Give chemical tests to distinguish between the following pairs of compounds. (i) Phenol and benzoic acid (ii) Acetaldehyde and Acetone (iii) Pentan-2-one and Pentan-3-one.

To differentiate between the pairs of compounds you've mentioned, we can use specific chemical tests that exploit the unique properties of each compound. Let’s break down each pair and the tests that can effectively distinguish them.

Distinguishing Phenol from Benzoic Acid

Phenol and benzoic acid can be differentiated using a couple of tests:

• Litmus Test: Phenol is a weak acid and will turn blue litmus paper red, while benzoic acid, being a stronger acid, will also turn blue litmus paper red. However, if you add sodium bicarbonate (NaHCO3) to each, only benzoic acid will produce carbon dioxide gas, resulting in effervescence. This is due to the reaction of the carboxylic acid group with the bicarbonate.
• Ferric Chloride Test: When phenol is treated with ferric chloride (FeCl3), it forms a violet complex, indicating the presence of the phenolic -OH group. Benzoic acid does not produce this color change.

Identifying Acetaldehyde and Acetone

To distinguish between acetaldehyde (an aldehyde) and acetone (a ketone), we can use:

• 2,4-Dinitrophenylhydrazine (DNPH) Test: Both compounds will react with DNPH to form yellow or orange precipitates, but the rate of reaction can differ. Acetaldehyde reacts more readily than acetone due to the presence of the aldehyde functional group.
• Oxidation Test: Acetaldehyde can be oxidized to acetic acid using mild oxidizing agents like Tollens' reagent (Ag(NH3)2OH), which will produce a silver mirror. Acetone, being a ketone, does not undergo oxidation under these conditions.

Differentiating Pentan-2-one from Pentan-3-one

For pentan-2-one and pentan-3-one, we can utilize:

• Iodoform Test: Pentan-2-one will give a positive iodoform test, producing a yellow precipitate of iodoform (CHI3) when treated with iodine and sodium hydroxide. This is because pentan-2-one has a methyl group adjacent to the carbonyl group. In contrast, pentan-3-one does not have this structure and will not yield a positive result.
• Infrared Spectroscopy: While this is more of a laboratory technique than a simple test, the IR spectra of these two compounds will show differences in the absorption peaks due to their structural variations. Pentan-2-one will show a distinct peak corresponding to the C=O stretch that can be compared to that of pentan-3-one.

These tests leverage the unique chemical properties of each compound, allowing for clear differentiation based on their functional groups and structural characteristics. Understanding these distinctions is crucial in organic chemistry for identifying and characterizing compounds effectively.