Question

Identify the wrong statement among the following (a) Molisch's test is used for the identification of carbohydrates. (b) Fehling solution \(A\) is an aqueous solution of \(\mathrm{CuSO} 4\). (c) Fehling solution \(\mathrm{B}\) is an alkaline solution of Rochelle salt. (d) All aldehydes reduce Fehling's solution to a red precipitate of Cu2O.

Short answer

(a) Molisch's test is used to detect the presence of carbohydrates. (b) Fehling solution A is an aqueous solution of CuSO4. (c) Fehling solution B is an alkaline solution containing Rochelle salt and a strong alkali. (d) All aldehydes reduce Fehling's solution to a red precipitate of Cu2O. Answer: (d) All aldehydes reduce Fehling's solution to a red precipitate of Cu2O.

Step-by-step solution

1. Information about Molisch's test

Molisch's test is a general test for carbohydrates. It indicates the presence of carbohydrates by forming a purple or violet ring at the junction when concentrated sulfuric acid is added to a solution containing carbohydrates. So, statement (a) is true.

2. Information about Fehling's solution A

Fehling solution A is an aqueous solution of copper(II) sulfate, represented by the chemical formula \(\mathrm{CuSO}_4\). Fehling solution A is blue in color. Thus, statement (b) is true.

3. Information about Fehling's solution B

Fehling solution B is an alkaline solution containing potassium sodium tartrate (also known as Rochelle salt) and a strong alkali - usually sodium hydroxide, NaOH. The chemical formula for Rochelle salt is KNaC\(_4\)H\(_4\)O\(_6\)·4H\(_2\)O. The purpose of Fehling solution B is to form a complex with Cu(II) ions, which causes the solution to be an effective oxidizing agent. So, statement (c) is true.

4. Information about the reactions of aldehydes with Fehling's solution

Aldehydes react with Fehling's solution, reducing it to a red precipitate of copper(I) oxide, \(\mathrm{Cu_2O}\). However, not all aldehydes can reduce Fehling's solution. Aromatic aldehydes (aldehydes with an aromatic ring in their structure) do not react with Fehling's solution, as it requires an alpha-hydrogen. Therefore, statement (d) is incorrect. Hence, the incorrect statement among the given options is: (d) All aldehydes reduce Fehling's solution to a red precipitate of Cu2O.

Key concepts

Molisch's Test

Molisch's Test is a classic chemical test used for the detection of carbohydrates. It is a simple yet effective test that can indicate the presence of various carbohydrates like sugars and starches. During this test, a few drops of Molisch's reagent, which is a solution of - naphthol in alcohol, are added to the carbohydrate solution. Upon the addition of concentrated sulfuric acid, a characteristic purple or violet ring appears at the interface. This is due to the dehydration of the carbohydrate by the acid, producing an aldehyde, which then reacts to form a colored complex. This color change is an indication that a carbohydrate is present in the sample. This test is very sensitive and can detect even small amounts of carbohydrates.

Carbohydrate Identification

Identifying carbohydrates in samples involves several tests, ranging from basic chemical reactions to more advanced analytical methods. Carbohydrates are organic compounds consisting of carbon, hydrogen, and oxygen, with the general formula C extsubscript{x}(H extsubscript{2}O) extsubscript{y}. These molecules are known for their diverse roles, including energy storage, structural functions, and biological processes. Analyzing carbohydrates usually starts with simple qualitative tests, such as:

• Molisch's Test: As described, it's effective for carbohydrate detection.
• Fehling's Test: Commonly used to identify reducing sugars.

These tests provide a quick and general indication of carbohydrate presence. However, for precise identification and differentiation between types of carbohydrates, methods like chromatography and mass spectrometry are often employed.

Aldehyde Reactions

Aldehydes are organic compounds characterized by the presence of a carbonyl group bonded to at least one hydrogen atom. They are highly reactive and can undergo various chemical reactions, one of which involves their interaction with Fehling's solution. Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate aldehydes and ketone functional groups. Reactions with Fehling's Solution:

• Typically, when aldehydes are introduced to Fehling's solution, the copper(II) ions are reduced to copper(I) oxide (Cu extsubscript{2}O), forming a red precipitate.
• This reduction reaction is easy to observe and is used to identify the presence of aldehydes.
• It’s important to note, however, that not all aldehydes can reduce Fehling's solution. Aromatic aldehydes, for instance, do not participate in this reaction because they lack an alpha-hydrogen necessary for the reduction.

Understanding these reactions plays an essential role in organic chemistry and helps in distinguishing between different organic compounds.