Problem 87 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Fehling's solution can be used for distinguishing between (a) \(\mathrm{CH}_{3} \mathrm{CHO}\) and \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CHO}\) (b) \(\mathrm{CH}_{3} \mathrm{CHO}\) and \(\mathrm{CH}_{3} \mathrm{COCH}_{2} \mathrm{OH}\) (c) Both (a) and (b) (d) None of these

Step-by-Step Solution

Answer

Fehling's solution can distinguish between (b) \\( ext{CH}_3 ext{CHO}\\) and \\( ext{CH}_3 ext{COCH}_2 ext{OH}\\).

Step 1: Understanding Fehling's Solution

Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrates and ketone functional groups, and specifically to detect aldehyde functional groups. It contains copper(II) ions which are reduced to copper(I) oxide in the presence of an aldehyde, producing a red precipitate.

Step 2: Identifying the Functional Groups

In the options provided, we need to identify the functional groups present in each compound. For (a), both \( ext{CH}_3 ext{CHO}\) and \( ext{C}_6 ext{H}_5 ext{CHO}\) are aldehydes. For (b), \( ext{CH}_3 ext{CHO}\) is an aldehyde while \( ext{CH}_3 ext{COCH}_2 ext{OH}\) is a ketone.

Step 3: Application of Fehling's Test

Fehling's solution can oxidize aldehydes to carboxylic acids and produce a red precipitate, but it does not react with ketones. Therefore, in pair (b), \( ext{CH}_3 ext{CHO}\) can be distinguished from \( ext{CH}_3 ext{COCH}_2 ext{OH}\) since the latter will not react with Fehling's solution.

Step 4: Conclusion

Since Fehling's solution cannot distinguish between two aldehydes (as both react similarly), option (a) cannot be distinguished. However, in option (b), the aldehyde reacts while the ketone does not. Therefore, Fehling's solution can distinguish between \( ext{CH}_3 ext{CHO}\) and \( ext{CH}_3 ext{COCH}_2 ext{OH}\).

Key Concepts

Aldehyde Detection

Fehling's solution is renowned for its role in aldehyde detection. Aldehydes are a type of organic compound that contain the functional group \(-CHO\). This group is highly reactive with oxidizing agents. When Fehling's solution, which contains copper(II) ions, is mixed with an aldehyde, it triggers a chemical reaction.

During this reaction, the aldehyde is oxidized to form a carboxylic acid. Simultaneously, the copper(II) ions in the solution are reduced to copper(I) ions, forming copper(I) oxide. This copper(I) oxide precipitates out of the solution as a red solid.

This change is visible and indicates the presence of an aldehyde. However, not all carbonyl compounds behave the same way. Ketones, which are structurally similar to aldehydes, do not produce a similar visible change with Fehling's solution. This distinction is incredibly useful in organic chemistry for identifying specific functional groups within a compound.

Chemical Reagent

Chemical reagents are substances or compounds added to a system to cause a chemical reaction or test if a reaction occurs. Fehling's solution is an excellent example of such a chemical reagent. It specifically targets aldehydes to help distinguish them from other organic groups.

Fehling's solution is made by combining two separate solutions: Fehling’s A and Fehling’s B. Fehling’s A contains aqueous copper(II) sulfate, which is blue in color. Fehling's B contains potassium sodium tartrate and a strong alkali, usually sodium hydroxide.

When preparing Fehling's reagent, Fehling’s A and Fehling’s B are mixed in equal parts. This creates the active solution, which is used for testing. This reagent only reacts with aldehydes, which makes it a selective test. It's important in organic chemistry when testing for the presence of specific functional groups.

• Fehling's solution only reacts with aldehydes, not ketones.
• This selectivity helps in identifying specific compounds in a mixture.

Organic Chemistry Functional Groups

Functional groups are specific groups of atoms within molecules that determine the characteristic chemical reactions of those molecules. In organic chemistry, understanding functional groups is pivotal because they define how a molecule will react.

Common functional groups include alcohols, ketones, carboxylic acids, and of course, aldehydes. Aldehydes contain the group \(\text{-CHO}\), typically positioned at the end of a carbon chain. This positioning makes aldehydes especially reactive.

Ketones, on the other hand, have a carbonyl group \(\text{-C(=O)-}\) located in the middle of the carbon chain. This structural difference between aldehydes and ketones contributes to their different reactivities with Fehling's solution. \[\begin{align*}\text{For aldehydes:} & \, \text{Can oxidize to carboxylic acids with Fehling's.} \ \text{For ketones:} & \, \text{Do not react with Fehling's solution.} \end{align*}\]Understanding these functional groups and their reactivity is crucial in organic chemistry, as it forms the basis for the design and synthesis of new molecules.