Question

(P) and (Q) can be distinguished by (A) \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\) (B) Tollen's test (C) 2,4-DNP (D) \(\mathrm{NaHSO}_{3}\)

Short answer

If one of the compounds (P) or (Q) is an aldehyde and the other is a ketone, Tollen's test (B) can be used to differentiate between them. This test is specifically used to detect aldehydes, which react with Tollen's reagent (ammoniacal silver nitrate) to produce a silver mirror on the test tube, while ketones do not give a positive test.

Step-by-step solution

Test (A): Bromine water test (Br₂/H₂O)

Bromine water is a reagent used commonly for the detection of alkenes or alkynes containing C=C or C≡C bonds, which react with bromine through an electrophilic addition reaction. If either (P) or (Q) has a double or triple bond, this test will help to differentiate them.

Test (B): Tollen's test

This test is specifically used to detect aldehydes, which react with Tollen's reagent (ammoniacal silver nitrate) to produce a silver mirror on the test tube. Ketones, on the other hand, do not give a positive test. If one of the compounds (P) or (Q) is an aldehyde and the other is a ketone, this test will help to differentiate them.

Test (C): 2,4-DNP test

The 2,4-dinitrophenylhydrazine (2,4-DNP) test is used to detect the presence of carbonyl compounds, such as aldehydes and ketones. They form yellow, orange, or red precipitates with 2,4-DNP, but if both (P) and (Q) are carbonyl compounds, this test will not distinguish between them.

Test (D): Sodium bisulfite (NaHSO₃) test

Sodium bisulfite is used to test for the presence of aldehydes, which form water-soluble addition products with NaHSO₃. This test cannot distinguish between (P) and (Q) if both are aldehydes or if neither is an aldehyde. Based on the information provided, we cannot definitively determine which test will distinguish between compounds (P) and (Q). This is because the nature of the compounds is not provided. However, if we assume that one of the compounds is an aldehyde and the other is a ketone, Tollen's test (B) can be used to differentiate between them.

Key concepts

Bromine water test

The Bromine water test is an essential chemical experiment in organic chemistry for detecting unsaturated compounds, specifically alkenes and alkynes. These compounds contain carbon-carbon double or triple bonds, also known as C=C or C≡C bonds. When bromine, often presented as a brown or reddish-brown solution, is added to a compound containing such bonds, it undergoes an electrophilic addition reaction.

During the reaction, the bromine interacts with the unsaturated compound, leading to the decolorization of the bromine water. This color change is a clear indication of the presence of carbon-carbon multiple bonds. This specific test is beneficial because it provides a visual cue and is generally straightforward to perform.

If you have two unknown compounds, let’s say (P) and (Q), containing potential unsaturations, using bromine water can help determine which one contains the unsaturated bonds if they behave differently upon the test.

Tollen's test

Tollen's test is a classic chemical reaction used primarily to differentiate between aldehydes and ketones. This is crucial, especially because both compounds look almost similar in everyday settings but have different chemical properties.

The test employs Tollen's reagent, which is ammoniacal silver nitrate. When introduced to an aldehyde, this reagent facilitates a unique reaction that results in the deposition of metallic silver, often referred to as the "silver mirror" on the test tube's inner surface. This striking occurrence is both a beautiful and effective way to confirm the presence of an aldehyde in a solution.

In contrast, ketones do not react in this manner, yielding no such silver mirror, effectively rendering this test as negative. Therefore, when faced with two compounds, one being an aldehyde and the other a ketone, such a test can be used to tell them apart with ease.

2,4-Dinitrophenylhydrazine test

The 2,4-Dinitrophenylhydrazine test, abbreviated as 2,4-DNP test, is a widely used method in organic chemistry for detecting carbonyl groups present in aldehydes and ketones. The test operates on the principle of condensation reaction, where carbonyl compounds react with 2,4-DNP to form yellow, orange, or red precipitates known as hydrazones.

This reaction is particularly helpful as it can confirm the presence of carbonyl groups. However, it may not help to differentiate between the two types of compounds if both (P) and (Q) are either aldehydes or ketones, as both would yield a positive result. 2,4-DNP serves more as a preliminary confirmation rather than a fine distinction between the carbonyl-containing compounds.

For example, in an experiment where you suspect the presence of a carbonyl group, this test is the perfect first step to an affirmation.

Sodium bisulfite test

The Sodium bisulfite test is another reaction used to identify and confirm the presence of aldehyde groups in organic compounds. It involves using sodium bisulfite (\(\text{NaHSO}_3\)), which selectively reacts with aldehydes, forming water-soluble bisulfite addition products. This reaction does not occur with most ketones, allowing for potential discrimination between aldehydes and ketones.

However, this test has limitations as it can only highlight the presence of aldehydes and is unhelpful when both substances, (P) and (Q), are aldehydes or when neither contains the target functional group. If you are comparing two unknown compounds that you suspect one might be an aldehyde, the Sodium bisulfite test provides a straightforward method to pursue this inquiry.

The utility of this test finds its strength in qualitative analysis, where knowing whether a sample contains an aldehyde can prompt more advanced analytical techniques.