CBSE Class 12 Chemistry Aldehydes Ketones And Carboxylic Acids VBQs

Question. Arrange the following compounds in the decreasing order of their reactivity in nucleophilic addition reactions.
(I) Benzaldehyde, (II) p-Tolualdehyde, (III) p-Nitro benzaldehyde,
(IV)Acetophenone.
(a) I >II >III > IV
(b) I >III >II > IV
(c) II >I >III > IV
(d) III >I >II > IV
Answer. D

Question. The IUPAC name of CH₃ COCH(CH₃)₂ is
(a) 3- methylbutan-2-one
(b) 3,3-dimethyl propanone
(c) 1,1-dimethyl propanone
(d) isopropyl methyl ketone
Answer. A

Question. The purpose of S in Rosemonds reduction is
(m) to prevent the reduction of aldehyde to alcohol
(n) to prevent the oxidation of aldehyde to carboxylic acid
(o) to increase the rate of reaction.
(d) None of the above
Answer. A

Question. Toluene is oxidized to benzaldehyde by
(a) alkaline KMnO₄
(b) PCC
(c) acidified K₂Cr₂O₇
(d) CrO₂Cl₂
Answer. D

Question. The oxidation of toluene to benzaldehyde using chromyl chloride is
(a) Stephan’s reaction
(b) Étards reaction
(c) Gatterman Koch reaction
(d) ozonolysis
Answer. B

Question. The IUPAC name of CH₃ CH(OH)COOH is
(a) lactic acid
(b) 2-hydroxypropanoic acid
(c) 3- carboxypropanol
(d) 3-carboxy propan -2-ol
Answer. B

Question. substance X is heated with soda lime and gives ethane ,the substance X is
(a) ethanol
(b) ethanoic acid
(c) propanoic acid
(d) methanoic acid
Answer. C

Question. The major product of nitration of benzoic acid is,
(a) 2-nitrobenzoic acid
(b) 4-nitrobenzoic acid
(c) 3-nitrobenzoic acid
(d) 2,4- dinitro benzoic acid
Answer. C

Question. Which of the following does not react with NaHSO₃
(a) Acetone
(b) Acetaldehyde
(c) Benzaldehyde
(d) benzophenone
Answer. D

Question. Carbonyl compounds undergo nucleophilic addition because of
(a) More stable anion with negative charge on oxygen and less stable carbocation
(b) Electromeric effect
(c) Electronegativity difference of carbon and oxygen atoms
(d) None of the above.
Answer. A

Question. Wolff Kishner reduction is carried with
(a) LiAlH₄ in ether
(b) Zn-Hg and HCl
(c) H₂ in the presence of Pd
(d) NH₂-NH₂/ethylene glycol and KOH
Answer. D

Question. The addition of HCN to carbonyl compounds is an example of
(a) Electrophilic addition
(b) Nucleophilic addition
(c) Nucleophilic substitution
(d) Electrophilic substitution
Answer. B

Question. Oxidation of cyclohexene using acidified KMnO₄ will give ………
(a) adipic acid
(b) hexane -1,6-dial
(c) cyclohexane carboxylic acid
(d) cyclopentane carboxylic acid
Answer. A

Question. Aldol condensation will not occur in
(a) HCHO
(b) CH₃ CHO
(c) CH₃ CO CH₃
(d) CH₃- CH₂- CHO
Answer. A

Question. Electrolysis of an aqueous solution of CH₃ –COONa gives
(a) methane
(b) ethane
(c) ethene
(d) butane
Answer. B

Question. Which of the following have the highest Ka value
(a) formic acid
(b) acetic acid
(c) chloroacetic acid
(d) fluroacetic acid
Answer. D

Question. Benzoyl chloride is prepared from benzoic acid by
(a) Cl₂/ FeCl₃
(b) Cl₂/ uv
(c) SOCl₂
(d) HCl
Answer. C

Question. Which of the following can be reduced with Zn/Hg and HCl to give the corresponding hydrocarbon.
(a) Butanal
(b) Butan-2-one
(c) Butanoic acid
(d) Benzoyl chloride
(a) a only
(b) both band d
(c) both a and c
(d) both a and b
Answer. D

ASSERTION - REASON TYPE QUESTIONS

Read the assertion and reason carefully to mark the correct option out of the options given below:
a. If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
b. If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
c. If the Assertion is correct but Reason is incorrect.
d. If both the Assertion and Reason are incorrect.

