Class 12 Chemistry Chapter 8 Aldehydes Ketones and Carboxylic Acids ncert solutions

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Aldehydes Ketones and Carboxylic Acids Class 12 ncert solutions: Chemistry Class 12 Chapter 8 exercise solutions

TextbookNCERT
ClassClass 12
SubjectChemistry
ChapterChapter 8
Chapter NameAldehydes Ketones and Carboxylic Acids ncert solutions
CategoryNcert Solutions
MediumEnglish

Are you looking for Class 12 Chemistry Chapter 8 Aldehydes Ketones and Carboxylic Acids ncert solutions? Now you can download Ncert Chemistry Class 12 Chapter 8 exercise solutions pdf from here.

Question 8.1: What is meant by the following terms? Give an example of the reaction in each case.

  • (i) Cyanohydrin
  • (ii) Acetal
  • (iii) Semicarbazone
  • (iv) Aldol
  • (v) Hemiacetal
  • (vi) Oxime
  • (vii) Ketal
  • (vii) Imine
  • (ix) 2,4-DNP-derivative
  • (x) Schiff’s base

Solution 8.1: (i) Cyanohydrin:

Cyanohydrins are organic compounds having the formula RR′C(OH)CN, where R and R′ can be alkyl or aryl groups.

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

Aldehydes and ketones react with hydrogen cyanide (HCN) in the presence of excess sodium cyanide (NaCN) as a catalyst to field cyanohydrin. These reactions are known as cyanohydrin reactions.

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

Cyanohydrins are useful synthetic intermediates.

(ii) Acetal:

Acetals are gem−dialkoxy alkanes in which two alkoxy groups are present on the terminal carbon atom. One bond is connected to an alkyl group while the other is connected to a hydrogen atom.

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

When aldehydes are treated with two equivalents of a monohydric alcohol in the presence of dry HCl gas, hemiacetals are produced that further react with one more molecule of alcohol to yield acetal.

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

(iii) Semicarbarbazone:

Semicarbazones are derivatives of aldehydes and ketones produced by the condensation reaction between a ketone or aldehyde and semicarbazide.

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

Semicarbazones are useful for identification and characterization of aldehydes and ketones.

(iv) Aldol:

A β-hydroxy aldehyde or ketone is known as an aldol. It is produced by the condensation reaction of two molecules of the same or one molecule each of two different aldehydes or ketones in the presence of a base.

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

(v) Hemiacetal:

Hemiacetals are α−alkoxyalcohols

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

General structure of a hemiacetal

Aldehyde reacts with one molecule of a monohydric alcohol in the presence of dry HCl gas.

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

(vi) Oxime:

Oximes are a class of organic compounds having the general formula RR′CNOH, where R is an organic side chain and R′ is either hydrogen or an organic side chain. If R′ is H, then it is known as aldoxime and if R′ is an organic side chain, it is known as ketoxime.

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

On treatment with hydroxylamine in a weakly acidic medium, aldehydes or ketones form oximes.

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

(vii) Ketal:

Ketals are gem−dialkoxyalkanes in which two alkoxy groups are present on the same carbon atom within the chain. The other two bonds of the carbon atom are connected to two alkyl groups.

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

Ketones react with ethylene glycol in the presence of dry HCl gas to give a cyclic product known as ethylene glycol ketals.

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

(viii) Imine:

Imines are chemical compounds containing a carbon nitrogen double bond.

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

Imines are produced when aldehydes and ketones react with ammonia and its derivatives.

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

(ix) 2, 4−DNP−derivative:

2, 4−dinitrophenylhydragones are 2, 4−DNP−derivatives, which are produced when aldehydes or ketones react with 2, 4−dinitrophenylhydrazine in a weakly acidic medium.

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

To identify and characterize aldehydes and ketones, 2, 4−DNP derivatives are used.

(x) Schiff’s base:

Schiff’s base (or azomethine) is a chemical compound containing a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group-but not hydrogen. They have the general formula R1R2C = NR3. Hence, it is an imine.

It is named after a scientist, Hugo Schiff.

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

Aldehydes and ketones on treatment with primary aliphatic or aromatic amines in the presence of trace of an acid yields a Schiff’s base.

