201. Which statement correctly connects electron withdrawal and the \(O-H\) bond in \(R-COOH\)?
ⓐ. Electron withdrawal by \(C=O\) weakens the \(O-H\) bond.
ⓑ. Electron withdrawal by \(C=O\) removes the \(O-H\) bond entirely.
ⓒ. Electron donation by \(C=O\) makes \(O-H\) non-polar.
ⓓ. Electron donation by \(C=O\) forms an aldehyde first.
Correct Answer: Electron withdrawal by \(C=O\) weakens the \(O-H\) bond.
Explanation: The carbonyl group in a carboxylic acid pulls electron density through the carboxyl system. This reduces electron density around the hydroxyl oxygen and makes the \(O-H\) bond more polar. A more polar \(O-H\) bond can release \(H^+\) more easily. The resulting carboxylate ion is resonance-stabilised, so the acid dissociation becomes more favourable.
202. Which statement is most accurate about the conjugate base of a carboxylic acid?
ⓐ. It is unstable because the negative charge is fixed on one carbon.
ⓑ. It is stabilised because the negative charge is shared by two oxygens.
ⓒ. It is neutral because the released proton carries the negative charge.
ⓓ. It is a ketone because it has two carbon groups attached to \(C=O\).
Correct Answer: It is stabilised because the negative charge is shared by two oxygens.
Explanation: The conjugate base of a carboxylic acid is the carboxylate ion, \(R-COO^-\). Its negative charge is delocalised over two oxygen atoms by resonance. This charge sharing makes the ion more stable than a species with charge fixed on a single atom. A stable conjugate base favours acid dissociation.
203. Which assertion-reason pair is correctly evaluated?
Assertion: Alkyl-substituted ketones are generally less reactive than aldehydes toward nucleophilic addition.
Reason: Alkyl groups decrease electrophilicity of the carbonyl carbon by electron donation and also increase crowding.
ⓐ. Both are true, and Reason explains Assertion.
ⓑ. Both are true, but Reason does not explain Assertion.
ⓒ. Assertion is true, but Reason is false.
ⓓ. Assertion is false, but Reason is true.
Correct Answer: Both are true, and Reason explains Assertion.
Explanation: Ketones usually have two carbon groups attached to the carbonyl carbon, while aldehydes have at least one hydrogen. Alkyl groups donate electron density through the \(+I\) effect, which reduces the partial positive character of the carbonyl carbon. They also increase steric crowding near the reaction centre. These two factors explain why aldehydes generally undergo nucleophilic addition more readily than ketones.
204. Which assertion-reason pair is correctly evaluated?
Assertion: Carboxylic acids can lose \(H^+\) from the \(O-H\) group more readily than alcohols.
Reason: The carboxylate ion formed is resonance-stabilised, whereas an alkoxide ion has a more localised negative charge.
ⓐ. Assertion is true, but Reason is false.
ⓑ. Assertion is false, but Reason is true.
ⓒ. Both are true, and Reason explains Assertion.
ⓓ. Both are true, but Reason does not explain Assertion.
Correct Answer: Both are true, and Reason explains Assertion.
Explanation: Carboxylic acids release \(H^+\) more readily than alcohols because their conjugate bases are more stable. The carboxylate ion spreads its negative charge over two oxygen atoms through resonance. An alkoxide ion usually keeps the negative charge mainly on one oxygen atom. Greater stabilisation of \(R-COO^-\) makes proton loss from \(R-COOH\) more favourable.
205. Which option gives the best electronic reason for the acidity of \(R-COOH\)?
ⓐ. \(R-COOH\) forms resonance-stabilised \(R-COO^-\).
ⓑ. \(R-COOH\) forms \(R-CO^+\), which is resonance-free.
ⓒ. \(R-COOH\) forms \(R-O-R'\), which is strongly basic.
ⓓ. \(R-COOH\) forms \(R-CHO\), which is fully ionic.
Correct Answer: \(R-COOH\) forms resonance-stabilised \(R-COO^-\).
