Q.Suggest a reagent for conversion of ethanol to ethanoic acid.

The key idea is the oxidation of a primary alcohol to a carboxylic acid. Ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) must be fully oxidised to ethanoic acid ($\text{CH}_3\text{COOH}$) without stopping at the aldehyde stage.

Reasoning:

1. Primary alcohols first oxidise to aldehydes, then further to carboxylic acids.
2. A strong oxidising agent that works under acidic conditions is needed to drive the reaction to completion. …

This is a classic organic chemistry conversion that tests your understanding of oxidation reactions of alcohols. Ethanol is a primary alcohol, and ethanoic acid is a carboxylic acid. The journey from one to the other involves a controlled oxidation.

Why This Approach Works

The core concept is oxidation. In organic chemistry, oxidation of an alcohol means increasing the number of bonds to oxygen (or decreasing bonds to hydrogen). For a primary alcohol like ethanol ($CH_3CH_2OH$), oxidation first gives an aldehyde (ethanal, $CH_3CHO$), and then further oxidation gives the carboxylic acid (ethanoic acid, $CH_3COOH$).

You cannot just "add" an oxygen atom directly. You need a reagent that can remove two hydrogen atoms from the alcohol and then add an oxygen atom to the resulting aldehyde. Strong oxidizing agents do exactly this.

Step-by-Step Reasoning

1. Identify the functional group change. Ethanol has an $-OH$ (alcohol) group. Ethanoic acid has a $-COOH$ (carboxylic acid) group. The carbon atom bonded to the $-OH$ in ethanol (the $C_1$ carbon) must be converted into the carboxyl carbon of the acid. This requires adding an oxygen atom and removing two hydrogen atoms from that carbon.

2. Recall the reagents for alcohol oxidation. For a primary alcohol, the most common and reliable oxidizing agents in your syllabus are:

   • Acidified potassium dichromate ($K_2Cr_2O_7/H_2SO_4$): This is the standard reagent. The orange dichromate ion ($Cr_2O_7^{2-}$) is reduced to green $Cr^{3+}$ ions. The colour change is a key observation.
   • Acidified potassium permanganate ($KMnO_4/H_2SO_4$): This is a stronger oxidising agent. The purple permanganate ion ($MnO_4^-$) is reduced to colourless $Mn^{2+}$ ions.

3. Understand the reaction mechanism (simplified). The reaction proceeds in two stages, but the reagent is used in excess so you don't isolate the intermediate aldehyde.

   • Stage 1: Ethanol is oxidised to ethanal.

    Here, $[O]$ represents the oxygen from the oxidising agent. …

Method: Controlled Oxidation of Primary Alcohols

This is an oxidation reaction (not electrophilic addition — important to distinguish reaction types for exams).

Concept First

• Ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) is a primary alcohol.
• Primary alcohols oxidise first to aldehydes, then to carboxylic acids.
• The reagent must be strong enough to push the oxidation all the way to the acid stage.

Steps

1. Identify the functional group change

   Ethanol ($\text{CH}_3\text{CH}_2\text{OH}$) → Ethanoic acid ($\text{CH}_3\text{COOH}$).

   The terminal carbon goes from $-\text{CH}_2\text{OH}$ to $-\text{COOH}$.

2. Choose the correct oxidising agent

   Use acidified potassium dichromate ($\text{K}_2\text{Cr}_2\text{O}_7 / \text{H}_2\text{SO}_4$).

   • Colour change: orange to green (Cr(VI) → Cr(III)) — a common exam observation.

3. Write the balanced equation

The $[\text{O}]$ represents the oxygen supplied by the dichromate. …

Common Mistakes: Ethanol → Ethanoic Acid (Electrophilic Addition Context)

Students often confuse this with electrophilic addition reactions (like alkene to alcohol), but this conversion is actually an oxidation reaction. Here are the most frequent errors:

✗ Mistake 1: Suggesting an electrophilic addition reagent

Wrong answer: Br₂/H₂O, H₂SO₄, H₂O/H⁺

Why it's wrong: These are reagents for electrophilic addition to alkenes/alkynes — they add atoms across a double bond. Ethanol has no π-bond; it's a saturated alcohol.

How to avoid:

• First identify the functional group change: alcohol (–OH) → carboxylic acid (–COOH)
• This is an oxidation (loss of H, gain of O), not an addition.
• Memorise: Electrophilic addition requires a multiple bond; oxidation requires an oxidising agent.

✗ Mistake 2: Using a weak or incorrect oxidising agent

Wrong answer: KMnO₄ (without specifying conditions), H₂O₂, O₂ (alone)

Why it's wrong:

• KMnO₄ in neutral/alkaline medium gives ethanal (aldehyde), not ethanoic acid.
• H₂O₂ or O₂ alone are too weak to oxidise ethanol directly to acid under normal conditions.

How to avoid:

• For primary alcohol → carboxylic acid, you need a strong oxidising agent under acidic conditions.
• Correct choice: Acidified potassium dichromate ($K_2Cr_2O_7/H_2SO_4$) or acidified potassium permanganate ($KMnO_4/H_2SO_4$).

✗ Mistake 3: Forgetting the colour change / observation

Wrong: Writing only the reagent without mentioning the visual change.

Why it's important: Exam questions often ask for "reagent and observation."

How to avoid:

• With $K_2Cr_2O_7/H_2SO_4$: orange → green (Cr⁶⁺ → Cr³⁺) …