Chapter 4 · Question 14

Describe the esterification reaction between ethanoic acid and ethanol. Write the chemical equation, list the conditions required, and give two practical uses of esters. What is saponification and how is it related to soap making?

Q14

Describe the esterification reaction between ethanoic acid and ethanol. Write the chemical equation, list the conditions required, and give two practical uses of esters. What is saponification and how is it related to soap making?

Answer Revealed

Direct Answer:

Esterification is the reaction of a carboxylic acid with an alcohol in the presence of an acid catalyst (concentrated

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

) to form an ester and water:

\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{Conc. H}_2\text{SO}_4} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}

. The mixture is warmed in a water bath for about 5 minutes and then poured into water — a sweet, fruity smell indicates ester formation. Esters are used in making perfumes and as flavouring agents. Saponification is the reverse — treating an ester with an alkali (

\text{NaOH}

) to regenerate the alcohol and the sodium salt of the carboxylic acid:

\text{CH}_3\text{COOC}_2\text{H}_5 \xrightarrow{\text{NaOH}} \text{C}_2\text{H}_5\text{OH} + \text{CH}_3\text{COONa}

. Soaps are sodium or potassium salts of long-chain carboxylic acids, prepared by the alkaline hydrolysis of esters (fats and oils).

Simple Explanation

Esterification is like building a bridge between an acid and an alcohol — ethanoic acid + ethanol → an ester (ethyl ethanoate) + water. The concentrated sulphuric acid helps the reaction along. Esters smell fruity and sweet — they are what give many perfumes and fruit flavours their smell. Saponification is taking the ester apart again using alkali (

\text{NaOH}

) — this is literally how soap is made: boil fat (an ester) with sodium hydroxide and you get soap (the sodium salt of the fatty acid) plus glycerol (the alcohol part).

Exam-Ready Structure

Esterification and saponification are complementary reactions central to the commercial preparation of esters and soaps: 1. Esterification: (a) Definition — esters are most commonly formed by the reaction of a carboxylic acid and an alcohol in the presence of an acid catalyst. (b) Equation —

\text{CH}_3\text{COOH} + \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{\text{Conc. H}_2\text{SO}_4} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}

. Ethanoic acid + ethanol → ethyl ethanoate (ester) + water. (c) Conditions (Activity 4.8): Take

1\text{ mL}

ethanol and

1\text{ mL}

glacial acetic acid with a few drops of concentrated

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

in a test tube. Warm in a water bath for at least 5 minutes. Pour into a beaker of water and smell — the sweet, fruity odour of the ester is evidence of its formation. (d) Uses of esters: (i) Sweet-smelling substances used in making perfumes. (ii) Used as flavouring agents in foods and beverages. 2. Properties of ethanoic acid: (a) Common name — acetic acid. A

5–8\%

solution in water is called vinegar, used as a preservative in pickles. Pure ethanoic acid (glacial acetic acid) has a melting point of

290\text{ K}

and freezes in cold climates. (b) It is a weak acid — only partially ionised in water, unlike mineral acids like

\text{HCl}

which ionise completely. (c) Reaction with a base:

\text{NaOH} + \text{CH}_3\text{COOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}

(sodium ethanoate/sodium acetate). (d) Reaction with carbonates:

2\text{CH}_3\text{COOH} + \text{Na}_2\text{CO}_3 \rightarrow 2\text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2

(Activity 4.9 — gas turns lime water milky). 3. Saponification: (a) Definition — the reaction in which an ester is treated with an alkali (such as

\text{NaOH}

) to convert it back into the corresponding alcohol and the sodium salt of the carboxylic acid. (b) Equation:

\text{CH}_3\text{COOC}_2\text{H}_5 \xrightarrow{\text{NaOH}} \text{C}_2\text{H}_5\text{OH} + \text{CH}_3\text{COONa}

. The ester is hydrolysed by the alkali. (c) Connection to soaps — soaps are sodium or potassium salts of long-chain carboxylic acids. Natural fats and oils are esters of long-chain carboxylic acids with glycerol (a triol). Treating these esters with

\text{NaOH}

(saponification) hydrolyses them to produce the soap (sodium salt of the fatty acid) and glycerol.

Key Points

Esterification: carboxylic acid + alcohol $\xrightarrow{\text{Conc. H}_2\text{SO}_4}$ ester + water
$\text{CH}_3\text{COOH} + \text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{Conc. H}_2\text{SO}_4} \text{CH}_3\text{COOC}_2\text{H}_5 + \text{H}_2\text{O}$
Esters: sweet-smelling; used in perfumes and flavouring agents
Saponification: ester + NaOH → alcohol + sodium salt of carboxylic acid (soap)
Soaps are sodium/potassium salts of long-chain carboxylic acids (fatty acids)
Ethanoic acid is a weak acid — vinegar is a 5–8% aqueous solution of acetic acid

Common Mistakes

Confusing esterification (acid + alcohol → ester) with saponification (ester + alkali → alcohol + salt)
Thinking concentrated $\text{H}_2\text{SO}_4$ in esterification is a reactant — it is a catalyst/dehydrating agent

Describe the esterification reaction between ethanoic acid and ethanol. Write the chemical equation, list the conditions required, and give two practical uses of esters. What is saponification and how is it related to soap making?