Quiz: Aldehydes and Ketones — 20 questions

1. Which structural feature identifies an aldehyde?

A. The carbonyl carbon is bonded to at least one hydrogen atom

B. The carbonyl carbon is bonded to one nitrogen group

C. The carbonyl carbon is bonded to two carbon groups

D. The carbonyl carbon is bonded to two oxygen atoms

Answer: (A)

Explanation

An aldehyde contains the general structure R–CHO, so its carbonyl carbon is attached to at least one hydrogen atom. A carbonyl carbon bonded to two carbon groups describes a ketone instead.

2. Which formula represents a ketone?

A. R–COOH, with a hydroxyl group attached to the carbonyl carbon

B. R–CHO, with one hydrogen attached to the carbonyl carbon

C. R–CO–R′, with two carbon groups attached to the carbonyl carbon

D. R–CH₂OH, with two hydrogen atoms attached to the carbonyl carbon

Answer: (C)

Explanation

A ketone has the general structure R–CO–R′, meaning that two carbon groups are attached to the carbonyl carbon. The R–CHO structure contains a carbonyl hydrogen and therefore represents an aldehyde.

3. Why is the carbonyl carbon electrophilic?

A. The carbonyl group contains two adjacent carbon atoms

B. Carbon is more electronegative and attracts electron density from oxygen

C. Oxygen is more electronegative and polarizes the C=O bond

D. The carbonyl carbon carries a permanent negative charge

Answer: (C)

Explanation

The greater electronegativity of oxygen polarizes the C=O bond, leaving the carbonyl carbon electron deficient and electrophilic. The carbonyl carbon is not negatively charged; it is the site attacked by nucleophiles.

4. Which order correctly ranks the reactivity of these carbonyl compounds?

A. HCHO > RCHO > RCOR′

B. RCOR′ > RCHO > HCHO

C. RCHO > RCOR′ > HCHO

D. HCHO > RCOR′ > RCHO

Answer: (A)

Explanation

The reactivity order is HCHO > RCHO > RCOR′ because aldehydes have less steric hindrance and fewer electron-donating alkyl groups than ketones. Ketones are therefore less reactive toward nucleophilic reactions than aldehydes.

5. What is the usual oxidation sequence for a primary alcohol?

A. Primary alcohol → ketone → carboxylic acid

B. Primary alcohol → aldehyde → carboxylic acid

C. Primary alcohol → carboxylic acid → aldehyde

D. Primary alcohol → alkene → ketone

Answer: (B)

Explanation

Oxidation of a primary alcohol first produces an aldehyde, which can undergo further oxidation to form a carboxylic acid. A ketone is typically formed by oxidation of a secondary alcohol instead.

6. Which preparation method converts an acid chloride into an aldehyde?

A. Friedel–Crafts acylation

B. Dehydrogenation with copper at 573 K

C. Stephen reduction

D. Rosenmund reduction

Answer: (D)

Explanation

Rosenmund reduction converts an acid chloride into an aldehyde. Stephen reduction instead begins with a nitrile, while Friedel–Crafts acylation is a method for preparing ketones.

7. Which reaction is a method for preparing ketones?

A. Oxidation of a primary alcohol followed by controlled distillation

B. Friedel–Crafts acylation of benzene using an acid chloride and AlCl₃

C. Stephen reduction of a nitrile followed by hydrolysis

D. Ozonolysis of an alkene followed by zinc and water

Answer: (B)

Explanation

Friedel–Crafts acylation of benzene with an acid chloride and AlCl₃ produces an aromatic ketone. Stephen reduction and ozonolysis followed by zinc and water are aldehyde preparation methods.

8. What occurs during nucleophilic addition to a carbonyl compound?

A. A nucleophile attacks the oxygen, forming a positively charged carbon intermediate

B. A nucleophile attacks the carbonyl carbon, forming an alkoxide that is then protonated

C. The carbonyl oxygen attacks an alkyl group, followed by elimination of hydride

D. A proton attacks the carbonyl carbon, forming a carbanion that is then oxidised

Answer: (B)

Explanation

The nucleophile attacks the electrophilic carbonyl carbon, producing an alkoxide intermediate that is subsequently protonated. The oxygen becomes negatively charged in the intermediate rather than serving as the initial attack site.

9. Why do aldehydes and ketones generally boil at higher temperatures than hydrocarbons of similar molar mass but lower temperatures than comparable alcohols?

A. They have dipole–dipole forces but cannot form self hydrogen bonds

B. They are nonpolar molecules but have stronger dispersion forces than alcohols

C. They form ionic networks that are weaker than the networks in hydrocarbons

D. They form extensive self hydrogen bonds but have weak permanent dipoles

Answer: (A)

Explanation

The polar carbonyl group gives aldehydes and ketones dipole–dipole attractions, raising their boiling points above those of similar hydrocarbons. They lack an O–H bond, so they cannot self hydrogen-bond as alcohols do.

10. After hydrolysis, what type of alcohol is formed when methanal reacts with a Grignard reagent?

A. A tertiary alcohol

B. A secondary alcohol

C. An aromatic alcohol

D. A primary alcohol

Answer: (D)

Explanation

Methanal has no carbon substituent attached to its carbonyl carbon, so Grignard addition followed by hydrolysis produces a primary alcohol. Other aldehydes produce secondary alcohols, while ketones produce tertiary alcohols.

