NCERT Solutions for Class 12 Chemistry Chapter 12 - Aldehydes, Ketones and Carboxylic Acids

Question 1:

Give names of the reagents to bring about the following conversions :

  1. Hexan-1-ol to hexanal
  2. Cyclohexanol to cyclohexanone
  3. p-Fluorotoluene to p-fluorobenzaldehyde
  4. Ethanenitrile to ethanal
  5. Allyl alcohol to propenal
  6. But-2-ene to ethanal
Answer:

Question 2:

Arrange the following compounds in the increasing order of their boiling points :

Answer:

Question 3:

Would you expect benzaldehyde to be less or more reactive in nucleophilic addition reactions than propanal ? Explain.

Answer:

The carbon atom of the carbonyl group of benzaldehyde is less electrophilic than carbon atom of the carbonyl group in the propanal. This is because the polarity of the carbonyl group is decreased in benzaldehyde due to resonance. Therefore, benzaldehyde is less reactive than propanal.

Question 4:

An organic compound (A) with molecular formula C8H8O forms an orange red precipitate with 2, 4-DNP reagent and gives yellow precipitate on heating with iodine in the presence of sodium hydroxide. It neither reduces Tollen's reagent or Fehling solution, nor does it decolourise bromine water or Baeyer's reagent. On drastic oxidation with chromic acid, it gives a carboxylic acid (B) having molecular formula C7H6O2. Identify the compound (A) and (B) and explain the reactions involved.

Answer:
  1. The compound A forms 2, 4-DNP derivative so, it is an aldehyde or ketone.
  2. The compound does not reduce Tollen's reagent or Fehling solution so it must be a ketone.
  3. The compound gives iodoform test so it should be a methyl ketone.
  4. The molecular formula of A, C8H8O indicates high degree of unsaturation, yet it does not docolourise Br2 water or Baeyer's reagent, this shows the presence of unsaturation due to the aromatic ring.
  5. The drastic oxidation with chromic acid, gives carboxylic acid (B) with molecular formula C7H6O2 and it should be benzoic acid. Therefore, the compound (A) should be monosubstituted aromatic methyl ketone. The molecular formula of (A) indicates that it should be methyl phenyl ketone.

Question 5:

Write chemical reactions to affect the following transformations:

  1. Butan-1-ol to butanoic acid
  2. Benzyl alcohol to phenyl ethanoic acid
  3. 3-Nitrobromobenzene to 3-nitrobenzoic acid
  4. 4-Methyl acetophenone to benzene-1, 4-dicarboxylic acid
  5. Cyclohexene to Hexane-1, 6-dioic acid
  6. Butanal to butanoic acid
Answer:

Question 6:

Write the structures of the following compounds :

  1. a-Methoxypropionaldehyde
  2. 3-Hydroxybutanal
  3. 2-Hydroxycyclopentanecarbaldehyde
  4. 4-Oxopentanal
  5. Di-sec-butyl ketone
  6. 4-Fluoroacetophenone
Answer:

Question 7:

Write the structures of products of the following reactions :

Answer:

Question 8:

Arrange the following compounds in the increasing order of their boiling points :
CH3CHO, CH3CH2OH, CH3OCH3, CH3CH2CH3

Answer:

Question 9:

Arrange the following compounds in increasing order of their reactivity in nucleophilic addition reactions:

  1. Ethanal, Propanal, Propanone, Butanone
  2. Benzaldehyde, p-Tolualdehyde, p-Nitrobenzaldehyde, Acetophenone
Answer:

(i) As we move from ethanal to propanal, to propanone to butanone, the +I inductive effect of alkyl group increases. As a result, the +ve charge on the carbon atom of the carbonyl group progressively decreases and hence attack by nucleophile becomes slower and slower.

Question 10:

Predict the products of the following reactions :

Answer:

Question 11:

Give the IUPAC names of the following compounds :
(i) PhCH2CH2COOH (ii) (CH3)2C==CHCOOH

Answer:
  1. 3-Phenylpropanoic acid
  2. 3-Methylbut-2-enoic acid
  3. 2-Methylcyclopentanecarboxylic acid
  4. 2,4,6-Trinitrobenzoic acid
Question 12:

Show how each of the following compounds could be converted to benzoic acid :
(i) Ethylbenzene (ii) Acetophenone (iii) Bromobenzene (iv) Phenylethene (Styrene)

Answer:

Question 13:

Which acid of each pair shown here would you expect to be stronger ?

