Q.Give simple chemical tests to distinguish between the following pairs of compounds.

Concept: Oxidation Reactions – The key is that aldehydes are easily oxidised to carboxylic acids (they reduce Tollen’s reagent and Fehling’s solution), while ketones are not. Also, methyl ketones give the iodoform test, and phenols give a colour with neutral FeCl₃.

Reasoning steps:

1. Propanal vs Propanone: Propanal (aldehyde) reduces Tollen’s reagent (silver mirror) and Fehling’s solution (red precipitate); propanone does not.
2. Acetophenone vs Benzophenone: Acetophenone (methyl ketone) gives a yellow precipitate of iodoform with I₂/NaOH; benzophenone does not.
3. Phenol vs Benzoic acid: Phenol gives a violet colour with neutral FeCl₃; benzoic acid does not.
4. Benzoic acid vs Ethyl benzoate: Benzoic acid reacts with NaHCO₃ to evolve CO₂ gas; ethyl benzoate does not.
5. Pentan-2-one vs Pentan-3-one: Pentan-2-one (methyl ketone) gives iodoform test positive; pentan-3-one gives negative.
6. Benzaldehyde vs Acetophenone: Benzaldehyde reduces Tollen’s reagent (silver mirror); acetophenone does not. …

(i) Propanal and Propanone

Concept: Both are carbonyl compounds, but propanal is an aldehyde (easily oxidised) while propanone is a ketone (resistant to mild oxidation). The Tollens' test (ammoniacal silver nitrate) is perfect here — aldehydes reduce Ag⁺ to a silver mirror, ketones do not.

Step-by-step:

1. Prepare Tollens' reagent: add dilute NaOH to AgNO₃ solution, then add just enough NH₄OH to dissolve the brown precipitate.
2. Add a small sample of each compound to separate test tubes containing the reagent.
3. Warm gently in a water bath (do not boil).
4. Observation: Propanal gives a bright silver mirror on the tube wall. Propanone gives no change. …

Here is the clear solution method for distinguishing the given pairs of compounds using simple chemical tests.

Method Name: Functional Group & Oxidation State Differentiation via Specific Reagent Tests

Core Concept: Different functional groups (aldehydes, ketones, carboxylic acids, phenols, esters) react differently to specific oxidizing agents or nucleophiles. The key is to identify a reagent that gives a positive test (color change, precipitate, gas evolution) with one compound but no reaction with the other.

(i) Propanal and Propanone

Method: Tollens' Test (Silver Mirror Test)

• Reagent: Ammoniacal silver nitrate ($[Ag(NH_3)_2]^+OH^-$).
• Steps:
  1. Take a small amount of each compound in separate clean test tubes.
  2. Add Tollens' reagent to each.
  3. Warm gently in a water bath (do not boil).
• Observation:
  • Propanal: A silver mirror forms on the inner wall of the test tube (aldehyde is oxidized to carboxylate; $Ag^+$ is reduced to $Ag$).
  • Propanone: No reaction (ketones are not easily oxidized by Tollens' reagent).

(ii) Acetophenone and Benzophenone

Method: Iodoform Test (Methyl Ketone Test)

• Reagent: Iodine ($I_2$) in the presence of sodium hydroxide ($NaOH$).
• Steps:
  1. Dissolve a small amount of each compound in a little ethanol or water.
  2. Add $I_2$ solution and then $NaOH$ solution dropwise until the brown color of iodine just disappears.
  3. Warm gently.
• Observation:
  • Acetophenone: A yellow precipitate of iodoform ($CHI_3$) forms (it has a $CH_3CO-$ group).
  • Benzophenone: No yellow precipitate (it lacks the $CH_3CO-$ group).

(iii) Phenol and Benzoic acid

Method: Neutral FeCl$_3$ Test (Ferric Chloride Test)

• Reagent: Neutral ferric chloride solution ($FeCl_3$).
• Steps:
  1. Dissolve a small amount of each compound in water (or a little alcohol if insoluble).
  2. Add a few drops of neutral $FeCl_3$ solution.
• Observation:
  • Phenol: A violet/blue color appears (due to formation of a colored iron-phenolate complex).
  • Benzoic acid: A buff-colored precipitate (or no color change) forms (carboxylic acids give a different, less intense color or precipitate).

(iv) Benzoic acid and Ethyl benzoate

Method: Sodium Bicarbonate Test (NaHCO$_3$ Test)

• Reagent: Aqueous sodium bicarbonate ($NaHCO_3$).
• Steps:
  1. Take a small amount of each compound in separate test tubes.
  2. Add a few mL of $NaHCO_3$ solution.
  3. Observe for effervescence.
• Observation:
  • Benzoic acid: Brisk effervescence of $CO_2$ gas (acid reacts with bicarbonate).
  • Ethyl benzoate: No effervescence (ester is not acidic enough to react with $NaHCO_3$).

