CBSE Class 12 Chemistry Aldehydes Ketones Carboxylic Acids Questions Answers

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ALDEHYDES,KETONES AND CARBOXYLIC ACIDS

SECTION-A 

1. Name one distinguishing test between aldehydes and ketones?

Ans. Aldehydes and ketones can be distinguished by Tollen’s test. Aldehydes give a silver mirror on reacting with Tollen’s reagent whereas ketones will not react.

2. Give reason why Formaldehyde does not undergo aldol condensation?

Ans. Formaldehyde does not have any α -hydrogen and therefore it cannot show aldol condensation.

3. Carboxylic acids have higher boiling points than alcohols of same no. of carbon atoms?

Ans. Carboxylic acids have more extensive association of molecules through intermolecular hydrogen bonding than alcohols.

4. Write IUPAC name .of CH₃COCH₂COCH₃.

Ans.Pentane-2,4-dione.

5. What product is obtained when Ethyl benzene is oxidized with alkaline KMnO4?

Ans. Benzoic acid is formed.

6. Give chemical test to distinguish between acetaldehyde and benzaldehyde.

Ans. Acetaldehyde will respond to Iodoform test whereas benzaldehyde does not.

7. Write one chemical to distinguish between Formic acid and Acetic acid.

 Ans.Formic acid gives silver mirror when treated with Tollen,’s reagent whereas acetic acid does not.

8. Give two important uses of formalin.

Ans.Used as a preservative.

         Used for the preparation of Bakelite.

9. How is formalin and trioxane related to methanal?

Ans. Formalin is 40%aqeous solution of methanol whereas trioxane is trimer of methanal.

10. Complete the following reaction and give the name of the major product.

HCHO+ CH₃MgX -----------------------------→ ?

Ans. HCHO +CH₃MgX --------→-CH₃CH₂OH + Mg(OH)X

CH₃CH₂OH- Ethanol

11. Draw the structural formula of Hex-2-en4-yn-oic acid.

Ans. CH₃-C≡C-CH=CHCOOH

12. Arrange the following in the increasing order of acidic character.

HCOOH, ClCH₂COOH, CF₃COOH, Cl₃CCOOH

Ans. CF₃COOH>CCl₃COOH>ClCH₂COOH>HCOOH

13. Complete the reaction:-

RCONH₂+4NaOH+Br₂ --------→

Ans.RCONH₂+4NaOH+Br₂-------→ RNH₂+ 2NaBr+ Na₂CO₃+2H₂O.

14. Give one chemical test to distinguish between Phenol and benzoic acid.

Ans. On treatment with neutral FeCl₃ solution Phenol gives a violet color whereas Benzoic acid does not.

15. Most of the aromatic acids are solids while acetic acids and others of this series are liquids. Why?

Ans. Aromatic acids have higher molecular weights. Therefore more Vander Waal’s force of attraction as compared to aliphatic acids and hence they are solids. 

SECTION –B 

1. Would you expect benzaldehyde to be more or less reactive in nucleophilic addition reaction

than Propanal? Explain your answer.

Ans. The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group present in propanal. The polarity of the carbonyl group is reduced in benzaldehyde due to resonance and hence is less reactive.

2. Describe the Trans esterification reaction giving an example.

Ans. When an ester reacts with alcohol to form another ester and another alcohol, the process is called trans-esterification.

CH₃COOC₂H₅ +CH₃OH ----------→ CH₃COOC₂H₅ + C₂H₅OH

3. Explain Hell- Volhard –Zelinsky reaction with an example.

Ans. Carboxylic acids having an a α hydrogen atom are halogenated at the α position on treatment with chlorine or bromine in the presence of small amount of red phosphorous to give α- halo carboxylic acids

                              (i)X₂/P (Red)

RCH₂COOH --------------→RCH(X) COOH.

                             (ii) H₂O/H⁺

4. Give simple chemical tests to distinguish between:-

(i) Pentan-2-one and Pentane-3-one

(ii) Ethanal and propanal

Ans. (i) Pentan-2-one gives Iodoform test on treatment with I₂/NaOHwhere as Pentane -3-one does not.

(ii)Ethanal gives Iodoform test whereas Propanal does not.

5. Although Phenoxide ion has more number of resonating structures than Carboxylate ion, Carboxylic

acids are more acidic than Phenol .Why?

Ans. In carboxylate ion (-)ve charge is delocalized over two oxygen atoms whereas in phenoxide ion (-)ve charge is delocalized over one oxygen atom .Therefore carboxylate ion is more stable than phenoxide ion .That is why Carboxylic acids are more acidic than Phenol.

6. Why is there a large difference in the boiling points of but anal and butan-1-ol?

Ans:-Butan-1-ol has higher boiling point due to intermolecular hydrogen bonding 

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