Explanation: Acid strength is closely related to the stability of the conjugate base. A carboxylic acid loses \(H^+\) to form \(R-COO^-\). This carboxylate ion is stabilised by resonance over two oxygen atoms. The stable conjugate base makes carboxylic acids noticeably more acidic than alcohols and many phenols.
206. Which statement is incorrect about electronic effects in carboxylic acids?
ⓐ. The carbonyl group withdraws electron density.
ⓑ. The \(O-H\) bond becomes easier to polarise.
ⓒ. The carboxylate ion gains resonance stabilisation.
ⓓ. The negative charge remains fixed on hydrogen.
Correct Answer: The negative charge remains fixed on hydrogen.
Explanation: When a carboxylic acid loses \(H^+\), hydrogen leaves as a proton and does not keep the negative charge. The negative charge remains on the carboxylate ion, \(R-COO^-\). Resonance delocalises this charge over two oxygen atoms. Electron withdrawal by the carbonyl group and conjugate-base stabilisation together explain the acidity of carboxylic acids.
207. Which statement best describes the physical nature of many lower aldehydes and ketones?
ⓐ. They are generally ionic solids with no smell.
ⓑ. They are often volatile liquids or gases with odour.
ⓒ. They are always waxy solids with very high melting points.
ⓓ. They are non-volatile salts that dissolve only in acids.
Correct Answer: They are often volatile liquids or gases with odour.
Explanation: Lower aldehydes and ketones have relatively small molecular sizes, so many of them are gases or volatile liquids. Their volatility is higher than that of compounds with extensive hydrogen bonding, such as alcohols and carboxylic acids. Many simple aldehydes and ketones also have characteristic odours. This physical recognition is useful, but classification should still be based on the functional group.
208. Which physical trend is generally observed as the molecular mass of aldehydes and ketones increases?
ⓐ. Boiling point decreases steadily.
ⓑ. Volatility increases without limit.
ⓒ. Odour disappears because \(C=O\) is lost.
ⓓ. Boiling point increases.
Correct Answer: Boiling point increases.
Explanation: As molecular mass increases, the size of the molecule and the surface area generally increase. This strengthens London dispersion forces between molecules. Stronger intermolecular attraction requires more energy to separate molecules into the vapour phase. Therefore, higher members of aldehydes and ketones generally have higher boiling points.
209. Which compound is expected to be more volatile among the following, mainly due to smaller molecular size?
ⓐ. Methanal
ⓑ. Hexanal
ⓒ. Heptanone
ⓓ. Decanone
Correct Answer: Methanal
Explanation: Methanal is the smallest aldehyde in the list. Smaller molecules generally have weaker dispersion forces than larger molecules of the same functional class. Weaker intermolecular forces allow molecules to escape more easily into the vapour phase. Therefore methanal is expected to be more volatile than higher aldehydes or ketones.
210. Which statement about the odour of simple aldehydes and ketones is most suitable?
ⓐ. All members are completely odourless.
ⓑ. Only carboxylic acids can show any smell.
ⓒ. Odour proves that a compound must be ionic.
ⓓ. Many lower members have distinct odours.
Correct Answer: Many lower members have distinct odours.
Explanation: Many lower aldehydes and ketones are known for characteristic odours. Odour alone, however, is not a reliable structural test because many organic compounds can smell. Functional-group identity must be decided from structural features such as \(CHO\) or \(CO\). Still, odour is often mentioned as a recognition-level physical property for lower members.
211. Which statement best describes lower carboxylic acids such as methanoic acid and ethanoic acid?
ⓐ. They are odourless crystalline salts.
ⓑ. They are pungent liquids.
ⓒ. They are neutral gases with no \(O-H\) bond.
ⓓ. They are hydrocarbons with fruity odours.
Correct Answer: They are pungent liquids.
Explanation: Lower carboxylic acids are commonly liquids with sharp or pungent smells. Their \(COOH\) group allows strong intermolecular association through hydrogen bonding. This makes their physical behaviour different from aldehydes and ketones of comparable size. The sharp smell of lower acids is a useful recognition feature, though the \(COOH\) group is the actual structural identifier.
