QB365 Provides the updated CASE Study Questions for Class 12 , and also provide the detail solution for each and every case study questions . Case study questions are latest updated question pattern from NCERT, QB365 will helps to get more marks in Exams

Download CBSE Class 12th Standard CBSE Chemistry question papers, sample papers, important questions, and previous year solved papers in PDF format. Get free study materials, NCERT solutions, and exam preparation resources for Class 12th Standard CBSE Chemistry

Questions:

Case Study Questions

5 x 4 = 20

Read the passage given below and answer the following questions:
Aldehydes and ketones undergo nucleophilic addition reactions .

Carbonyl carbon is electron deficient hence acts as an electrophile. Nucleophile attacks on the electrophilic carbon atom of the carbonyl group from a direction perpendicular to the plane of the molecule.

In this process, hybridisation of carbon atom changes from sp² to sp³ and a tetrahedral alkoxide ion is formed as intermediate. This intermediate captures proton from the reaction medium to give the neutral product. Aldehydes are generally more reactive than ketones in nucleophilic addition reactions.
In these questions ( i - iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
(i) Assertion: Benzaldehyde is more reactive than ethanal towards nucleophilic attack.
Reason: The overall effect of -land +R effect of phenyl group decreases the electron density on the carbon atom of ＞C=O group in benzaldehyde
(ii) Assertion: (CH₃)₃CCOC(CH₃)₃ and acetone can be distinguished by the reaction with NaHSO₃.
Reason: HSO₃ is the nucleophile in bisulphite addition.
(iii) Assertion: Ease of nucleophilic addition of the compounds (I). CH₃CHO(II) and CH₃COCH₃(III) is I > II > III.
Reason : Aldehydes and ketones undergo nucleophilic addition reactions.
(iv) Assertion: is more reactive towards nucleophilic addition reaction than
Reason : Reactivity of carbonyl group is due to electrophilic nature of carbonyl carbon.

Read the passage given below and answer the following questions:
Fehling's reagent: Fehling's reagent is a mixture of two solutions. Fehling's solution A is aqueous copper sulphate solution. Fehling's solution B is alkaline sodium potassium tartarate (Rochelle salt).

It is a mild oxidising agent. It is weaker than Tollens' reagent. It oxidises only aliphatic aldehydes to carboxylate ions and'itself gets reduced to reddish brown precipitate of cuprous oxide.
Aromatic aldehydes do not respond to Fehling's test. This reaction is used for the test of aliphatic aldehydes known as Fehling's reagent test.
In these questions ( i - iv) a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
(i) Assertion: Fehling's solution can be used to distinguish between acetaldehyde and acetone.
Reason: Fehling's reagent is a mixture of two solutions.
(ii) Assertion: Aromatic aldehydes can be distinguished from aliphatic aldehydes by Fehling's solution.
Reason: Aromatic 'aldehydes reduce Fehling's solution, but aliphatic aldehydes do not.
(iii) Assertion: CH₃CHO and C₆H₅CH₂CHO cannot be distinguished chemically by Fehling's solution.
Reason: CH₃CHO and C₆H₅CH₂CHO can be distinguished by iodoform test.
(iv) Assertion: Formaldehyde, when heated with Fehling's reagent produces a reddish brown ppt. of Cu.
Reason: Fehling's reagent oxidises formaldehyde to formate ion.

Read the passage given below and answer the following questions:
Carboxylic acids having an a-hydrogen atom when treated with chlorine or bromine in the presence of small amount of red phosphorus gives a-halo carboxylic acids. The reaction is known as Hell- Volhard-Zelinsky reaction.

When sodium salt of carboxylic acid is heated with soda lime it loses carbon dioxide and gives hydrocarbon with less number of C-atoms.