Answer:

Question 14:

What is meant by the following terms ? Give an example in each case.

  1. Cyanohydrin
  2. Acetal
  3. Semicarbazone
  4. Aldol
  5. Hemiacetal
  6. Oxime
  7. Ketal
  8. Imine
  9. 2,4-DNP-derivative
  10. Schiff’s base
Answer:

The compounds containing hydroxyl and cyano groups on the same carbon atom are called cyanohydrins. These are also called gem-hydroxynitriles. These are obtained by the addition of HCN to a carbonyl group in weakly basic medium :

Question 15:

Name the following compounds according to IUPAC system of nomenclature:

Answer:
  1. 4-Methylpentanal
  2. 6-Chloro-4-ethylhexan-3-one
  3. But-2-enal
  4. Pentane-2,4-dione
  5. 3,3,5-Trimethylhexan-2-one
  6. 3,3-Dimethylbutanoic acid
  7. Benzene-1,4-dicarbaldehyde

Question 16:

Write the IUPAC names of the following ketones and aldehydes. Wherever possible, give also common names.

Answer:

Question 17:

Draw structures of the following derivatives :

  1. The 2,4-dinitrophenylhydrazone of benzaldehyde
  2. Cyclopropanone oxime
  3. Acetaldehydedimethylacetal
  4. The semicarbazone of cyclobutanone
  5. The ethylene ketal of hexan-3-one
  6. The methyl hemiacetal of formaldehyde
Answer:

Question 18:

Predict the product formed when cyclohexanecarbaldehyde reacts with following reagents :

  1. PhMgBr and then H3O+
  2. Tollen’s reagent
  3. Semicarbazide and weak acid
  4. Excess ethanol and acid
  5. Zinc amalgam and dilute hydrochloric acid
Answer:

Question 19:

Which of the following compounds would undergo aldol condensation? Which the Cannizzaro reaction and which neither? Write the structures of the expected products of aldol condensation and Cannizzaro reaction.

  1. Methana
  2. 2-Methylpentanal
  3. Benzaldehyde
  4. Benzophenone
  5. Cyclohexanone
  6. 1-Phenylpropanone
  7. Phenylacetaldehyde
  8. Butan-1-ol
  9. 2,2-Dimethylbutanal
Answer:

(ii) 2-Methylpentanal (v) 1-cyclohexanone, (vi) 1-Phenylpropanone and (vii) phenylacetaldehyde contain one or more a-hydrogen atoms and therefore, undergo aldol condensation.

Question 20:

How will you convert ethanal into the following compounds ?

  1. Butane-1,3-diol
  2. But-2-enal
  3. But-2-enoic acid
Answer:

Question 21:

Write structural formulas and names of the four possible aldol condensation products from propanal and butanal. In each case, indicate which aldehyde served as nucleophile and which as electrophile.

Answer:

Question 22:

An organic compound with the molecular formula C9H10O forms 2,4-DNP derivative, reduces Tollens’ reagent and undergoes Cannizzaro reaction. On vigorous oxidation, it gives 1, 2-benzenedicarboxylic acid. Identify the compound.

Answer:

The given compound forms 2,4-DNP derivative. Therefore, it is an aldehyde or ketone. Since it reduces Tollen’s reagent, it must be an aldehyde. The compound undergoes Cannizzaro’s reaction, so it does not contain a-hydrogen. On vigorous oxidation, it gives 1,2-benzendicarboxylic acid, it means that it must be containing alkyl group at 2-position with respect to CHO group on the benzene ring.

Question 23:

An organic compound (A) (molecular formula C8H16O2) was hydrolysed with dilute sulphuric acid to give a carboxylic acid (B) and an alcohol (C). Oxidation of (C) with chromic acid produced (B). (C) on dehydration gives but-1-ene. Write equations for the reactions involved.

Answer:

(a) Since the given compound on hydrolysis with dil. H2SO4 gives carboxylic acid (B) and an alcohol (C), it must be an ester.