212. Which description best fits higher members of carboxylic acids?
ⓐ. Waxy solids
ⓑ. Noble gases
ⓒ. Ionic solutions
ⓓ. Volatile ethers
Correct Answer: Waxy solids
Explanation: Higher carboxylic acids have long hydrocarbon chains attached to the \(COOH\) group. The larger non-polar part increases dispersion forces and reduces volatility. As a result, higher members are often waxy solids. Their physical state differs from lower carboxylic acids, which are commonly pungent liquids.
213. Which functional group is mainly responsible for strong intermolecular association in carboxylic acids?
ⓐ. \(R-O-R'\)
ⓑ. \(R-CO-R'\)
ⓒ. \(R-CHO\)
ⓓ. \(R-COOH\)
Correct Answer: \(R-COOH\)
Explanation: The \(COOH\) group contains both a carbonyl oxygen and a hydroxyl hydrogen. These allow carboxylic acid molecules to form strong intermolecular hydrogen bonds. Two acid molecules can associate as a dimer through two hydrogen bonds. This strong association explains their unusually high boiling points compared with many compounds of similar molecular mass.
214. Why do aldehydes and ketones have higher boiling points than hydrocarbons of comparable molecular mass?
ⓐ. They form ionic lattices in the liquid state.
ⓑ. They contain no polar bonds at all.
ⓒ. Their \(C=O\) group gives dipole attractions.
ⓓ. Their molecules always form carboxylate ions.
Correct Answer: Their \(C=O\) group gives dipole attractions.
Explanation: Aldehydes and ketones contain the polar \(C=O\) bond. Oxygen pulls electron density away from carbon, creating a molecular dipole. These dipoles attract one another in the liquid state, giving dipole-dipole interactions. Hydrocarbons of comparable mass lack such strong permanent dipoles, so their boiling points are generally lower.
215. Why do aldehydes and ketones usually boil at lower temperatures than alcohols of similar molecular mass?
ⓐ. They lack strong self-hydrogen bonding.
ⓑ. They have no intermolecular forces of any kind.
ⓒ. They contain only carbon and hydrogen atoms.
ⓓ. They always exist as gases at room temperature.
Correct Answer: They lack strong self-hydrogen bonding.
Explanation: Aldehydes and ketones have a polar \(C=O\) group, so they can show dipole-dipole attractions. However, they do not have an \(O-H\) bond, so they cannot form strong intermolecular hydrogen bonds with their own molecules in the same way alcohols do. Alcohols can form extended hydrogen-bonded networks. This stronger association usually gives alcohols higher boiling points than comparable aldehydes and ketones.
216. Which comparison is generally correct for compounds of comparable molecular mass?
ⓐ. Hydrocarbons \(>\) aldehydes \(>\) alcohols in boiling point
ⓑ. Ethers \(>\) carboxylic acids \(>\) alcohols in boiling point
ⓒ. Aldehydes \(>\) carboxylic acids \(>\) hydrocarbons in boiling point
ⓓ. Carboxylic acids \(>\) alcohols \(>\) aldehydes in boiling point
Correct Answer: Carboxylic acids \(>\) alcohols \(>\) aldehydes in boiling point
Explanation: Carboxylic acids form strong hydrogen-bonded dimers, so their boiling points are very high for their molecular mass. Alcohols form intermolecular hydrogen bonds, but usually not the same stable dimer association seen in carboxylic acids. Aldehydes have dipole-dipole attractions due to \(C=O\), but they lack self-hydrogen bonding through \(O-H\). Hence the general order is carboxylic acids \(>\) alcohols \(>\) aldehydes.
217. Which statement best explains why carboxylic acids often have unusually high boiling points?
ⓐ. They contain only non-polar \(C-H\) bonds.
ⓑ. They cannot attract neighbouring molecules.
ⓒ. They form hydrogen-bonded dimers.
ⓓ. They break into free radicals before boiling.
Correct Answer: They form hydrogen-bonded dimers.
Explanation: Carboxylic acid molecules can pair through two intermolecular hydrogen bonds. This produces a dimer-like association in which two molecules are held together strongly. Extra energy is required to separate these associated molecules before boiling. