CBSE Class 12 Chemistry Notes: Aldehydes & Ketones

Prob 1. The correct order of reactivity towards nucleophilic addition reaction will be

Sol. Aldehydes are more reactive than ketones due to lesser number of +I group & again the reactivity of carbonyl group depends on the major nature of alkyl group attached to it.

So correct order is

Prob 2. Which of the following will not give aldol condensation?

(A) RCH₂CHO (B) RCOCH₂R

(C) C₆H₅CHO (D) C₆H₅CH₂CHO

Sol. Aldol condensation is given by those carbonyl compounds, which possess at least one a – H.

∴(C)

Prob 3. Aldehydes & ketones can be reduced to corresponding hydrocarbons by

(A) refluxing with strong acids

(B) passing the vapours under heated PbO₂

(C) Refluxing with zinc amalgam

(D) Refluxing with strong alkali

Sol. ∴(C)

Prob 8. Arrange the following compounds in order of their decreasing activity towards Tollen’s reagent:

HCHO CH₃CHO CH₃COCH₃ C₆H₅CHO

I II III IV

(A) I > II > IV > III (B) I > II > III > IV

(C) II > I > III > IV (D) none

Sol. Aldehydes are more reactive than ketones

∴ (A)

Prob 9. The most reactive compound towards formation of cyanohydrin on treatment with KCN followed by acidification is

(A) benzaldehyde (B) p – nitrobenzaldehyde

(C) phenylacetaldehyde (D) p – hydroxybenzaldehyde

Sol. Presence of electron withdrawing group in para position increases nucleophilic addition.

∴ (B)

Prob 10. Which of the following chemical system has most acidic hydrogen?

(A) 3 – hexanone (B) 2, 4 – hexanedione

(C) 2, 5 – hexanedione (D) 2, 3 – hexanedione

Sol. 2, 4 – hexanedione has ¾CH₂¾ group surrounded on both sides of electronegative (>C=O) group, hence it is acidic in nature, the carbanions

∴ (D)

Prob 12. Acetophenone can be prepared by

(I) Oxidation of 1 – phenylethanol

(II) Reduction of benzaldehyde with methyl magnesium bromide

(III) Friedel – crafts reaction of benzene with acetylchloride

(IV) Distillation of calcium benzoate

Which of above is correct?

(A) I & III (B) II & IV

(C) III & IV (D) I & II

Prob 13. An organic compound ‘A’ has the molecular formula C₃H₆O. It undergoes iodoform test. When saturated with HCl it gives ‘B’ of molecular formula C₉H₁₄O, ‘A’ & ‘B’ respectively are

(A) propanal & mesitylene

(B) propanone & mesityl oxide

(C) propanone & 2, 6 – dimethyl – 2, 5 – heptadien – 4 – one

(D) propane & meistylene oxide

Sol. Since the compound A has the molecular formula C₃H₆O & undergoes iodoform test, it should be CH₃COCH₃. Further reaction of B with HCl gives C₉H₁₄O, this should be trimer of acetone, i.e. It should be phorone.

∴ (C)

Prob 14. What will be the product when 2-Butenal reacts with sodiumborohydride?

(A) Butanal (B) Butanol

(C) But-2-ene-1-ol (D) None

Sol. ∴ (C)

Prob 15. Which will give a yellow precipitate with iodine & alkali?

(A) 2 – hydroxyl propane (B) benzophenone

(C) o – methyl toluene (D) acetamide

Sol. ∴ (A)

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