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

Question 8.2: Name the following compounds according to the IUPAC system of nomenclature:

  • (i) CH3CH(CH3)CH2CH2CHO
  • (ii) CH3CH2COCH(C2H5)CH2CH2Cl
  • (iii) CH3CH=CHCHO
  • (iv) CH3COCH2COCH3
  • (v) CH3CH(CH3)CH2C(CH3)2COCH3
  • (vi) (CH3)3CCH2COOH
  • (vii) OHCC6H4CHO-p

Solution 8.2:

  • (i) 4-methylpentanal
  • (ii) 6-Chloro-4-ethylhexan-3-one
  • (iii) But-2-en-1-al
  • (iv) Pentane-2,4-dione
  • (v) 3,3,5-Trimethylhexan-2-one
  • (vi) 3,3-Dimethylbutanoic acid
  • (vii) Benzene-1,4-dicarbaldehyde

Question 8.3: Draw the structures of the following compounds.

  • (i) 3-Methylbutanal
  • (ii) p-Nitropropiophenone
  • (iii) p-Methylbenzaldehyde
  • (iv) 4-Methylpent-3-en-2-one
  • (v) 4-Chloropentan-2-one
  • (vi) 3-Bromo-4-phenylpentanoic acid
  • (vii) p,p’-Dihydroxybenzophenone
  • (viii) Hex-2-en-4-ynoic acid

Solution 8.3: (i)

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

(ii)

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

(iii)

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

(iv)

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

(v)

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

(vi)

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

(vii)

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

(viii)

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

Question 8.4: Write the IUPAC names of the following ketones and aldehydes. Wherever possible, also give common names.

(i) CH3CO(CH2)4CH3

(ii) CH3CH2CHBrCH2CH(CH3)CHO

(iii) CH3(CH2)5CHO

(iv) Ph-CH=CH-CHO

(v)

(vi) PhCOPh

Solution 8.4: (i) CH3CO(CH2)4CH3

IUPAC name: Heptan-2-one

Common name: Methyl pentyl ketone

(ii) CH3CH2CHBrCH2CH(CH3)CHO

IUPAC name: 4-Bromo-2-methylhaxanal

Common name: (γ-Bromo-α-methyl-caproaldehyde)

(iii) CH3(CH2)5CHO

IUPAC name: Heptanal

(iv) Ph-CH=CH-CHO

IUPAC name: 3-phenylprop-2-enal

Common name: β-Pheynolacrolein

(v)

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

IUPAC name: Cyclopentanecarbaldehyde

(vi)PhCOPh

IUPAC name: Diphenylmethanone

Common name: Benzophenone

Question 8.5: Draw structures of the following derivatives.

  • (i) The 2,4-dinitrophenylhydrazone of benzaldehyde
  • (ii) Cyclopropanone oxime
  • (iii) Acetaldehydedimethylacetal
  • (iv) The semicarbazone of cyclobutanone
  • (v) The ethylene ketal of hexan-3-one
  • (vi) The methyl hemiacetal of formaldehyde

Solution 8.5: (i)

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

(ii)

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

(iii)

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

(iv)

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

(v)

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

(vi)

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

Question 8.6: Predict the products formed when cyclohexanecarbaldehyde reacts with following reagents.

  • (i) PhMgBr and then H3O+
  • (ii) Tollens’ reagent
  • (iii) Semicarbazide and weak acid
  • (iv) Excess ethanol and acid
  • (v) Zinc amalgam and dilute hydrochloric acid

Solution 8.6: (i)

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

(ii)

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

(iii)

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

(iv)

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

(v)

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

Question 8.7: 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.

  • (i) Methanal
  • (ii) 2-Methylpentanal
  • (iii) Benzaldehyde
  • (iv) Benzophenone
  • (v) Cyclohexanone
  • (vi) 1-Phenylpropanone
  • (vii) Phenylacetaldehyde
  • (viii) Butan-1-ol
  • (ix) 2, 2-Dimethylbutanal

Solution 8.7: Aldehydes and ketones having at least one α-hydrogen undergo aldol condensation. The compounds (ii) 2−methylpentanal, (v) cyclohexanone, (vi) 1-phenylpropanone, and (vii) phenylacetaldehyde contain one or more α-hydrogen atoms. Therefore, these undergo aldol condensation.

Aldehydes having no α-hydrogen atoms undergo Cannizzaro reactions. The compounds (i) Methanal, (iii) Benzaldehyde, and (ix) 2, 2-dimethylbutanal do not have any α-hydrogen. Therefore, these undergo cannizzaro reactions.

Compound (iv) Benzophenone is a ketone having no α-hydrogen atom and compound (viii) Butan-1-ol is an alcohol. Hence, these compounds do not undergo either aldol condensation or cannizzaro reactions.

