NCERT Exemplar Class 12 Chemistry Solutions chapter 12 Aldehydes, Ketones and Carboxylic Acids

Question 1. Addition of water to alkynes occurs in acidic medium and the presence of Hg²⁺ ions as a catalyst. Which of the following products will be formed on addition of water to but-1-yne under these conditions.

Answer:

The answer is the option (ii)

On addition of water to but-1-yne in the presence of H₂SO₄ and HgSO₄ , 2-butanone is obtained (by Markovnikoff’s rule).

Question 2. Which of the following compounds is most reactive towards nucleophilic addition reactions?

Answer:

The answer is the option (i)

CH₃CHO (an aldehyde) is generally considered more reactive than ketones towards nucleophilic addition reactions. Ketones have 2 large substituents compared to 1 in aldehydes, that hinder the nucleophilic approach. Aldehydes are also considered more reactive because the 2 alkyl groups lead to the reduction of electrophilicity of the carbonyl carbon of ketone by releasing electron in its direction.
The reactivity order can be given as:

Question 3. The correct order of increasing acidic strength is _____________.
(i) Phenol < Ethanol < Chloroacetic acid < Acetic acid
(ii) Ethanol < Phenol < Chloroacetic acid < Acetic acid
(iii) Ethanol < Phenol < Acetic acid < Chloroacetic acid
(iv) Chloroacetic acid < Acetic acid < Phenol < Ethanol
Answer:

The answer is the option (iii)

Due to formation of more stable conjugate base after removal of H⁺ phenol is more stable than the alcohol.
On the other hand, the stable conjugate base formed after removing H⁺ makes carboxylic acid more acidic than phenol.
In case chloroacetic acid, the presence of electron withdrawing chlorine group makes it more acidic compared to the acetic acid.

Question 4. Compound can be prepared by the reaction of _____________.

(i) Phenol and benzoic acid in the presence of NaOH
(ii) Phenol and benzoyl chloride in the presence of pyridine
(iii) Phenol and benzoyl chloride in the presence of ZnCl₂
(iv) Phenol and benzaldehyde in the presence of palladium
Answer:

The answer is the option (ii)

The compound C₆H₅OCOC₆H₅ can be prepared by the given below reaction. In this, the phenol is made to react with acid chloride in presence of an alkali. The alkali used is pyridine and acid chloride is benzoyl chloride.

Question 5. The reagent which does not react with both, acetone and benzaldehyde.
(i) Sodium hydrogen sulphite
(ii) Phenyl hydrazine
(iii) Fehling’s solution
(iv) Grignard reagent
Answer:

The answer is the option (iii)

Fehling’s solution is the reagent that does not react with both acetone and benzaldehyde.

Question 6. Cannizaro’s reaction is not given by _____________.

Answer:

The answer is the option (iv)
CH₃CHO contains 3 α-hydrogens while other three compounds have no any α -hydrogen. Hence, Cannizzaro’s reaction is given by them. Aldehydes without any alpha hydrogen undergo reduction and self-oxidation reaction when treated with conc. Alkali.

Question 7. Which product is formed when the compound is treated with concentrated aqueous KOH solution?

Answer:

The answer is the option (ii)

Benzaldehyde does not have any α hydrogen. When reacted with aqueous KOH solution, it gives Cannizzaro’s reaction.

Question 8.

Structure of ‘A’ and type of isomerism in the above reaction are respectively.
(i) Prop-1-en-2-ol, metamerism
(ii) Prop-1-en-1-ol, tautomerism
(iii) Prop-2-en-2-ol, geometrical isomerism
(iv) Prop-1-en-2-ol, tautomerism
Answer:

The answer is the option (iv)

A undergoes tautomerism to form acetone.

Question 9. Compounds A and C in the following reaction are __________.

(i) identical
(ii) positional isomers
(iii) functional isomers
(iv) optical isomers
Answer:

The answer is the option (ii) positional isomers

Thus, and CH₃−CH₂−CH₂OH are positional isomers.
We know that acetaldehyde when treated with Grignard reagent followed by hydrolysis gives propane 2-ol.

Question 10. Which is the most suitable reagent for the following conversion?

(i) Tollen’s reagent
(ii) Benzoyl peroxide
(iii) I₂ and NaOH solution
(iv) Sn and NaOH solution
Answer:

The answer is the option (iii)

Iodoform test is most suitable for the given reaction.

Question 11. Which of the following compounds will give butanone on oxidation with alkaline KMnO4 solution?
(i) Butan-1-ol
(ii) Butan-2-ol
(iii) Both of these
(iv) None of these
Answer:

The answer is the option (ii).

Aldehydes, when treated with common oxidising agents get readily oxidized to carboxylic acids.

