Chapter 4 · Question 13

Describe two important chemical reactions of ethanol. Explain how ethanol reacts with sodium and how it can be converted to ethene. What is the role of concentrated sulphuric acid in the dehydration reaction?

Q13

Describe two important chemical reactions of ethanol. Explain how ethanol reacts with sodium and how it can be converted to ethene. What is the role of concentrated sulphuric acid in the dehydration reaction?

Answer Revealed

Direct Answer:

Two key reactions of ethanol: (1) Reaction with sodium: Ethanol reacts with sodium metal to produce sodium ethoxide and hydrogen gas:

2\text{Na} + 2\text{CH}_3\text{CH}_2\text{OH} \rightarrow 2\text{CH}_3\text{CH}_2\text{O}^-\text{Na}^+ + \text{H}_2 \uparrow

. The gas can be tested by the pop sound with a burning splinter (characteristic of hydrogen). (2) Dehydration to ethene: When ethanol is heated at

443\text{ K}

with excess concentrated sulphuric acid, it undergoes dehydration — a water molecule is removed, forming ethene:

\text{CH}_3\!-\!\text{CH}_2\text{OH} \xrightarrow[\text{H}_2\text{SO}_4]{\text{Hot Conc.}} \text{CH}_2\!=\!\text{CH}_2 + \text{H}_2\text{O}

. Concentrated

\text{H}_2\text{SO}_4

acts as a dehydrating agent — it removes the elements of water from ethanol, converting the saturated alcohol into an unsaturated hydrocarbon.

Simple Explanation

Ethanol does two interesting things in the lab. First, drop a tiny piece of sodium into ethanol and it fizzes — hydrogen gas bubbles out, just like sodium reacts with water (but more gently). The product is sodium ethoxide. Second, heat ethanol strongly with concentrated sulphuric acid and it becomes ethene (

\text{CH}_2\!=\!\text{CH}_2

), the same starting material for polythene. The sulphuric acid pulls water out of the ethanol molecule, leaving the double bond behind — that is why it is called a dehydrating agent.

Exam-Ready Structure

Ethanol (

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

), commonly called alcohol, is a commercially important carbon compound with well-characterised reactions: 1. Physical properties of ethanol: It is a liquid at room temperature, soluble in water in all proportions, and is the active ingredient of all alcoholic drinks. It is also used as a solvent in medicines like tincture iodine, cough syrups, and tonics. 2. Reaction with sodium (Activity 4.6): (a) A small piece of sodium (size of a couple of rice grains) is dropped into absolute ethanol in a test tube. (b) Observation: Bubbles of a gas are produced — the sodium reacts with ethanol. (c) Equation:

2\text{Na} + 2\text{CH}_3\text{CH}_2\text{OH} \rightarrow 2\text{CH}_3\text{CH}_2\text{O}^-\text{Na}^+ + \text{H}_2 \uparrow

. The product is sodium ethoxide and hydrogen gas. (d) The hydrogen gas evolved can be tested by bringing a burning splinter near the mouth of the test tube — it burns with a pop sound. (e) Note: This is similar to the reaction of sodium with water or acids, which also produce hydrogen. 3. Dehydration to ethene (unsaturated hydrocarbon): (a) Ethanol is heated at

443\text{ K}

with excess concentrated sulphuric acid. (b) Equation:

\text{CH}_3\!-\!\text{CH}_2\text{OH} \xrightarrow[\text{H}_2\text{SO}_4]{\text{Hot Conc.}} \text{CH}_2\!=\!\text{CH}_2 + \text{H}_2\text{O}

. The ethanol molecule loses a molecule of water (

\text{H}_2\text{O}

) and the resulting molecule is ethene, which has a carbon–carbon double bond. (c) Role of concentrated

\text{H}_2\text{SO}_4

: It acts as a dehydrating agent — it removes water from ethanol, facilitating the formation of the double bond. 4. Additional context — health effects: (a) Ethanol in small quantities causes drunkenness; pure ethanol (absolute alcohol) can be lethal. Long-term consumption causes serious health problems. (b) Denatured alcohol: Industrial ethanol is made unfit for drinking by adding poisonous substances (like methanol) and blue dye. (c) Methanol is extremely toxic — oxidised in the liver to methanal, which coagulates cellular protoplasm and attacks the optic nerve, causing blindness.

Key Points

Ethanol + sodium: $2\text{Na} + 2\text{C}_2\text{H}_5\text{OH} \rightarrow 2\text{C}_2\text{H}_5\text{ONa} + \text{H}_2 \uparrow$
Hydrogen gas test: pop sound with a burning splinter
Dehydration: $\text{C}_2\text{H}_5\text{OH} \xrightarrow[\text{H}_2\text{SO}_4]{\text{Hot Conc.}} \text{CH}_2\!=\!\text{CH}_2 + \text{H}_2\text{O}$
Conc. $\text{H}_2\text{SO}_4$ is a dehydrating agent — removes water from ethanol
Ethanol is the active ingredient of all alcoholic drinks; also a good solvent (tincture iodine, cough syrups)

Common Mistakes

Writing the product as sodium hydroxide ( $\text{NaOH}$ ) — sodium ethoxide ( $\text{C}_2\text{H}_5\text{ONa}$ ) is the correct product
Confusing dehydration (removing water) with oxidation (adding oxygen) — both change the carbon skeleton differently

Describe two important chemical reactions of ethanol. Explain how ethanol reacts with sodium and how it can be converted to ethene. What is the role of concentrated sulphuric acid in the dehydration reaction?