Aldol condensation

(ii)

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

(v)

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

(vi)

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

(vii)

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

Cannizzaro reaction

(i)

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

(iii)

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

(ix)

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

Question 8.8: How will you convert ethanal into the following compounds?
(i) Butane-1, 3-diol, (ii) But-2-enal, (iii) But-2-enoic acid

Solution 8.8: (i) On treatment with dilute alkali, ethanal produces 3-hydroxybutanal gives butane-1, 3-diol on reduction.

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

(ii) On treatment with dilute alkali, ethanal gives 3-hydroxybutanal which on heating produces but-2-enal.

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

(iii) When treated with Tollen’s reagent, But-2-enal produced in the above reaction produces but-2-enoic acid .

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

Question 8.9: Write structural formulas and names of four possible aldol condensation products from propanal and butanal. In each case, indicate which aldehyde acts as a nucleophile and which as an electrophile.

Solution 8.9: (i) Taking two molecules of propanal, one which acts as a nucleophile and the other as an electrophile.

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

(ii) Taking two molecules of butanal, one which acts as a nucleophile and the other as an electrophile.

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

(iii) Taking one molecule each of propanal and butanal in which propanal acts as a nucleophile and butanal acts as an electrophile.

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

(iv) Taking one molecule each of propanal and butanal in which propanal acts as an electrophile and butanal acts as a nucleophile.

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

Question 8.10: An organic compound with the molecular formula C9H10O forms a 2,4-DNP derivative, reduces Tollens’ reagent and undergoes the Cannizzaro reaction. On vigorous oxidation, it gives 1,2-benzenedicarboxylic acid. Identify the compound.

Solution 8.10: The compound having molecular formula C9H10O forms a 2,4-DNP derivative and reduces Tollen’s reagent. Therefore, the given compound must be an aldehyde. Again, the compound gives 1, 2 benzene dicarboxylic acids and undergoes a Cannizzaro reaction followed by oxidation. Therefore, the −CHO group is directly attached to a benzene ring, and this benzaldehyde is ortho-substituted. Hence, the compound is found to be 2-ethylbenzaldehyde.

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

The given reactions can be explained by the following equations.

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

Question 8.11: An organic compound (A) (molecular formula C8H16O2) was hydrolysed with dilute sulphuric acid to give carboxylic acid (B) and alcohol (C). Oxidation of (C) with chromic acid produced (B). (C) on dehydration gives but-1-ene. Write equations for the reactions involved.

Solution 8.11: A is an organic compound with the molecular formula C8H16O2. This gives carboxylic acid (B) and alcohol (C) on hydrolysis with dilute sulphuric acid. Thus, compound A must be an ester.

Further, the oxidation of alcohol (C) with chromic acid gives acid B. Thus, B and C must contain an equal number of carbon atoms.

A total of 8 carbon atoms are present in compound A, and each of B and C contain 4 carbon atoms.

Again, alcohol C gives but-1-ene on dehydration. Therefore, C is of straight-chain, and hence, it is butan-1-ol.

On oxidation, Butan-1-ol gives butanoic acid. Hence, acid B is butanoic acid.

Hence, the ester with the molecular formula C8H16O2 is butylbutanoate.

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

All the given reactions can be explained by the following equations.

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

Question 8.12: Arrange the following compounds in increasing order of their property as indicated:

  • (i) Acetaldehyde, Acetone, Di-tert-butyl ketone, Methyl tert-butyl ketone (reactivity towards HCN)
  • (ii) CH3CH2CH(Br)COOH, CH3CH(Br)CH2COOH, (CH3)2CHCOOH, CH3CH2CH2COOH (acid strength)
  • (iii) Benzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid, 4-Methoxybenzoic acid (acid strength)

Solution 8.12: (i) When HCN reacts with a compound, the attacking species is a nucleophile, CN. Therefore, as the negative charge on the compound increases, its reactivity with HCN decreases. In the given compounds, the +I effect increases as shown below. It can be observed that steric hindrance also increases in the same

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

Hence, the given compounds can be arranged according to their increasing reactivities toward HCN as:

Di-tert-butyl ketone < Methyl tert-butyl ketone < Acetone < Acetaldehyde

(ii) After losing a proton, carboxylic acids gain a negative charge as shown:

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

Now, any group that will help stabilise the negative charge will increase the stability of the carboxyl ion and as a result, will increase the strength of the acid. Thus, groups having +I effect will decrease the strength of the acids and groups having −I effect will increase the strength of the acids. In the given compounds, −CH3 group has +I effect and Br− group has −I effect. Thus, acids containing Br− are stronger.

