Our Lady of the Rosary College Chemistry Practical Manual

OUR LADY OF THE ROSARY COLLEGE CHEMISTRY PRACTICAL MANUAL  Advanced Level, F.7  Experiment should be carried out under the supervision of a Chemistry teacher  Safety precaution must be noted. Division of work: individual Reaction of carboxylic acids Introduction

Saturated aliphatic monocarboxylic acid form a homologous series. General molecular formula is Cn22n+1COOH, in which CnH2n+1 is the carbon skeleton while the functional group, carboxylic acid has the following structure

This functional group contains both carbonyl and hydroxyl functional groups. These groups modify each other to such an extent that the chemical reactivity of the carboxylic acid group bears little resemblance to its constituent groups. In the carbonyl linkage there is mesomeric withdrawal of electrons away from the carbon atom towards the more electronegative atoms creating relative positive charge on the carbon atom. This deficiency of electrons on the carbon atom intensifies the inductive pull of electron cloud away from the hydrogen atom in the oxygen-hydrogen bond enhancing the stability of the carboxylate anion RCOO- and promoting the release of proton. In other words, the proton is exposed to the environment and it is susceptible to the attack of a base, e.g. water molecule which is, here, behaves as a weak acid.

Reagents required Sodium ethanoate (2g), ethanol (6 cm3), sodium methanoate, (2g), pH paper, sodium ethanedioate (2g), dilute sulphuric(VI) acid (15 cm3), concentrated sulphuric(VI) acid (12 cm3), iron (III) chloride solution (5 cm3), dilute ammonia solution (10 cm3), potassium permanganate solution (1 cm3), silver nitrate solution (3 cm3-), calcium chloride solution (6 cm3), glacial ethanoic acid (5 cm3), anhydrous methanoic acid (5 cm3), ethanedoic acid crystals (5g) Additional Apparatus: nil

Procedure Part A: ethanoic acid 1. Dissolve about 1 g of sodium ethanoate in 2-3 cm3 of distilled water. Add a similar volume dilute sulphuric(VI) acid and warm. Smell the vapour evolved and examine its effect on damp pH paper. 2. Make 2-3 cm3 of an aqueous solution of sodium ethanoate as described above. To this add a few drops of neutral iron(III) chloride solution. (The latter can be made by adding dilute ammonia solution dropwise to about 2 cm3 of bench aqueous iron (III) chloride solution until the first trace of permanent precipitate appears). Perform a blank experiment by adding the same number of drops of neutral iron(III) chloride solution to 2-3 cm3 of distilled water. 3. Mix 1-2 cm3 of dilute sulphuric acid with a like volume of glacial ethanoic acid. Add 3 drops of bench potassium permanganate solution and warm the mixture for about a minute. 4. Adding about 5 drops of glacial acid to a mixture of about 1 cm3 of silver nitrate solution with a like volume of dilute ammonia solution. Try warming the mixture. 5. Run 1-2 cm3 of concentrated sulphuric acid into a test-tube and warm it gently. (Take care: do not boil) Using a dropper, add 3 drops of glacial ethanoic acid. Observe closely for signs of gas evolution. If you detect a gas, try to identify it. Remember to pour the contents of the tube into a sink of cold water. Never add water to the tube. 6. To 1-2 cm3 of ethanol add a like volume of concentrated sulphuric(VI) acid. pour a few drops of glacial ethanoic acid onto the mixture and warm it gently. Note the smell of the vapour evolved. 7. To 1-2 cm3 of glacial ethanoic acid add a like volume of bench calcium chloride solution.

Part B: methanoic and ethanedioic acid Repeat experiments (a) to (g) on both methanoic acid and ethanedioic acid. Record all your observations in the Data Section. Data section Experiment Methanoic acid Ethanoic acid Ethanedioic acid (a) pH value (b) with iron(III) chloride (c) reducing power (d) with Tollen’s reagent (e) with warm and concentrated sulphuric(VI) acid (f) with ethanol (g) tested with calcium chloride

Questions for discussion 1. Tabulate solubilities and volatilities of the three carboxylic acid. 2. Write equation to explain the reaction between a carboxylic acid and iron (III) chloride solution. 3. Could you use the iron(III) chloride test to distinguish between (a) methanoic acid and ethanoic acid (b) ethanoic acid and ethanedioic acid? 4. Which carboxylic acid shows reducing power towards (a) Tollen's reagent (b) acidified potassium permanganate (c) warm and concentrated sulphuric acid? 5. Is there any evidence to show that warm and concentrated sulphuric(VI) acid is a dehydrating agent toward the carboxylic acids? Give the evidence. 6. Explain, with the aid of an equation, the reactions in question 4. 7. Can you distinguish the three acids?