Question 12. In Clemmensen Reduction carbonyl compound is treated with _____________.
(i) Zinc amalgam + HCl
(ii) Sodium amalgam + HCl
(iii) Zinc amalgam + nitric acid
(iv) Sodium amalgam + HNO₃
Answer:

The answer is the option (i)

Clemmensen reduction is used in the conversion of carbonyl group to CH₂ group. In this reaction, zinc amalgam and HCl act as reagent.

Question 14. Treatment of compound with NaOH solution yields

(i) Phenol
(ii) Sodium phenoxide
(iii) Sodium benzoate
(iv) Benzophenone
Answer:

The answer is the option (ii, iii)

We get sodium phenoxide and sodium benzoate by nucleophilic substitution reaction.

Question 15. Which of the following conversions can be carried out by Clemmensen Reduction?
(i) Benzaldehyde into benzyl alcohol
(ii) Cyclohexanone into cyclohexane
(iii) Benzoyl chloride into benzaldehyde
(iv) Benzophenone into diphenyl methane
Answer:

The answer is the option (ii, iv)
Clemmensen reduction is used in the conversion of carbonyl group of aldehydes and ketones to CH₂ group when treated with zinc amalgam and conc. HCl. It is used to convert cyclohexanone into cyclohexane and benzophenone into diphenyl methane.

Question 16. Through which of the following reactions number of carbon atoms can be increased in the chain?
(i) Grignard reaction
(ii) Cannizaro’s reaction
(iii) Aldol condensation
(iv) HVZ reaction
Answer:

The answer is the option (i,iii)

Grignard reaction is used to increase the number of carbon atom in chain. Aldol condensation increases the number of carbon atom in the chain for aldehydes and ketones.

Question 17. Benzophenone can be obtained by ____________.
(i) Benzoyl chloride + Benzene + AlCl₃
(ii) Benzoyl chloride + Diphenyl cadmium
(iii) Benzoyl chloride + Phenyl magnesium chloride
(iv) Benzene + Carbon monoxide + ZnCl₂
Answer:

The answer is the option (i,ii)

Benzophenone can be obtained by either of benzoyl chloride + diphenylcadmium or benzoyl chloride+ benzene+ AlCI₃ . The latter is referred to as Friedel-Craft acylation reaction.

Question 18. Which of the following is the correct representation for intermediate of nucleophilic addition reaction to the given carbonyl compound (A) :

Answer:

The answer is the option (i,ii)

The 2 possible orientation of a planar molecule of carbonyl compound in case of nucleophilic attack are:

The nucleophililic attack can occur either from front or rear side.

Question 20. Write a test to differentiate between pentan-2-one and pentan-3-one.

Answer:

The answer is Idoform test. There is a CH₃CO group in Pentan-2-one and that is why it will give a positive iodoform test.

Question 21. Give the IUPAC names of the following compounds

Answer:

(i) 3-Phenylprop-2-en-1 -al
(ii) Cyclohexanecarbaldehyde
(iii) 3-Oxopentan-l-al
(iv) But-2-en-l-al

Question 22. Give the structure of the following compounds.
(i) 4-Nitropropiophenone
(ii) 2-Hydroxycyclopentanecarbaldehyde
(iii) Phenyl acetaldehyde
Answer:

Question 23. Write IUPAC names of the following structures.

Answer:

(i) Ethane-1,2-dial
(ii) Benzene-1, 4-dicarbaldehyde
(iii) 3-Bromobenzaldehyde

Question 24. Benzaldehyde can be obtained from benzal chloride. Write reactions for obtaining benzal chloride and then benzaldehyde from it.
Answer:

Reactions for obtaining benzalchloride and benzaldehyde from it are:

Question 25. Name the electrophile produced in the reaction of benzene with benzoyl chloride in the presence of anhydrous AlCl3. Name the reaction also.
Answer:

The electrophile produced in the reaction of benzene with benzoyl chloride in the presence of anhydrous AlCl₃ is benzoylium cation. The product formed in this reaction is benzophenone. This reaction is called Friedel Craft's acylation reaction.

Question 26. Oxidation of ketones involves carbon-carbon bond cleavage. Name the products formed on oxidation of 2, 5-dimethylhexan-3-one.
Answer:

The products formed on oxidation of 2, 5-dimethylhexan-3-one are: -
2-Methylpropanoic acid
Ethanoic acid and
Methanoic acid.
3-Methylbutanoic acid

Question 27. Arrange the following in decreasing order of their acidic strength and give the reason for your answer.
CH₃CH₂OH,CH₃COOH,ClCH₂COOH,FCH₂COOH,C₆H₅CH₂COOH
Answer:

FCH₂COOH>ClCH₂COOH>C₆H₅CH₂COOH>CH₃COOH>CH₃CH₂OH
The electron-withdrawing group increase the acidity of carboxylic acids since they stabilize the conjugate base by delocalization of charge. Whereas the electron donating group decrease the acidity as the resulting conjugate base is unstable.
Higher the electronegativity, greater the acidic strength.
(i.e,F>Cl>C₆H₅>H)

Question 28. What product will be formed on reaction of propanal with 2-methyl propanal in the presence of NaOH? What products will be formed? Write the name of the reaction also.
Answer:

On reaction of propanal with 2-methyl propanal in the presence of NaOH, aldehyde mixture is formed.

