydroxide. The Rochelle salt acts as a complexing agent keeping the copper ions in solution, while the NaOH is there to provide the alkaline conditions necessary for the redox reaction to occur.

As was indicated in the given exercise, a positive result is depicted by the formation of a red or brick red precipitate—copper(I) oxide—when aldehydes reduce the blue Cu^2+ ions from the Fehling's A solution.

Aromatic Aldehydes Reactivity

Understanding the reactivity of aromatic aldehydes is important when predicting the outcome of many organic reactions. Aromatic aldehydes are less reactive than their aliphatic counterparts due to the resonance stabilization of the aromatic ring with the carbonyl group. This conjugation distributes the positive charge across the ring, reducing the electrophilic character of the carbonyl carbon.

In context with the Fehling's test as mentioned in the step by step solution, the resonance effect in aromatic aldehydes hampers the reduction of Cu^2+ to Cu2O, thus they often give a negative result with Fehling's test, contrary to aliphatic aldehydes. This decreased reactivity should be kept in mind when performing other chemistry reactions involving aromatic aldehydes as well, since this characteristic influences the way they behave under various chemical conditions. It is also worth noting that certain substituents on the aromatic ring can affect the reactivity in different ways, either by donating or withdrawing electrons.