Now, the +I effect of the isopropyl group is more than that of the n-propyl group. Hence, CH3CH2CH2COOH is a stronger acid than (CH3)2CHCOOH.

Also, as the distance increases, the +I effect grows weaker. Hence, CH3CH2CH(Br)COOH is a stronger acid than CH3CH(Br)CH2COOH.

Hence, the strengths of the given acids increase as:

(CH3)2CHCOOH < CH3CH2CH2COOH < CH3CH(Br)CH2COOH < CH3CH2CH(Br)COOH

(iii) As we have seen in the previous case, electron-donating groups decrease the strengths of acids, while electron-withdrawing groups increase the strengths of acids. As methoxy group is an electron-donating group, 4-methoxybenzoic acid is a weaker acid than benzoic acid. Nitro group is an electron-withdrawing group and will increase the strengths of acids. As 3,4-dinitrobenzoic acid contains two nitro groups, it is a slightly stronger acid than 4-nitrobenzoic acid. Hence, the strengths of the given acids increase as:

4-Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3,4-Dinitrobenzoic acid

Question 8.13: Give simple chemical tests to distinguish between the following pairs of compounds.

  • (i) Propanal and propanone
  • (ii) Acetophenone and benzophenone
  • (iii) Phenol and benzoic acid
  • (iv) Benzoic acid and ethyl benzoate
  • (v) Pentan-2-one and pentan-3-one
  • (vi) Benzaldehyde and acetophenone
  • (vii) Ethanal and propanal

Solution 8.13: (i) Propanal and propanone can be distinguished by the following tests.

(a) Tollen’s test

Propanal is an aldehyde. Thus, it reduces Tollen’s reagent. But, propanone being a ketone does not reduce Tollen’s reagent.

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

(b) Fehling’s test

Aldehydes respond to Fehling’s test, but ketones do not.

Propanal being an aldehyde reduces Fehling’s solution to a red-brown precipitate of Cu2O, but propanone being a ketone does not.

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

(c) Iodoform test:

Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom respond to iodoform test. They are oxidized by sodium hypoiodite (NaOI) to give iodoforms. Propanone being a methyl ketone responds to this test, but propanal does not.

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

(ii) Acetophenone and Benzophenone can be distinguished using the iodoform test.

Iodoform test:

Methyl ketones are oxidized by sodium hypoiodite to give yellow ppt. of iodoform. Acetophenone being a methyl ketone responds to this test, but benzophenone does not.

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

(iii) Phenol and benzoic acid can be distinguished by ferric chloride test.

Ferric chloride test:

Phenol reacts with neutral FeCl3 to form an iron-phenol complex giving violet colouration.

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

But benzoic acid reacts with neutral FeCl3 to give a buff coloured ppt. of ferric benzoate.

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

(iv) Benzoic acid and Ethyl benzoate can be distinguished by sodium bicarbonate test.

Sodium bicarbonate test:

Acids react with NaHCO3 to produce brisk effervescence due to the evolution of CO2 gas.

Benzoic acid being an acid responds to this test, but ethylbenzoate does not.

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

(v) Pentan-2-one and pentan-3-one can be distinguished by iodoform test.

Iodoform test:

Pentan-2-one is a methyl ketone. Thus, it responds to this test. But pentan-3-one not being a methyl ketone does not respond to this test.

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

(vi) Benzaldehyde and acetophenone can be distinguished by the following tests.

(a) Tollen’s Test

Aldehydes respond to Tollen’s test. Benzaldehyde being an aldehyde reduces Tollen’s reagent to give a red-brown precipitate of Cu2O, but acetophenone being a ketone does not.

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

(b) Iodoform test

Acetophenone being a methyl ketone undergoes oxidation by sodium hypoiodite (NaOI) to give a yellow ppt. of iodoform. But benzaldehyde does not respond to this test.

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

(vii) Ethanal and propanal can be distinguished by iodoform test.

Iodoform test:

Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom responds to the iodoform test. Ethanal having one methyl group linked to the carbonyl carbon atom responds to this test. But propanal does not have a methyl group linked to the carbonyl carbon atom and thus, it does not respond to this state.