Question 29. Compound ‘A’ was prepared by oxidation of compound ‘B’ with alkaline KMnO₄. Compound ‘A’ on reduction with lithium aluminium hydride gets converted back to compound ‘B’. When compound ‘A’ is heated with compound B in the presence of H₂SO₄ it produces the fruity smell of compound C to which family the compounds ‘A’, ‘B’ and ‘C’ belong to?
Answer:

It is given that B and A on heating together in the presence of acid produces a fruity smelling compound which is ester.

1. Carboxylic Acid

2. Alcohol

3. Ester Group

Question 30. Arrange the following in decreasing order of their acidic strength. Give explanation for the arrangement.
C₆H₅COOH,FCH₂COOH,NO₂CH₂COOH
Answer:

NO₂CH₂COOH>FCH₂COOH>C₆H₅COOH

-NO2 and -F groups are electron withdrawing groups that increase the acidity due to stable nature of resulting conjugate base. Whereas the electron-donating group decrease the acidity as the resulting conjugate base is unstable.
Higher the electronegativity, greater the acidic strength.

Question 31. Alkenes and carbonyl compounds both contain a π bond but alkenes show electrophilic addition reactions whereas carbonyl compounds show nucleophilic addition reactions. Explain.
Answer:

In carbonyl groups, carbon acquires a partial positive charge because oxygen is more electronegative. Thus, carbonyl group is polar in nature and hence susceptible to nucleophilic addition reaction. Alkenes, on the other hand are non-polar and electron abundant; making them susceptible to electrophilic addition reaction.

Question 32. Carboxylic acids contain carbonyl group but do not show the nucleophilic addition reaction like aldehydes or ketones. Why?
Answer:

Carboxylic acid doesnt show nucleophilic substitution reaction as because of presence of lone pair atoms on O atom of OH group decreases electrophilic character of carbonyl carbon by resonance.. Hence partial positive charge decreases and therefore it doesnt show nucleophilic substitution reactions

Question 33. Identify the compounds A, B and C in the following reaction.

Answer:

• Compound A is CH₃−MgBr

• Compound B is CH₃−COOH

• Compound C is CH₃−COOCH₃

Question 34. Why are carboxylic acids more acidic than alcohols or phenols although all of them have a hydrogen atom attached to an oxygen atom (−O−H)?
Answer:

Acidity depends upon the stability of the resulting conjugate base. In case of carboxylic acid, equivalent resonance structures stabilize the carboxylate ion as the negative charge is at the more electronegative oxygen atom. Phenoxide ion, conjugate base of phenol does not have equivalent resonance structure as the negative charge is on the less electronegative carbon atom.

Question 35. Complete the following reaction sequence.

Answer:

Question 36. Ethylbenzene is generally prepared by acetylation of benzene followed by reduction and not by direct alkylation. Think of a possible reason.

Answer:

The reason is formation of polysubstituted products. Friedel-craft’s acylation reaction can be used to avoid the formation of these products.

Question 37. Can Gatterman-Koch reaction be considered similar to Friedel Craft’s acylation? Discuss.
Answer:

Yes, the Gattermann-Koch reaction can be considered similar to Friedel Craft’s acylation reaction. In Gattermann-Koch reaction introduction of -CHO group to the benzene nucleus takes place by reacting CO with HCl gas. In Friedel-Crafts acylation reactions, treatment of benzene or any arene with an acid chloride in presence of anhydrous AlCl₃ takes place. In this RCO group is introduced.

Question 39. Match the acids given in Column I with their correct IUPAC names given in Column II.

Answer:

(i → b), (ii → e), (iii → d), (iv → a), (v → c)

Question 40. Match the reactions given in Column I with the suitable reagents given in Column II.

Answer:

(i → c), (ii → d), (iii → a), (iv → b)

Question 41. Match the example given in Column I with the name of the reaction in Column II

Answer:

(i → e), (ii → d), (iii → a), (iv →b), (v → f), (vi → c)

Question 43. In the following question a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
Assertion: Compounds containing -CHO group are easily oxidised to corresponding carboxylic acids.
Reason: Carboxylic acids can be reduced to alcohols by treatment with LiAlH₄
(i) Assertion and reason both are correct and the reason is the correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) The assertion is a correct statement but the reason is the wrong statement.
(iv) The assertion is a wrong statement but the reason is the correct statement.
(v) Assertion and reason both are correct statements but the reason is not the correct
explanation of assertion

Answer:

The answer is the option (v). Due to high electronegativity of oxygen, carbonyl group in aldehydes are a strong electron-withdrawing group. This weakens the C−H bond in aldehydes. Even a weak oxidizing agent can oxidize the aldehydes into the corresponding carboxylic acids.

