Chapter 4 · Question 11

How is ethanol converted to ethanoic acid? Why is this reaction classified as an oxidation reaction? Name the oxidising agents used and explain the observation that indicates the reaction is over.

Q11

How is ethanol converted to ethanoic acid? Why is this reaction classified as an oxidation reaction? Name the oxidising agents used and explain the observation that indicates the reaction is over.

Answer Revealed

Direct Answer:

Ethanol (

\text{CH}_3\text{CH}_2\text{OH}

) is oxidised to ethanoic acid (

\text{CH}_3\text{COOH}

) using alkaline potassium permanganate (

\text{KMnO}_4

) or acidified potassium dichromate (

\text{K}_2\text{Cr}_2\text{O}_7

) as oxidising agents, with heating:

\text{CH}_3\text{CH}_2\text{OH} \xrightarrow[\text{or acidified }\text{K}_2\text{Cr}_2\text{O}_7 + \text{Heat}]{\text{Alkaline }\text{KMnO}_4 + \text{Heat}} \text{CH}_3\text{COOH}

. This is an oxidation reaction because ethanol gains oxygen — the oxygen atom from the oxidising agent adds to the ethanol molecule. During the activity, the purple colour of

\text{KMnO}_4

disappears initially as it oxidises ethanol. When excess

\text{KMnO}_4

is added and the purple colour persists, it indicates that all ethanol has been oxidised and the reaction is complete.

Simple Explanation

To turn ethanol into ethanoic acid, you need to add oxygen to it — this is oxidation. Potassium permanganate (

\text{KMnO}_4

, purple) acts like an oxygen donor: it gives oxygen to ethanol, converting it to ethanoic acid. As

\text{KMnO}_4

does its job, its purple colour fades. When you keep adding drops and the purple colour stops disappearing and stays, you know all the ethanol has been used up — the oxidising agent has nothing left to oxidise.

Exam-Ready Structure

The controlled oxidation of ethanol to ethanoic acid is a key laboratory demonstration of the oxidation of alcohols: 1. Oxidation reaction definition: Some substances are capable of adding oxygen to other compounds. These substances are known as oxidising agents. When ethanol gains oxygen to form ethanoic acid, the reaction is classified as an oxidation reaction. 2. Reaction:

\text{CH}_3\text{CH}_2\text{OH} \xrightarrow[\text{or acidified }\text{K}_2\text{Cr}_2\text{O}_7 + \text{Heat}]{\text{Alkaline }\text{KMnO}_4 + \text{Heat}} \text{CH}_3\text{COOH}

. The ethanol molecule (

\text{C}_2\text{H}_5\text{OH}

) is converted to ethanoic acid (

\text{CH}_3\text{COOH}

) — the

\!-\!\text{CH}_2\text{OH}

group is oxidised to the

\!-\!\text{COOH}

(carboxylic acid) group. 3. Oxidising agents used: (a) Alkaline potassium permanganate (

\text{KMnO}_4

in basic medium). (b) Acidified potassium dichromate (

\text{K}_2\text{Cr}_2\text{O}_7

in acid medium). Both require heating. 4. Activity 4.5 — visual observation: (a) About

3\text{ mL}

of ethanol is warmed gently in a water bath. (b) A

5\%

solution of alkaline

\text{KMnO}_4

is added drop by drop. (c) Initially, the purple colour of the

\text{KMnO}_4

disappears because the

\text{KMnO}_4

is being consumed to oxidise ethanol. (d) When all ethanol has been oxidised to ethanoic acid, further addition of

\text{KMnO}_4

will not be consumed — the purple colour persists in the test tube. (e) The persistence of the purple colour is the indicator that the reaction (oxidation) is complete. 5. This is a controlled oxidation — in contrast, combustion (uncontrolled oxidation) would burn ethanol completely to

\text{CO}_2

and

\text{H}_2\text{O}

Key Points

Ethanol to ethanoic acid: $\text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Alk. }\text{KMnO}_4 + \Delta} \text{CH}_3\text{COOH}$
Oxidising agents: alkaline $\text{KMnO}_4$ or acidified $\text{K}_2\text{Cr}_2\text{O}_7$ (both with heat)
This is oxidation because the ethanol molecule gains oxygen (goes from $\!-\!\text{CH}_2\text{OH}$ to $\!-\!\text{COOH}$ )
Purple $\text{KMnO}_4$ colour disappears initially → purple persists when all ethanol is oxidised → reaction complete
Controlled oxidation (alcohol → acid) versus uncontrolled oxidation (combustion → $\text{CO}_2$ + $\text{H}_2\text{O}$ )

How is ethanol converted to ethanoic acid? Why is this reaction classified as an oxidation reaction? Name the oxidising agents used and explain the observation that indicates the reaction is over.