Question

Column-I Column-II (Pair of compounds) (Can be distinguished by) (A) \(\mathrm{CH}_{3}-\mathrm{CHO}\) and \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (P) \(\quad \mathrm{AgNO}_{3}\) (B) \(\mathrm{CH}_{3}-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}_{3}\) and \(\mathrm{Ph}-\mathrm{CHO}\) (Q) Iodoform test (C) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (R) Fehling solution (D) \(\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{OH}\) and \(\mathrm{HCHO}\) (S) \(\quad 2,4-\mathrm{DNP}\) (T) \(\mathrm{NaHSO}_{3}\)

Short answer

The short answer is: (A) matches with (T) (B) matches with (R) (C) matches with (Q) (D) matches with (P)

Step-by-step solution

Identifying functional groups in the given compounds

First, we need to determine the functional groups in each of the given organic compounds to help us understand which tests may be applicable to them. (A) \(\mathrm{CH}_{3}-\mathrm{CHO}\) (Acetaldehyde) - Aldehyde \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (Acetone) - Ketone (B) \(\mathrm{CH}_{3}-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}_{3}\) (2-Propanol) - Secondary alcohol \(\mathrm{Ph}-\mathrm{CHO}\) (Benzaldehyde) - Aldehyde (C) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}_{3}\) (2-Butanol) - Secondary alcohol \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (Acetone) - Ketone (D) \(\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{OH}\) (Ethanol) - Primary alcohol \(\mathrm{HCHO}\) (Formaldehyde) - Aldehyde

Understanding the chemical tests

Next, we need to know the characteristics and reaction capabilities of the given chemical tests in Column-II, which helps to determine which of these tests can be used to differentiate between the given compound pairs. (P) \(\mathrm{AgNO}_{3}\) (Tollens' Test) - Can distinguish between Aldehydes and Ketones. (Q) Iodoform test - Can distinguish between Methyl ketones and other carbonyl compounds. (R) Fehling solution - Can distinguish between Aldehydes and Ketones. (S) 2,4-DNP (2,4-Dinitrophenylhydrazine) - Can distinguish between Aldehydes and Ketones. (T) \(\mathrm{NaHSO}_{3}\) (Sodium bisulfite) - Can distinguish between Aldehydes and Ketones.

Matching the compound pairs with the chemical tests

Now, we will match the pairs of compounds with the corresponding chemical tests that can be used to distinguish between them. (A) \(\mathrm{CH}_{3}-\mathrm{CHO}\) and \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) can be distinguished by: (T) \(\mathrm{NaHSO}_{3}\), as it differentiates between Aldehydes and Ketones. (B) \(\mathrm{CH}_{3}-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}_{3}\) and \(\mathrm{Ph}-\mathrm{CHO}\) can be distinguished by: (R) Fehling solution, as it differentiates between Aldehydes and Ketones. (C) \(\mathrm{CH}_{3} \mathrm{CH}(\mathrm{OH})\mathrm{CH}_{2}\mathrm{CH}_{3}\) and \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) can be distinguished by: (Q) Iodoform test, as it differentiates between Methyl ketones and other carbonyl compounds. (D) \(\mathrm{CH}_{3}-\mathrm{CH}_{2} \mathrm{OH}\) and \(\mathrm{HCHO}\) can be distinguished by: (P) \(\mathrm{AgNO}_{3}\), as it differentiates between Aldehydes and Ketones. In summary: (A) matches with (T) (B) matches with (R) (C) matches with (Q) (D) matches with (P)

Key concepts

Chemical Tests in Organic Chemistry

Chemical tests play a vital role in organic chemistry. They help identify and differentiate various organic compounds.
This is crucial because some compounds share similar physical properties, making them hard to distinguish. Chemical tests provide distinct results based on the reactive nature of the functional groups within compounds.
When conducting these tests, specific reagents interact with functional groups, leading to observable changes like color shifts, precipitate formation, or gas evolution.
Students must understand these tests, as they are indispensable tools in characterizing and distinguishing compounds in laboratory settings.

Functional Groups Identification

Identifying functional groups is the first step in understanding any organic compound. Functional groups are specific groups of atoms within molecules that have characteristic properties.
Depending on the types of bonding and the atoms involved, these groups react predictably, allowing chemists to infer the type of compound they are dealing with.
For example:

• Aldehydes (\( \text{-CHO} \)) and ketones (\( \text{-CO-} \)) contain the carbonyl group with different structures affecting their reactivity.
• Alcohols (such as primary alcohols like ethanol) have the hydroxyl group (\( \text{-OH} \)), making them reactive as well.

Recognizing these groups aids in applying suitable chemical tests to confirm compound identity.

Aldehydes and Ketones Differentiation

Aldehydes and ketones share a common functional group – the carbonyl group (\( \text{-C=O} \)) – but they differ in structure and reactivity.
Aldehydes have the carbonyl group at the end of the carbon chain, while ketones have it within the chain.
This difference affects their chemical behavior and the tests used to identify them:

• Aldehydes are generally more reactive than ketones due to the presence of hydrogen attached to the carbonyl carbon.
• Certain tests, such as Tollens' test and Fehling's solution, exploit these differences to distinguish between the two.

Understanding this differentiation is key in organic chemistry.

Iodoform Test

The iodoform test is a specific chemical test used to identify methyl ketones and some secondary alcohols.
When a compound contains the \( \text{CH}_3\text{CO}- \) group, it will react with iodine in the presence of a base, producing a yellow precipitate of iodoform (\( \text{CHI}_3 \)).
Key points about the iodoform test:

• It detects the presence of methyl ketones, which include compounds like acetone.
• In secondary alcohols that oxidize to methyl ketones, such as 2-propanol, the test will also give a positive result.

This test is useful for confirming the presence of specific functional groups quickly and effectively.

Tollens' Test

Tollens' test is a classic method used to differentiate aldehydes from ketones.
Known for its "silver mirror" effect, this test utilizes a reagent made from silver nitrate (\( \text{AgNO}_3 \)) in ammonia, forming the complex silver diamine ion.
Key aspects of Tollens' test:

• When an aldehyde is present, it reduces the silver ions to metallic silver, which deposits on the container's surface, creating a reflective layer.
• Ketones do not react with Tollens' reagent, so no silver mirror forms.

This makes the test practical and visually straightforward for identifying the presence of aldehydes in a mixture.