Question. Assertion : Benzoic acid does not undergo Freifel Craft reaction
Reason : Carbonyl group in benzoic acid donate electron into thr ring through resonance
Answer. C

Question. Assertion : Although phenoxide ion has more number of resonating structures than Carboxylate ion, Carboxylic acid is a stronger acid than phenol.
Reason : In carboxylate ions, negative charge is delocalised over two oxygen atoms which is more stable. Phenoxide is less stable as negative charge is delocalised over one oxygen atom and carbon atoms of benzene ring.
Answer. A

Question. Assertion : Carboxylic acids have greater boiling point than alcohols
Reason : Carboxilic acid have the greater ability to form dimers in solution
Answer. A

Question. Assertion: Aldehydes react with Tollen's reagent to form silver mirror.
Reason: Both, aldehydes and ketones contain a carbonyl group.
Answer. B

Question. Assertion: Aldol condensation can be catalysed both by acids and bases.
Reason: ββ Hydroxy aldehydes or ketones readily undergo acid catalysed dehydration.
Answer. B

Question. Assertion: Ethanal and Propanone both gives iodoform test
Reason: both having –CO- CH₃ group
Answer. A

Question. Assertion: The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses.
Reason: There is a weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions.
Answer. A

Question. Assertion: Compounds containing –CHO group are easily oxidised to corresponding carboxylic acids.
Reason : Carboxylic acids can be reduced to alcohols by treatment with LiAlH₄
Answer. B

Question. Assertion: Acetic acid does not undergo a haloform reaction.
Reason: Acetic acid has no alpha hydrogens.
Answer. C

Question. Assertion: 2, 2–Dimethyl propanal undergoes Cannizzaro reaction with concentrated NaOH.
Reason: Cannizzaro is a disproportionation reaction.
Answer. B

Question. Assertion: Acetaldehyde on treatment with dil NaOH it gives aldol.
Reason: Acetaldehyde molecules contain alpha hydrogen atom.
Answer. A

Question. Assertion : The boiling points of aldehydes and ketones are higher than hydrocarbons and ethers of comparable molecular masses.
Reason : There is a weak molecular association in aldehydes and ketones arising out of the dipole-dipole interactions.
Answer. A

Question. Assertion : Butan -2 – ol will give butanone on oxidation with alkaline KMnO₄ solution.
Reason : Aldehydes are easily oxidised to carboxylic acids on treatment with common oxidising agents like nitric acid, potassium permanganate, potassium dichromat, etc.
Answer. B

Question. Assertion : Benzaldehyde gives a positive test with Tollens reagent but not with Fehling’s solution
Reason: Under normal circumstances, aldehydes that lack alpha hydrogens and so cannot form an enolate do not produce a positive test using Fehling's solution, which is a weaker oxidising agent than Tollen's reagent.
Answer. A

Question. Assertion: Propanal is more reactive than propanone.
Reason- Due to the presence of alkyl groups on both sides of the carbonyl carbon, propanone is sterically more hindered than propanal, making it less reactive to nucleophilic attack.
Answer. A

Question. Assertion : Carboxylic acid is a stronger acid than phenol.
Reason : Carboxylate ions are more polar and has less stearic hindrance
Answer. C

SHORT ANSWER QUESTIONS 

Question. What is Tollens’ reagent? Write one usefulness of this reagent.
Answer. Tollens’ reagent is an ammoniacal silver nitrate solution. Tollens’ reagent is used to test an aldehyde. Both aliphatic and aromatic aldehydes reduce Tollens ’reagent and give silver mirror.

Question. The aldehydes and ketones undergo a number of addition reactions. Explain.
Answer. Aldehydes and ketones undergo a number of addition reactions as both possess the carbonyl functional group which reacts with a number of nucleophiles such as HCN, NaHSO₃, alcohols,ammonia derivatives and Grignard reagents. 

Question. A and B are two functional isomers of compound C₂H₆O. On heating with NaOH and I 2, isomer B forms yellow precipitate of iodoform whereas isomer A does not form any precipitate. Write the formulae of A and B .
Answer. Formula of compounds A and B = C₃H₆O & B forms yellow precipitate of iodoform. Hence,B must contain —COCH₃ group Therefore, compound ‘B’ must be .
A does not give iodoform test and it is functional isomer of B thus, it may be CH₃CH₂CHO.