11. Which product forms when an aldehyde reacts with an alcohol under acidic conditions?

A. An acetal

B. A ketal

C. A bisulfite addition compound

D. A cyanohydrin

Answer: (A)

Explanation

Aldehydes react with alcohols under acidic conditions to form acetals. Ketones undergo the analogous reaction to form ketals, while cyanohydrins require hydrogen cyanide.

12. What product results when a ketone is reduced with sodium borohydride?

A. A primary alcohol

B. A hydrocarbon

C. A secondary alcohol

D. A carboxylic acid

Answer: (C)

Explanation

Reduction of a ketone with sodium borohydride converts the carbonyl group into a secondary alcohol. Aldehyde reduction under the same type of conditions gives a primary alcohol.

13. Which reagent conditions distinguish the Clemmensen reduction from the Wolff–Kishner reduction?

A. Zn(Hg) and concentrated HCl versus hydrazine, KOH, and heat

B. Iodine and hydroxide versus silver nitrate and ammonia

C. Hydrazine and concentrated HCl versus Zn(Hg), KOH, and cooling

D. Sodium borohydride and water versus hydrogen, nickel, and acid

Answer: (A)

Explanation

The Clemmensen reduction uses Zn(Hg) in concentrated HCl, whereas the Wolff–Kishner reduction uses hydrazine with KOH and heat. Thus, the two methods differ chiefly in acidic versus basic conditions.

14. Which observation most directly identifies an aldehyde in the Tollens test?

A. A yellow precipitate of iodoform

B. A silver mirror on the inside of the vessel

C. A brick-red precipitate of copper(I) oxide

D. A blue solution that remains unchanged

Answer: (B)

Explanation

Tollens’ reagent is reduced by aldehydes to produce a silver mirror. The brick-red copper(I) oxide precipitate is associated with Fehling’s test, particularly for aliphatic aldehydes.

15. What positive result is expected when a methyl ketone gives the iodoform test?

A. A yellow precipitate of CHI₃ forms

B. A colorless solution of an acetal forms

C. A silver mirror forms on the glass

D. A brick-red precipitate of Cu₂O forms

Answer: (A)

Explanation

Methyl ketones and related compounds give yellow iodoform, CHI₃, in the iodoform test. A silver mirror instead indicates a positive Tollens test.

16. Which structural feature is required for an aldol condensation to occur?

A. An aromatic ring attached directly to the carbonyl group

B. At least one α-hydrogen adjacent to the carbonyl group

C. An aldehyde lacking any carbonyl substituent

D. A strongly acidic medium with no heating

Answer: (B)

Explanation

Aldol condensation requires at least one α-hydrogen, which allows formation of the reactive enolate or enol species. The resulting β-hydroxy carbonyl compound may dehydrate upon heating.

17. What products are formed when an aldehyde without α-hydrogen undergoes the Cannizzaro reaction in concentrated alkali?

A. A β-hydroxy carbonyl compound and water

B. One alcohol and one carboxylate from disproportionation

C. A hydrocarbon and an oxidizing agent

D. Two molecules of the corresponding secondary alcohol

Answer: (B)

Explanation

Cannizzaro reaction disproportionates an aldehyde lacking α-hydrogen, reducing one molecule to an alcohol and oxidizing another to a carboxylate. Aldol condensation instead involves carbonyl compounds that have α-hydrogen.

18. How do the –CHO and –COR groups affect electrophilic aromatic substitution on a benzene ring?

A. They activate the ring and direct substitution meta

B. They deactivate the ring and direct substitution meta

C. They activate the ring and direct substitution ortho and para

D. They deactivate the ring and direct substitution ortho and para

Answer: (B)

Explanation

The –CHO and –COR groups withdraw electron density from the aromatic ring, making it less reactive and directing electrophilic substitution to the meta position. Their deactivating effect distinguishes them from activating ortho–para directors.

19. Which starting-material and product pairing correctly identifies the Rosenmund and Stephen transformations?

A. Acid chloride to aldehyde in Rosenmund; nitrile to aldehyde in Stephen

B. Nitrile to aldehyde in Rosenmund; acid chloride to aldehyde in Stephen

C. Acid chloride to alcohol in Rosenmund; nitrile to ketone in Stephen

D. Aldehyde to acid chloride in Rosenmund; aldehyde to nitrile in Stephen

Answer: (A)

Explanation

Rosenmund reduction converts an acid chloride into an aldehyde, whereas Stephen reduction converts a nitrile into an aldehyde. The second choice reverses their starting materials, which is the documented confusion between these reactions.

20. What product is formed when toluene undergoes controlled oxidation with $\mathrm{CrO_2Cl_2}$ in the Etard reaction?

A. Benzoic acid

B. Benzyl alcohol

C. Acetophenone

D. Benzaldehyde

Answer: (D)

Explanation

The Etard reaction selectively oxidizes the methyl group of toluene to produce benzaldehyde. Benzoic acid can result from stronger or further oxidation, but it is not the controlled Etard product.