15 extra multiple-choice questions for Carbonyl Compounds and Carboxylic Acids (12th Standard Chemistry, Samacheer Kalvi), beyond the ones printed in the textbook — each with the correct option highlighted and a clear, worked explanation. Free to read in English and Tamil.
Q1
What is the IUPAC name for the compound commonly known as mesityl oxide?
- A. 4-methylpent-3-en-2-oneCorrect
- B. 2-methylpent-2-en-4-one
- C. 4-methylpent-2-en-3-one
- D. 3-methylpent-3-en-2-one
Explanation. In the IUPAC system, numbering starts from the end closer to the carbonyl group, making it a pent-2-one with a methyl substituent at carbon 4 and a double bond starting at carbon 3.
Q2
In a carbonyl group, the carbon atom uses which of the following hybridisations to form its sigma bonds?
- A. sp
- B. \(sp^{2}\)Correct
- C. \(sp^{3}\)
- D. \(dsp^{2}\)
Explanation. The carbonyl carbon is bonded to three other atoms in a planar arrangement using three sp2 hybridised orbitals, while the remaining unhybridised p-orbital forms the pi bond with oxygen.
Q3
Which of the following reagents is specifically used to oxidize a primary alcohol to an aldehyde while preventing further oxidation to a carboxylic acid?
- A. Acidified \(K_{2}Cr_{2}O_{7}\)
- B. Alkaline \(KMnO_{4}\)
- C. Pyridinium chlorochromate (PCC)Correct
- D. Hot concentrated \(HNO_{3}\)
Explanation. PCC is a mild oxidising agent that stops the reaction at the aldehyde stage, unlike stronger agents that continue to oxidise aldehydes into carboxylic acids.
Q4
The dry distillation of which of the following calcium salts produces propanone (acetone)?
- A. Calcium formate
- B. Calcium acetateCorrect
- C. Calcium propionate
- D. Calcium benzoate
Explanation. The dry distillation of calcium acetate, a calcium salt of a carboxylic acid other than formic acid, yields a symmetrical ketone, which in this case is acetone.
Q5
Aldehydes and ketones have significantly higher boiling points than non-polar hydrocarbons of similar molar mass primarily due to:
- A. Strong covalent bonding
- B. Intermolecular hydrogen bonding
- C. Weak dipole-dipole interactionsCorrect
- D. Vigorous van der Waals forces
Explanation. The polarity of the carbonyl group allows for dipole-dipole interactions between molecules, requiring more energy to separate them compared to the non-polar forces in hydrocarbons.
Q6
What is the final organic product obtained when a cyanohydrin undergoes complete acid hydrolysis?
- A. An \(\alpha\)-hydroxy acidCorrect
- B. An \(\alpha\)-amino acid
- C. A primary amine
- D. An aliphatic nitrile
Explanation. During acid hydrolysis, the nitrile group (-CN) of the cyanohydrin is converted into a carboxylic acid group (-COOH), resulting in the formation of an alpha-hydroxy acid.
Q7
According to Popoff's rule, during the vigorous oxidation of an unsymmetrical ketone, which part of the molecule is favored to retain the carbonyl group?
- A. The larger alkyl group
- B. The smaller alkyl groupCorrect
- C. The group with the least hydrogens
- D. The group with branching
Explanation. Popoff's rule states that the C-CO bond is cleaved such that the keto group remains associated with the smaller alkyl group during oxidation.
Q8
In the mechanism of aldol condensation of acetaldehyde, what is the specific role of the dilute NaOH base in the first step?
- A. To protonate the carbonyl oxygen
- B. To act as a nucleophile attacking the carbon
- C. To remove an \(\alpha\)-hydrogen atom as a protonCorrect
- D. To dehydrate the final aldol product
Explanation. The base abstracts an acidic alpha-hydrogen atom as a proton from the aldehyde, leading to the formation of a nucleophilic carbanion or enolate ion.
Q9
Which of the following aromatic aldehydes undergoes the Cannizzaro reaction when treated with concentrated alkali?
- A. Acetaldehyde
- B. BenzaldehydeCorrect
- C. Propanal
- D. Phenylacetaldehyde
Explanation. Aldehydes that lack an alpha-hydrogen atom, like benzaldehyde, undergo self-oxidation and reduction (disproportionation) instead of aldol condensation when treated with concentrated base.
Q10
What is the correct IUPAC name for the dicarboxylic acid commonly known as succinic acid?
- A. Ethane-1,2-dioic acid
- B. Propane-1,3-dioic acid
- C. Butane-1,4-dioic acidCorrect
- D. Pentane-1,5-dioic acid
Explanation. Succinic acid consists of a four-carbon chain with two carboxylic acid groups located at the terminal positions, hence it is named butane-1,4-dioic acid.
Q11
The reaction of a Grignard reagent with dry ice followed by acid hydrolysis is used to synthesize which class of compounds?
- A. Ketones
- B. Carboxylic acidsCorrect
- C. Primary alcohols
- D. Acid anhydrides
Explanation. Grignard reagents react with carbon dioxide to form a carboxylate salt, which yields the corresponding carboxylic acid upon acidification with a mineral acid.
Q12
The Hell-Volhard-Zelinsky (HVZ) reaction is specifically used to introduce which substituent into a carboxylic acid?
- A. A hydroxyl group at the \(\beta\)-position
- B. A halogen atom at the \(\alpha\)-positionCorrect
- C. An amino group at the terminal position
- D. An alkyl group into the benzene ring
Explanation. The HVZ reaction involves treating a carboxylic acid having an alpha-hydrogen with a halogen and red phosphorus to substitute the alpha-hydrogen with a halogen atom.
Q13
Which of the following substituted acetic acids is the most acidic?
- A. Fluoroacetic acidCorrect
- B. Chloroacetic acid
- C. Bromoacetic acid
- D. Iodoacetic acid
Explanation. Acidity increases with the increasing electronegativity of the substituent. Fluorine is the most electronegative halogen, creating the strongest electron-withdrawing effect to stabilise the carboxylate ion.
Q14
What is the correct decreasing order of reactivity of carboxylic acid derivatives towards nucleophilic acyl substitution?
- A. Acid halide > Acid anhydride > Ester > AmideCorrect
- B. Amide > Ester > Acid anhydride > Acid halide
- C. Acid halide > Ester > Acid anhydride > Amide
- D. Acid anhydride > Acid halide > Amide > Ester
Explanation. The order is determined by the basicity of the leaving group; weaker bases are better leaving groups. Halide ions are the weakest bases, making acid halides the most reactive.
Q15
Which named reaction involves the self-condensation of an ester containing at least one \(\alpha\)-hydrogen in the presence of a strong base to form a \(\beta\)-keto ester?
- A. Aldol condensation
- B. Perkin's reaction
- C. Claisen condensationCorrect
- D. Kolbe's reaction
Explanation. Claisen condensation occurs when two ester molecules react in the presence of a base like sodium ethoxide to produce a beta-keto ester by losing an alcohol molecule.