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

Question 8.14: 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

  • (i) Methyl benzoate
  • (ii) m-Nitrobenzoic acid
  • (iii) p-Nitrobenzoic acid
  • (iv) Phenylacetic acid
  • (v) p-Nitrobenzaldehyde

Solution 8.14: (i)

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

(ii)

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

(iii)

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

(iv)

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

(v)

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

Question 8.15: How will you bring about the following conversions in not more than two steps?

  • (i) Propanone to propene
  • (ii) Benzoic acid to benzaldehyde
  • (iii) Ethanol to 3-hydroxybutanal
  • (iv) Benzene to m-nitroacetophenone
  • (v) Benzaldehyde to benzophenone
  • (vi) Bromobenzene to 1-phenylethanol
  • (vii) Benzaldehyde to 3-phenylpropan-1-ol
  • (viii) Benazaldehyde to α-hydroxyphenylacetic acid
  • (ix) Benzoic acid to m-nitrobenzyl alcohol

Solution 8.15: (i)

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

(ii)

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

(iii)

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

(iv)

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

(v)NCERT Solutions for Class 12 Chemistry Chapter 12 - Aldehydes, Ketones And Carboxylic Acids

(vi)

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

(vii)

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

(viii)

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

(ix)

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

Question 8.16: Describe the following:

  • (i) Acetylation
  • (ii) Cannizzaro reaction
  • (iii) Cross aldol condensation
  • (iv) Decarboxylation

Solution 8.16: (i) Acetylation :

The introduction of an acetyl functional group into an organic compound is known as acetylation. It is usually carried out in the presence of a base such as pyridine, dirnethylaniline, etc. This process involves the substitution of an acetyl group for an active hydrogen atom. Acetyl chloride and acetic anhydride are commonly used as acetylating agents.

For example, acetylation of ethanol produces ethyl acetate.

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

(ii) Cannizzaro reaction: 

The self oxidation-reduction (disproportionation) reaction of aldehydes having no α-hydrogens on treatment with concentrated alkalis is known as the Cannizzaro reaction. In this reaction, two molecules of aldehydes participate where one is reduced to alcohol and the other is oxidized to carboxylic acid.

For example, when ethanol is treated with concentrated potassium hydroxide, ethanol and potassium ethanoate are produced.

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

(iii) Cross-aldol condensation: 

When aldol condensation is carried out between two different aldehydes, or two different ketones, or an aldehyde and a ketone, then the reaction is called a cross-aldol condensation. If both the reactants contain α-hydrogens, four compounds are obtained as products.

For example, ethanal and propanal react to give four products.

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

(iv) Decarboxylation: 

Decarboxylation refers to the reaction in which carboxylic acids lose carbon dioxide to form hydrocarbons when their sodium salts are heated with soda-lime.

NCERT Solutions for Class 12 Chemistry Chapter

Decarboxylation also takes place when aqueous solutions of alkali metal salts of carboxylic acids are electrolyzed. This electrolytic process is known as Kolbe’s electrolysis.

Question 8.17: Complete each synthesis by giving the missing starting material, reagent or products

(i) 

NCERT Solutions for Class 12 Chemistry Chapter

(ii) 

NCERT Solutions for Class 12 Chemistry Chapter

(iii) 

NCERT Solutions for Class 12 Chemistry Chapter

(iv) 

NCERT Solutions for Class 12 Chemistry Chapter

(v) 

NCERT Solutions for Class 12 Chemistry Chapter

(vi) 

NCERT Solutions for Class 12 Chemistry Chapter

(vii) 

NCERT Solutions for Class 12 Chemistry Chapter

(viii) 

NCERT Solutions for Class 12 Chemistry Chapter

(ix) 

NCERT Solutions for Class 12 Chemistry Chapter

(x) 

NCERT Solutions for Class 12 Chemistry Chapter

(xi) 

NCERT Solutions for Class 12 Chemistry Chapter

Solution 8.17: (i)

NCERT Solutions for Class 12 Chemistry Chapter

(ii)

NCERT Solutions for Class 12 Chemistry Chapter

(iii)

NCERT Solutions for Class 12 Chemistry Chapter

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NCERT Solutions for Class 12 Chemistry Chapter

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NCERT Solutions for Class 12 Chemistry Chapter

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NCERT Solutions for Class 12 Chemistry Chapter

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NCERT Solutions for Class 12 Chemistry Chapter

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NCERT Solutions for Class 12 Chemistry Chapter

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NCERT Solutions for Class 12 Chemistry Chapter

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NCERT Solutions for Class 12 Chemistry Chapter

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NCERT Solutions for Class 12 Chemistry Chapter

Question 8.18: Give a plausible explanation for each of the following:

  • (i) Cyclohexanone forms cyanohydrin in good yield, but 2,2,6-trimethylcyclohexanone does not.
  • (ii) There are two −NH2 groups in semicarbazide. However, only one is involved in the formation of semicarbazones.
  • (iii) 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.