In these questions (i - iv), a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices.
(a) Assertion and reason both are correct statements and reason is correct explanation for assertion.
(b) Assertion and reason both are correct statements but reason is not correct explanation for assertion.
(c) Assertion is correct statement but reason is wrong statement.
(d) Assertion is wrong statement but reason is correct statement.
(i) Assertion: (CH₃)₃CCOOH does not give H.V.Z reaction.
Reason: (CH₃)₃CCOOH does not have \(\alpha\)-hydrogen atom.
(ii) Assertion: H.V.Z. reaction involves the treatment of carboxylic acids having \(\alpha\)-hydrogens with Cl₂ or Br₂in presence of small amount of redphosphorus.
Reason : Phosphorus reacts with halogens to form phosphorus trihalides.
(iii) Assertion: C₆H₅COCH₂COOH undergoes decarboxylation easily than C₆H₅COCOOH.
Reason : C₆H₅COCH₂COOH is a 13-ketoacid.
(iv) Assertion: On heating 3-methylbutanoic acid with soda lime, isobutane is obtained.
Reason: Soda lime is a mixture of NaOH + CaO in the ratio 3 : 1.

Read the passage given below and answer the following questions:
Aldehydes and ketones are reduced to primary and secondary alcohols respectively by NaBH₄ or LiAIH₄ as well as catalytic hydrogenation. The carbonyl group of aldehydes and ketones is reduced to group on treatment with Zn-Hg and cone, HCI (Clemmensen reduction) or with hydrazine followed by NaOH or KOH in highly boiling solvent such, as ethylene glycol (Wolff- Kishner reduction).
Aldehydes differ from ketones in their oxidation reactions. Aldehydes are easily oxidised to carboxylic acids on treatment with HNO₃, KMnO₄, K₂Cr₂O₇ etc. Even mild oxidising agents mainly Tollens' reagent and Fehling's solution also oxidise aldehydes. Ketones are generally oxidised under vigorous conditions i.e., strong oxidising agents and at elevated temperatures, to give mixture of carboxylic acids having lesser number of C-atoms than the parent ketone.
The following questions are multiple choice questions. Choose the most appropriate answer:
(i) Which of the following cannot be made by reduction of ketone or aldehyde with NaBH₄ in methanol?

(a) 1-Butanol	(b) 2-Butanol
(c) 2-Methyl-I-propanol	(d) 2-Methyl-2-propanol

(ii) The carbonyl compound producing an optically active product by reaction with LiAlH₄ is

(a) propanone

(b) butanone

(c) 3-pentanone

(d) benzophenone

(iii) A substance C₄H₁₀O(X) yields on oxidation a compound C₄H₈O which gives an oxime and a positive iodoform test. The substance X on treatment with cone. H2S04 gives C4Hs. The structure of the compound (X) is

(a) CH₃CH₂CH₂CH₂OH	(b) CH₃CH(OH)CH₂CH₃
(c) (CH₃)₃COH	(d) CH₃CH₂-O-CH₂CH₃

(iv) In the oxidation of by acidified K₂Cr₂O₇, the products are

Read the passage given below and answer the following questions:
(A), (B) and (C) are three non-cyclic functional isomers of a carbonyl compound with molecular formula C₄H₈O. Isomers (A) and (C) give positive Tollen's test whereas isomer (B) does not give Tollen's test but gives positive iodoform test. Isomers (A) and (B) on reduction with Zn(Hg)/conc. HCI give the same product (D).
The following questions are multiple choice questions. Choose the most appropriate answer:
(i) Compound A is

(ii) Compound (B) can be obtained by

\({ (a) } \ \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{2}-\mathrm{CH}_{3} \frac{\text { dil. } \mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{HgSO}_{4}}{333 \mathrm{~K}}\)	\((b) \ \left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COO}\right)_{2} \mathrm{Ca} \stackrel{\text { Dry distill }}{\longrightarrow}\)
\((c) \ \mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3} \frac{\mathrm{B}_{2} \mathrm{H}_{6} / \mathrm{THF}}{\mathrm{H}_{2} \mathrm{O}_{2} / \mathrm{NaOH}}\)	\((d) \ \mathrm{CH}_{3}-\mathrm{CH}=\mathrm{CH}-\mathrm{CH}_{3} \frac{\mathrm{O}_{3}}{\mathrm{Zn} / \mathrm{H}_{2} \mathrm{O}}\)

(iii) Out of (A), (B) and (C) isomers, which one is least reactive towards addition of HCN ?