Question. Arrange the following compounds in increasing order of their boiling points.
CH₃CHO, CH₃CH₂OH, CH₃OCH₃ , CH₃CH₂CH₃
Answer. The molecular masses of the given compounds are in the range 44 to 46. CH₃CH₂OH undergoes extensive intermolecular H-bonding, resulting in the association of molecules. Therefore, it has the highest boiling point and is more polar than CH₃OCH₃ and so CH₃CHO has stronger intermolecular dipole - dipole attraction than CH₃OCH₃ and so CH₃CH₂CH₃ has only weak van der Waals force. Thus, the arrangement of the given compounds in the increasing order of their boiling points is given by:
CH₃CH₂CH₃<CH₃OCH₃<CH₃CHO<CH₃CH₂OH

Question. Give chemical tests to distinguish between :Benzoic acid and Phenol
Answer. Phenol and benzoic acid can be distinguished by their reactions with sodium bicarbonate solution. Benzoic acid will give effervescence with NaHCO₃ but phenol will not react.

Question. Account for the following :Cl -CH₂COOH is a stronger acid than CH₃COOH.
Answer. Chloroacetic acid has lower pKa value than acetic acid; ‘Cl’ in chloroacetic acid shows
–I effect, it creates less electron density on oxygen of carboxylic aci(d) thus, release of proton becomes easier. In case of acetic acid, the state of affair is just opposite. Hence, chloroacetic acid is stronger than acetic

Question. Monochloroethanoic acid has a higher pKa value than dichloroethane aci)Why?
Answer. The strength of an acid is indicated by pKa value, where, pKa = – log Ka
Since monochloroethanoic acid is weaker than dichloroethanoic acid , it has lower value of dissociation constant Ka. ,therefore, it has higher value of pKa.

Question. Ethanoic acid is a weaker acid than benzoic aci(d) Explain.
Answer. The —COOH group in benzoic acid is attached to sp² - carbon of the phenyl ring and is more acidic than acetic acid in which —COOH group is attached to sp³ – carbon atom of CH₃ group. So, benzoic acid is stronger than acetic or acetic acid is weaker acid than benzoic acid

Question. Why does solubility decrease with increasing molecular mass in carboxylic acid?
(ii) Why PCC cannot oxidize methanol to methanoic acid while KMnO₄ can? .
(iii) Benzaldehyde is less reactive in nucleophilic addition reactions than propanal ? Why?
Answer.
 (i) Solubility decrease with increasing molecular mass in carboxylic acid as with increase of molecular mass size the hydrophobic carbon chain length increases.
(ii) PCC (pyridinium chlorochromate )is a mild oxidizing agent and can oxidize methanol to methanal only, while KMnO₄ is strong oxidizing agent which oxidizes it to methanoic aci(d)
(iii) Carbon atom of carbonyl is C₆H₅CHO is less reactive than that of propanal.
C₆H₅CHO less polar due to resonance

Question. The term “Green chemistry” as adopted by the IUPAC working pastly on synthetic pathways and processes in green chemistry is getting awareness even among common people. My mother who has retired twenty years ago, as working with a laboratory synthesizing aldehydes using.
RCOCl + H₂ RCHO + HCl
Myself, working in the same lab adopted this method but my mother advised not to use this method and suggested another one Now answer the following questions:
(i) Why did my mother advise not to use given method
(ii) What was another method? Did you consider it to be environmental friendly and economically?
Answers:  (i) BaSO₄ causes toxic problems. My mother having recent knowledge about green chemistry advised me to discard this method.
(ii) Another method does not use BaSO4

Question. Formaldehyde (Methanal) is gas at room temperature. It is soluble in water because it is polar. It has pungent smell.
I. What is formalin? Give its use.
II. Which thermoplastics can be made from formalin? Give their uses also.
III. Name a medicine prepared from formaldehyde? Give chemical equation and its use.
IV. What is the problem in using formalin?
Answers: i) 40% aqueous solution of formaldehyde is called Formalin.
ii) a) Urea – Formaldehyde resin: It is used for making unbreakable crockery.
b) Phenol – Formaldehyde resin (Bakelite): It is used for making electrical switches and switch boards.
iii) It is used for the preparation of Urotropine ( urinary antiseptic)
iv) It is strongly pungent smelling, which is difficult 

Please refer to the link below for CBSE Class 12 Chemistry -  Aldehydes, Ketones And Carboxylic Acids VBQs