Question 44. In the following question a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
Assertion: The α-hydrogen atom in carbonyl compounds is less acidic.
Reason: The anion formed after the loss of the α-hydrogen atom is resonance stabilised.
(i) Assertion and reason both are correct and the reason is the correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) The assertion is a correct statement but the reason is the wrong statement.
(iv) The assertion is a wrong statement but the reason is the correct statement.
(v) Assertion and reason both are correct statements but the reason is not the correct
explanation of assertion
Answer:

The answer is the option (iv). The α -hydrogen atom in carbonyl compounds is more acidic. Carboxylic acids on dissociation in water give resonance stabilized carboxylate anions and hydronium ion.

Question 45. In the following question a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
Assertion: Aromatic aldehydes and formaldehyde undergo Cannizaro reaction.
Reason: Aromatic aldehydes are almost as reactive as formaldehyde.
(i) Assertion and reason both are correct and the reason is the correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) The assertion is a correct statement but the reason is the wrong statement.
(iv) The assertion is a wrong statement but the reason is the correct statement.
(v) Assertion and reason both are correct statements but the reason is not the correct
explanation of assertion.
Answer:

The answer is the option (iii). Aromatic aldehydes and formaldehyde do not have a-hydrogen and thus undergo Cannizzaro’s reaction. Aldehydes with no alpha hydrogen atom undergo oxidation and reduction reaction when treated with conc. Alkali.

Question 46. In the following question a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
Assertion: Aldehydes and ketones, both react with Tollen’s reagent to form a silver mirror.
Reason: Both, aldehydes and ketones contain a carbonyl group.
(i) Assertion and reason both are correct and the reason is the correct explanation of assertion.
(ii) Assertion and reason both are wrong statements.
(iii) The assertion is a correct statement but the reason is the wrong statement.
(iv) The assertion is a wrong statement but the reason is the correct statement.
(v) Assertion and reason both are correct statements but the reason is not the correct
explanation of assertion.
Answer:

The answer is the option (iv) Aldehyde and ketones both have carbonyl group but only the former undergoes reaction with Tollen’s reagent to give silver mirror.

Question 48. An aromatic compound ‘A’ (Molecular formula C₈H₈O) gives positive 2, 4-DNP test. It gives a yellow precipitate of compound ‘B’ on treatment with iodine and sodium hydroxide solution. Compound ‘A’ does not give Tollens or Fehling’s test. On drastic oxidation with potassium permanganate it forms a carboxylic acid ‘C’ (Molecular formula C₇H₆O₂), which is also formed along with the yellow compound in the above reaction. Identify A, B and C and write all reactions involved.

Answer:

Molecular formula of the compound in question is C8H8O. It is given that ‘A’ does not give Tollens’ or Fehling’s test, making it a ketone. Since we get positive test with 2, 4-DNP and iodoform test, It means it is methyl ketone.

Question 49. Write down functional isomers of a carbonyl compound with molecular formula C₃H₆O. Which isomer will react faster with HCN and why? Explain the mechanism of the reaction also. Will the reaction lead to the completion with the conversion of whole reactant into product at reaction conditions? If a strong acid is added to the reaction mixture what will be the effect on concentration of the product and why?
Answer:

C₃H₆O will have following isomer.

Compound 1 (Propanal) is more susceptible to a reaction with HCN as carbon atom in carbonyl group acquires more positive charge than carbon atom in compound 2 (propan-2-one). Moreover, there is much less static hindrance in compound 1 compared to compound 2.
Equilibrium is established in this reaction as it is reversible in nature. Addition of an acid hinders the formation of CN^- ion and thus, can stop the reaction.

Question 50. When liquid ‘A’ is treated with a freshly prepared ammoniacal silver nitrate solution, it gives bright silver mirror. The liquid forms a white crystalline solid on treatment with sodium hydrogen sulphite. Liquid ‘B’ also forms a white crystalline solid with sodium hydrogen sulphite but it does not give test with ammoniacal silver nitrate. Which of the two liquids is aldehyde? Write the chemical equations of these reactions also.
Answer:

Since liquid A forms a bright silver mirror when treated with ammoniacal AgNO₃ solution it is an aldehyde.
In the same way, liquid ‘B’ must be a methyl ketone for it does not give test with ammoniacal AgNO₃ solution.
Chemical equations for these reactions are :