(a) A	(b) B
(c) C	(d) All are equally reactive

(iv) What will be the product when (B) reacts with ethylene glycol in presence ofHCI gas?

Visit QB365-SLMS - https://www.qb365.in/slms/

Answers Key:

Case Study Questions

5 x 4 = 20

(i) (a)
(ii) (b): \(\mathrm{HSO}_{3}^{-}\) is a bulky nucleophile, hence, cannot attack on sterically hindered ketones.
(iii) (d): Aromatic aldehydes and ketones are less reactive than the corresponding aliphatic analogues towards nucleophilic addition reactions due to the +R effect of benzene ring. Further, aldehydes are more reactive than ketones due to +I effect and steric effect of alkyl group. Therefore, the ease of nucleophilic addition will follow the order:

(iv) (b): Electron withdrawing group (-NO₂) increases the reactivity towards nucleophilic addition reactions, whereas electron donating group (-CH₃) decreases the reactivity towards nucleophilic addition reactions.

(i) (b): All aliphatic aldehydes give red ppt. with Fehling's solution, but ketones do not reduce Fehling's solution.
(ii) (c): Aliphatic aldehydes reduce Fehling's solution, but aromatic aldehydes do not.
(iii) (b): CH₃CHO and C₆H₅CH₂CHO both are aliphatic aldehydes, hence cannot be distinguished by Fehling's solution. CH₃CHO contains CH₃COgroup whereas C₆H₅CH₂CHO does not contain any CH₃CO- group. Thus, CH₃CHO will give yellow ppt. with 12 and NaOH but C₆H₅CH₂CHO will not.
(iv) (d): Formaldehyde when heated with Fehling's reagent, undergo oxidation to give formate ion and produce reddish brown ppt. of Cu₂O.
\(\mathrm{HCHO}+2 \mathrm{Cu}^{2+}+5 \mathrm{OH}^{-} \rightarrow \mathrm{HCOO}^{-}+\mathrm{Cu}_{2} \mathrm{O}+3 \mathrm{H}_{2} \mathrm{O}\\ \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \text { Reddish brown ppt. } \)

(i) (a)
(ii) (c): Phosphorus converts a little .of the acid into acid chloride which is more reactive than the parent carboxylic acid. Thus, it is the acid chloride, not the acid itself, that undergoes chlorination at the \(\alpha\)-carbon.
(iii) (a): \(\beta\)-ketoacids are unstable acids. They readily undergo decarboxylation through a cyclic transition state.

(i) (b): All aliphatic aldehydes give red ppt. with
Fehling's solution, but ketones do not reduce Fehling's
solution.
(ii) (e): Aliphatic aldehydes reduce Fehling's solution,
but aromatic aldehydes do not.

(i) (d): 2-Methyl-2-propanol is It cannot be obtained by reduction of an aldehyde or ketone with NaBH₄·

2-Butanone forms oxime on reaction with hydroxylamine (NH₂OH) and also gives positive iodoform test.

(i) (b): As (A) and (C) gives positive Tollens' test thus these two should be aldehydes while (B) should be a ketone (does not give Tollen's test) with

group (as it gives positive iodoform test). Three isomers are.

(iii) (b): (B) is least reactive among the three isomers towards addition of HCN. Aldehydes are more reactive than ketones towards nucleophilic addition reactions.
(iv) (a): When butanone reacts with ethylene glycol in presence of HCl, it forms a ketal.