Solution 8.18: (i) Cyclohexanones form cyanohydrins according to the following equation.

NCERT Solutions for Class 12 Chemistry Chapter

In this case, the nucleophile CN− can easily attack without any steric hindrance. However, in the case of 2, 2, 6 trimethylcydohexanone, methyl groups at α-positions offer steric hindrances and as a result, CN− cannot attack effectively.

NCERT Solutions for Class 12 Chemistry Chapter

For this reason, it does not form a cyanohydrin.

(ii) Semicarbazide undergoes resonance involving only one of the two −NH2 groups, which is attached directly to the carbonyl-carbon atom.

NCERT Solutions for Class 12 Chemistry Chapter

Therefore, the electron density on −NH2 group involved in the resonance also decreases. As a result, it cannot act as a nucleophile. Since the other −NH2 group is not involved in resonance; it can act as a nucleophile and can attack carbonyl-carbon atoms of aldehydes and ketones to produce semicarbazones.

(iii) Ester along with water is formed reversibly from a carboxylic acid and an alcohol in presence of an acid.

NCERT Solutions for Class 12 Chemistry Chapter

If either water or ester is not removed as soon as it is formed, then it reacts to give back the reactants as the reaction is reversible. Therefore, to shift the equilibrium in the forward direction i.e., to produce more ester, either of the two should be removed.

Question 8.19: 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 hydrogensulphite and give positive iodoform test. On vigorous oxidation it gives ethanoic and propanoic acid. Write the possible structure of the compound.

Solution 8.19: % of carbon = 69.77 %

% of hydrogen = 11.63 %

% of oxygen = {100 − (69.77 + 11.63)}%

= 18.6 %

Thus, the ratio of the number of carbon, hydrogen, and oxygen atoms in the organic compound can be given as:

\(C:H:O=\frac{69.77}{12}:\frac{11.63}{1}:\frac{18.6}{16}\)

= 5.81:11.63:1.16

= 5:10:1

Therefore, the empirical formula of the compound is C5H10O. Now, the empirical formula mass of the compound can be given as:

= 5 x 12 + 10 x 1 + 1 x 16

= 86

The molecular mass of the compound = 86

Therefore, the molecular formula of the compound is given by C5H10O.

Tollen’s reagent is not reduced by the given compound; hence it is not an aldehyde. A positive iodoform test is given by the compound and also forms sodium hydrogen sulphate addition products. Since the compound is not an aldehyde, it must be a methyl ketone.

The given compound also gives a mixture of ethanoic acid and propanoic acid.

Hence, the given compound is Pentan-2-one.

NCERT Solutions for Class 12 Chemistry Chapter

The given reactions can be explained by the following equations:

NCERT Solutions for Class 12 Chemistry Chapter

Question 8.20: Although phenoxide ion has more number of resonating structures than carboxylate ion, carboxylic acid is a stronger acid than phenol. Why?

Solution 8.20: Resonance structures of phenoxide ion are:

NCERT Solutions for Class 12 Chemistry Chapter

It can be observed from the resonance structures of phenoxide ion that in II, III and IV, less electronegative carbon atoms carry a negative charge. Therefore, these three structures contribute negligibly towards the resonance stability of the phenoxide ion. Hence, these structures can be eliminated. Only structures I and V carry a negative charge on the more electronegative oxygen atom.

Resonance structures of carboxylate ion are:

NCERT Solutions for Class 12 Chemistry Chapter

In the case of carboxylate ion, resonating structures I′ and II′ contain a charge carried by a more electronegative oxygen atom.

Further, in resonating structures I′ and II′, the negative charge is delocalized over two oxygen atoms. But in resonating structures I and V of the phexoxide ion, the negative charge is localized on the same oxygen atom. Therefore, the resonating structures of carboxylate ion contribute more towards its stability than those of phenoxide ion. As a result, carboxylate ion is more resonance-stabilized than phenoxide ion. Hence, carboxylic acid is a stronger acid than phenol.

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