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Here are the common mistakes students make when tackling these distinction tests in oxidation reactions, along with how to avoid each.

General Mistake: Confusing "Chemical Test" with "Physical Property"

• The Mistake: Trying to distinguish compounds by smell, colour, or solubility (e.g., "Propanal smells different").
• Why it fails: Exam questions demand a reagent and a visible observation (precipitate, colour change, gas evolution). Physical properties are not considered "chemical tests."
• How to Avoid: Always ask yourself: "What reagent do I add, and what do I see?" If you can't name a reagent, your answer is incomplete.

(i) Propanal vs. Propanone

Common Mistake: Using Tollen’s reagent or Fehling’s solution but forgetting that both are aldehydes/ketones.

• The Error: Students write "Propanal gives silver mirror, Propanone does not" — which is correct, but they often mislabel which is which.
• How to Avoid: Remember: Only aldehydes reduce Tollen’s reagent. Propanal ($\text{CH}_3\text{CH}_2\text{CHO}$) is an aldehyde; Propanone ($\text{CH}_3\text{COCH}_3$) is a ketone.
• Correct Test: Add Tollen’s reagent (ammoniacal silver nitrate) and warm.
  • Propanal: Silver mirror forms.
  • Propanone: No reaction.

(ii) Acetophenone vs. Benzophenone

Common Mistake: Assuming both are ketones, so no test works.

• The Error: Students forget that methyl ketones (like Acetophenone, $\text{C}_6\text{H}_5\text{COCH}_3$) give the iodoform test, while Benzophenone ($\text{C}_6\text{H}_5\text{COC}_6\text{H}_5$) does not.
• How to Avoid: Check the structure: Does the carbonyl have a $\text{CH}_3$ group attached? If yes → iodoform test positive.
• Correct Test: Add iodine ($\text{I}_2$) and NaOH (or NaOI) and warm.
  • Acetophenone: Yellow precipitate of $\text{CHI}_3$ (iodoform).
  • Benzophenone: No yellow precipitate.

(iii) Phenol vs. Benzoic acid

Common Mistake: Using $\text{NaHCO}_3$ test but forgetting that phenol is a weaker acid.

• The Error: Students think both react with $\text{NaHCO}_3$ because both are acidic. Phenol does not react with $\text{NaHCO}_3$ (it needs $\text{NaOH}$).
• How to Avoid: Remember the acid strength order: Benzoic acid ($\text{p}K_a \approx 4.2$) > Carbonic acid ($\text{p}K_a \approx 6.4$) > Phenol ($\text{p}K_a \approx 10$). Only benzoic acid displaces $\text{CO}_2$ from $\text{NaHCO}_3$.
• Correct Test: Add aqueous $\text{NaHCO}_3$.
  • Benzoic acid: Effervescence of $\text{CO}_2$ gas.
  • Phenol: No effervescence.

(iv) Benzoic acid vs. Ethyl benzoate

Common Mistake: Using $\text{NaHCO}_3$ test but forgetting that esters are neutral.

• The Error: Students think Ethyl benzoate ($\text{C}_6\text{H}_5\text{COOC}_2\text{H}_5$) might react because it has a carbonyl group. It does not — esters do not release $\text{H}^+$ ions.
• How to Avoid: Only free carboxylic acids ($-\text{COOH}$) react with $\text{NaHCO}_3$ to give $\text{CO}_2$. Esters are not acidic.
• Correct Test: Add aqueous $\text{NaHCO}_3$.
  • Benzoic acid: Effervescence of $\text{CO}_2$.
  • Ethyl benzoate: No reaction.

(v) Pentan-2-one vs. Pentan-3-one

Common Mistake: Using Tollen’s or Fehling’s test — both are ketones.

• The Error: Students forget that both are ketones, so neither reduces Tollen’s reagent. They need a test that distinguishes a methyl ketone from a non-methyl ketone.
• How to Avoid: Check the position of the carbonyl. Pentan-2-one ($\text{CH}_3\text{COCH}_2\text{CH}_2\text{CH}_3$) has a $\text{CH}_3\text{CO}-$ group → iodoform positive. Pentan-3-one ($\text{CH}_3\text{CH}_2\text{COCH}_2\text{CH}_3$) does not.
• Correct Test: Iodoform test (iodine + NaOH).
  • Pentan-2-one: Yellow precipitate of $\text{CHI}_3$.
  • Pentan-3-one: No precipitate.

(vi) Benzaldehyde vs. Acetophenone

Common Mistake: Using Tollen’s test but forgetting that Benzaldehyde is an aldehyde and Acetophenone is a methyl ketone.

• The Error: Students often use only one test and miss the fact that both can give positive results with different reagents. …