Question

Of the following the number of compounds which answer Fehling's test is Glucose, \(\alpha\)-hydroxyl ketone, glyoxal, p-chlorobenzaldehyde, glyoxalic acid, succinaldehyde.

Short answer

Five compounds answer Fehling's test: Glucose, Alpha-hydroxy ketone, Glyoxal, Glyoxalic acid, and Succinaldehyde.

Step-by-step solution

Understand Fehling's Test

Fehling's test is a chemical test used to differentiate between water-soluble carbohydrates and ketones. Specifically, it identifies aldehyde groups, as aldehydes are oxidized by the Fehling's solution, turning it from blue to a red precipitate of cuprous oxide. This test is not typically positive for ketones except for alpha-hydroxy ketones.

Identify Aldehydes and Special Cases

Among the given compounds, we need to identify which ones have aldehyde groups or are special cases of ketones that can give a positive Fehling's test. The given compounds are: 1. Glucose (contains an aldehyde group) 2. Alpha-hydroxy ketone (an exception that gives a Fehling's positive test) 3. Glyoxal (contains an aldehyde group) 4. p-Chlorobenzaldehyde (contains an aldehyde group) 5. Glyoxalic acid (functions as an aldehyde in presence of Fehling's agent) 6. Succinaldehyde (contains an aldehyde group)

Key concepts

Aldehyde Compounds

Aldehyde compounds are a fascinating class of organic molecules containing a carbonyl group with a hydrogen atom bonded to the carbon. This gives them the general formula R-CHO, where R represents a hydrogen atom or any other hydrocarbon group. Their reactivity stems from the presence of the double-bonded oxygen in the carbonyl group, which is highly electronegative, hence attracting electrons. One of the remarkable reactions involving aldehydes is their ability to be easily oxidized. This is especially important in the context of Fehling's test, a classic test in organic chemistry. Aldehydes, in the presence of Fehling's solution, can be oxidized to carboxylic acids. During the process, the blue copper(II) ions in the solution are reduced to red copper(I) oxide, forming a precipitate. This distinct color change signifies a positive test result, confirming the presence of an aldehyde in a compound.

Alpha-Hydroxy Ketones

Alpha-hydroxy ketones are a unique subset of ketones, characterized by the presence of a hydroxyl group (OH) attached to the alpha carbon next to the ketone group. Despite ketones traditionally not reacting with Fehling's solution, alpha-hydroxy ketones are a special case. The presence of the hydroxyl group facilitates a reaction with Fehling's reagent, resulting in a similar red precipitate as observed with aldehydes. This unique reactivity makes alpha-hydroxy ketones an intriguing exception in organic chemistry tests like Fehling's test, allowing them to be identified even though they are structurally different from aldehydes.

Glucose Oxidation

Glucose is a prime example of an aldehyde-containing compound that reacts positively with Fehling's solution. Its structure includes an aldehyde group, which is crucial for its identification in oxidation reactions. When glucose undergoes Fehling's test, the aldehyde group in its open-chain form becomes oxidized. This reaction is significant in biological processes, where glucose oxidation results in the production of energy. In experimental settings, the color change from blue to red during Fehling's test not only indicates the presence of glucose but also its reducing sugar capabilities.

Organic Chemistry Tests

Organic chemistry tests, like Fehling's test, are invaluable tools for identifying various functional groups in organic molecules. Fehling's test specifically is designed to distinguish between aldehydes and ketones, with some exceptions like alpha-hydroxy ketones. These tests utilize specific chemical reactions that produce visible changes, often in color, to confirm the presence of particular functional groups. Such tests are crucial in labs for quick qualitative analyses. By systematically performing these tests, chemists can determine the structure and components of unknown substances, aiding in everything from academic research to industrial applications. The predictability and specificity of organic chemistry tests make them a staple in any chemist's arsenal.