preparation MCQs for UPSC Prelims

31 practice questions on preparation from the Aldehydes, Ketones and Carboxylic Acids section of the UPSC Prelims syllabus. 31 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

9 Easy 16 Medium 6 Hard

Sample questions

Q1
easy

Benzaldehyde, $\text{C}_6\text{H}_5\text{CHO}$, is to be made by the Rosenmund reduction. Which compound should be hydrogenated over $\text{Pd/BaSO}_4$?

  1. A Benzoyl chloride, $\text{C}_6\text{H}_5\text{COCl}$
  2. B Benzyl chloride, $\text{C}_6\text{H}_5\text{CH}_2\text{Cl}$
  3. C Chlorobenzene, $\text{C}_6\text{H}_5\text{Cl}$
  4. D Benzoic acid, $\text{C}_6\text{H}_5\text{COOH}$
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Correct answer: A - Benzoyl chloride, $\text{C}_6\text{H}_5\text{COCl}$

The Rosenmund reduction turns an acyl chloride $\text{RCOCl}$ into $\text{RCHO}$, so the starting material must carry chlorine on a carbonyl carbon, as benzoyl chloride does. In benzyl chloride the chlorine sits on a $\text{CH}_2$ group, so hydrogenation cannot build a $\text{CHO}$ group. Chlorobenzene holds its chlorine on the ring and is untouched, and a carboxylic acid is not reduced by hydrogen over poisoned palladium.

Q2
easy

Acetone is the common name of the simplest ketone, $\text{CH}_3\text{COCH}_3$. What is its IUPAC name?

  1. A Propanal
  2. B Butanone
  3. C Propanone
  4. D Ethanal
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Correct answer: C - Propanone

Acetone is a three-carbon chain with the carbonyl on carbon 2, so its IUPAC name is propanone. Propanal has the same three carbons but the oxygen at the end of the chain, which makes it an aldehyde rather than a ketone. Ethanal is the IUPAC name of acetaldehyde, a different common name. Butanone has four carbons and is the IUPAC name of methyl ethyl ketone.

Q3
medium

Ethanoyl chloride is treated with dimethylcadmium, $(\text{CH}_3)_2\text{Cd}$. What is the organic product?

  1. A Ethanoic acid
  2. B Ethanal
  3. C Propanone
  4. D Propanal
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Correct answer: C - Propanone

A dialkylcadmium hands its alkyl group to the acyl carbon in place of chlorine, so $\text{CH}_3\text{COCl}$ becomes $\text{CH}_3\text{COCH}_3$. Ethanal would need a reduction, which cadmium reagents do not carry out. Propanal has its carbonyl at the end of the chain, which attack at the acyl carbon cannot produce, and no water is supplied to give an acid.

Q4
easy

Toluene is treated with chromyl chloride, $\text{CrO}_2\text{Cl}_2$, in carbon disulphide, and the brown chromium complex that separates out is then hydrolysed. What is the organic product?

  1. A Benzyl chloride
  2. B Benzaldehyde
  3. C Benzyl alcohol
  4. D Benzoic acid
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Correct answer: B - Benzaldehyde

This is the Etard reaction. Chromyl chloride attacks only the methyl group and traps the oxidised carbon as a chromium complex, so hydrolysis releases benzaldehyde and the oxidation goes no further. Benzoic acid is what hot alkaline potassium permanganate would give. Benzyl alcohol is a reduction product of benzaldehyde, and benzyl chloride comes from chlorinating toluene in sunlight.

Q5
medium

Ethylmagnesium bromide is poured on to solid carbon dioxide and the mixture is then treated with dilute acid. Which acid is obtained?

  1. A Ethanoic acid
  2. B Butanoic acid
  3. C Propanoic acid
  4. D Ethanedioic acid
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Correct answer: C - Propanoic acid

The ethyl group of the Grignard reagent adds to carbon dioxide, so the chain gains one carbon and $\text{C}_2\text{H}_5-$ becomes $\text{C}_2\text{H}_5\text{COOH}$. Ethanoic acid would mean no carbon was gained and butanoic acid would mean two were. Ethanedioic acid needs a $-\text{COOH}$ group at each end, which one addition to one $\text{CO}_2$ molecule cannot give.

Q6
medium

Four hydrocarbons are refluxed separately with hot alkaline potassium permanganate and the mixtures are then acidified. Which one does NOT give benzoic acid?

  1. A tert-Butylbenzene
  2. B Ethylbenzene
  3. C Toluene
  4. D Propylbenzene
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Correct answer: A - tert-Butylbenzene

A side chain is cut back to $-\text{COOH}$ only when the carbon joined to the ring carries at least one hydrogen. In tert-butylbenzene that carbon holds three methyl groups and no hydrogen, so the side chain survives. Toluene, ethylbenzene and propylbenzene all have a benzylic hydrogen, so however long the chain, permanganate trims it to benzoic acid.

Q7
medium

Ketones are prepared from acyl chlorides using a dialkylcadmium rather than a Grignard reagent. Why is the cadmium reagent preferred?

  1. A A Grignard reagent would attack the ketone as it forms and convert it into a tertiary alcohol
  2. B A Grignard reagent would reduce the acyl chloride to an aldehyde
  3. C Dialkylcadmium reagents work without solvent, while Grignard reagents need water
  4. D A Grignard reagent does not react with an acyl chloride at all
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Correct answer: A - A Grignard reagent would attack the ketone as it forms and convert it into a tertiary alcohol

Grignard reagents are so reactive that a second alkyl group adds to the ketone as fast as the ketone appears, ending in a tertiary alcohol; the milder dialkylcadmium stops after one addition. Grignard reagents certainly do attack acyl chlorides, and they deliver alkyl groups rather than reduce. Water destroys a Grignard reagent, so the claim that it needs water is the reverse of the truth.

Q8
medium

Benzophenone has a carbonyl group joining two benzene rings. It is to be made from benzene by Friedel-Crafts acylation. Which reagent should be used along with anhydrous aluminium chloride?

  1. A Ethanoyl chloride, $\text{CH}_3\text{COCl}$
  2. B Benzoyl chloride, $\text{C}_6\text{H}_5\text{COCl}$
  3. C Benzyl chloride, $\text{C}_6\text{H}_5\text{CH}_2\text{Cl}$
  4. D Chlorobenzene, $\text{C}_6\text{H}_5\text{Cl}$
Show answer and explanation

Correct answer: B - Benzoyl chloride, $\text{C}_6\text{H}_5\text{COCl}$

Benzoyl chloride gives the acylium ion $\text{C}_6\text{H}_5\text{CO}^+$, which attaches a $-\text{COC}_6\text{H}_5$ group to benzene, so the product is benzophenone. Benzyl chloride carries no carbonyl and would only attach a $-\text{CH}_2\text{C}_6\text{H}_5$ group. Ethanoyl chloride gives acetophenone, with a methyl group in place of the second ring. Chlorobenzene has its chlorine on the ring itself and is not an acylating agent.

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