(b) Since the oxidation of alcohol (C) gives the acid B, therefore, both the carboxylic acid B and alcohol C must contain same number of C atoms.

(c) Since ester (A) contains 8 carbon atoms, therefore, both carboxylic acid (B) and the alcohol (C) must contain 4 C atoms each.

(d) Alcohol (C) on dehydration gives but-1-ene and therefore, C must be a straight chain alcohol i.e., butan-1-ol.

(e) (B) is obtained by the oxidation of (C) and therefore, B must be butanoic acid.

This also suggests that the ester (A) must be butyl butanoate. The relevant reactions are:

Question 24:

Arrange the following compounds in increasing order of their property as indicated :

  1. Acetaldehyde, Acetone, Di-tert-butyl ketone, Methyl tert-butyl ketone (reactivity towards HCN)
  2. CH3CH2CH(Br)COOH, CH3CH(Br)CH2COOH, (CH3)2CHCOOH, CH3CH2CH2COOH (acid strength)
  3. Benzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid, 4-Methoxybenzoic acid (acid strength).
Answer:

(i) The reactivity towards HCN addition decreases as + I effect of alkyl group (or groups) increases and /or the steric hindrance to the nucleophilic attack by CN– at the carbonyl carbon increases.br> Thus, the increasing order of reactivity is :

Question 25:

Give simple chemical tests to distinguish between the following pairs of compounds :

  1. Propanal and propanone
  2. Acetophenone and Benzophenone
  3. Phenol and Benzoic acid
  4. Benzoic acid and Ethyl benzoate
  5. Pentan-2-one-and pentan-3-one
  6. Benzaldehyde and Acetophenone
  7. Ethanal and propanal.
Answer:

(i) Propanal and propanone
(a) Propanal gives silver mirror with Tollen’s reagent.

Question 26:

How will you prepare the following compounds from benzene? You may use any inorganic reagent and any organic reagent having not more than one carbon atom.

  1. Methyl benzoate
  2. m-Nitrobenzoic acid
  3. p-Nitrobenzoic acid
  4. Phenylacetic acid
  5. p-Nitrobenzaldehyde
Answer:

Question 27:

Answer:

Question 28:

Describe the following:

  1. Acetylation
  2. Cannizzaro reaction
  3. Cross aldol condensation
  4. Decarboxylation
Answer:

(i) Acetylation
The replacement of an active hydrogen of alcohols, phenols or amines with acetyl group to form corresponding esters or amides is called acetylation. It is carried out by using acid chlorides or anhydrides in the presence of a base such as pyridine, dimethyl aniline, etc. For example,

Question 29:

Complete each synthesis by giving missing starting material, reagent or products

Answer:

Question 30:

Give plausible explanation for each of the following:

  1. Cyclohexanone forms cyanohydrin in good yield but 2,2,6 trimethylcyclo-hexanone does not.
  2. There are two –NH2 groups in semicarbazide. However, only one is involved in the formation of semicarbazones.
  3. During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst, the water or the ester should be removed as soon as it is formed.
Answer:

(i) Cyclohexanone forms cyanohydrin in good yield.

Question 31:

An organic compound contains 69.77% carbon, 11.63% hydrogen and rest oxygen. The molecular mass of the compound is 86. It does not reduce Tollens’ reagent but forms an addition compound with sodium hydrogen sulphite and give positive iodoform test. On vigorous oxidation it gives ethanoic and propanoic acid. Write the possible structure of the compound.

Answer:

Step 1: To determine the molecular formula of the compound
%C = 69.77, % H = 11.63
%O = 100 – (69.77 + 11.63) = 18.60
Simple atomic ratio

Question 32:

Although phenoxide ion has more number of resonating structure than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why?

Answer:

The phenoxide ion has more number of resonating structures but carboxylic acid is more stronger acid than phenol. This is because the electron charge in the carboxylate ion is more dispersed in comparison to phenate ion since there are two electronegative oxygen atoms in carboxylate ion as compared to only one oxygen atom in phenate ion.

In other words, carboxylate ion is relatively more stable as compared to phenate ion. Thus, the release of H+ ion from carboxylic acid is comparatively easier and therefore, it behaves as stronger acid than phenol.

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