CHEM BRAINS: Carboxylic Acids Three Marks

1. Account the reducing nature of formic acid with suitable illustration.

Formic acid is unique because it contains both an aldehyde group and carboxyl group also. Hence it can act as a reducing agent. It reduces Fehling’s solution, Tollen’s reagent and decolourises pink coloured KMnO4 solution.

In all cases formic acid is oxidised to CO2 and water.

(a) Formic acid reduces ammoniacal silver nitrate solution (Tollen’s reagent) to metallic silver.

HCOOH + Ag2O → H2O + CO2 + 2Ag (metallic silver)

(b) Formic acid reduces Fehling’s solution. It reduces blue coloured cupric ions to red coloured cuprous ions.

2. Formic acid reduces Tollen’s reagent, but acetic acid does not. Give reasons.

HCOO– + 2Cu2+ + 5OH – → CO32– + Cu2O + 3H2O

(blue) (red)

Formic acid has

- C = O group an aldehyde group but acetic acid has no aldehydic group. Hence formic acid reduces - Tollen’s solution.

Or Formic acid is unique because it contains both an aldehyde group and carboxyl group also. Hence it can act as a reducing agent. It reduces Tollen’s reagent. But acetic acid has no aldehydic group hence it does not reduces Tollen’s reagent.

Formic acid reduces ammoniacal silver nitrate solution (Tollen’s reagent) to metallic silver.

HCOOH + Ag2O →H2O + CO2 + 2Ag (metallic silver)

3. Give tests for Salicylic acid.

1. Gives violet colour with neutral ferric chloride.

2. Gives effervescence with the sodium bicarbonate.

3. Soluble in sodium hydroxide and reprecipitated on acidification.

4. With Bromine water the colour is discharged with the formation of white precipitate.

4. Give the source and trivial names of i) C3H7COOH ii) HCOOH and iii) C11H23COOH
Formula Source Trivial names

i) C3H7COOH - Butter (Butyrum) - Butyric acid

ii) HCOOH - Red ant (formica) - Formic acid

iii) C11H23COOH - Laurel oil - Lauric acid

5. How is methyl salicylate prepared?

On heating Salicylic acid with methyl alcohol in presence of conc. H2SO4 a pleasant smelling liquid-methyl salicylate is formed.

6. Mention the uses of benzoic acid.

1. Benzoic acid is used as an urinary antiseptic

2. Sodium benzoate is used as food preservative

3. Benzoic acid vapours are used to disinfect bronchial tube.

4. It is used for the manufacture of dyes.

7. Mention the uses of oxalic acid.

Oxalic acid is used

1. for removing ink stains and iron stains.

2. as mordant in dyeing and calico printing.

3. in manufacture of ink and metal polishes.

4. Redox titration

8. What is aspirin? How is it prepared?

Aspirin is Acetyl salicylic acid

Salicylic acid undergoes acetylation by heating with acetic anhydride to form aspirin which is used as an analgesic and antipyretic.

9. What is meant by esterification reaction? Write the equation.

Carboxylic acid reacts with alcohols in presence of mineral acid as catalyst and forms esters. This reaction is called esterification.

H+
CH3COOH + C2H5OH → CH3COOC2H5 + H2O
10. What is the reaction of lactic acid with dil. H2SO4?

With dil. H2SO4 lactic acid dissociates to acetaldehyde and formic acid.

dil. H2SO4

CH3CH(OH)COOH → CH3CHO + HCOOH

11. What is trans-esterification?

In presence of a little acid, methyl acetate is cleaved by ethyl alcohol to form ethyl acetate.This reaction is called trans-esterification.

H+

CH3COOCH3 + C2H5OH → CH3COOC2H5 + CH3OH
12. Write a note on HVZ reaction.

When the halogenation of Carboxylic acid is carried out with halogen and phosphorous trihalide, this reaction is known as Hell-Volhard Zelinsky reaction. (HVZ-reaction).

Br2/PBr3 H2O

RCH2COOH → RCH2COBr → RCHBrCOBr → RCHBrCOOH

13. Write two tests to identify carboxylic acids.

1. Aqueous solution of carboxylic acids turn blue litmus into red colour.

2. Carboxylic acids give brisk effervescence with sodium bi-carbonate due to the evolution of carbon-di-oxide.

3. On warming carboxylic acids with alcohol and concentrated sulphuric acid it forms ester which is identified from its fruity odour.
14. Mention the uses of formic acid

1) In Textile Industry for preparing ‘mordants’.

2) In leather tanning for removing lime from the hides.

3) In coagulating rubber latex.

4) Nickel formate as hydrogenation catalyst.

5) As a stimulant for the growth of yeast.

6) As an antiseptic and in preservation of fruits.

7) In the treatment of gout.

15. Write the uses of Lactic acid

1. In Tanning industry.

2. In Soft drinks.

3. In the Treatment of Digestive disorder in children.

4. Preparation ofLactates, Ethyl lactate as a solvent.

5. Silver lactate as an Antiseptic and Astringent

16. Compare the strength of mono, di, trichloro acetic acid

An acid is a proton donor. The base with which a proton can be released determines the strength of an acid. Organic acids are weak acids. Greater the stability of the anion of the acid, higher is its strength. Thus, acetic acid.

When the bonding pair of electrons between oxygen and hydrogen in – O – H group, is shifted towards oxygen, easier the hydrogen is released as a proton.

Thus when chloroacetic acid is taken, the (– I) effect of chlorine Increases its strength.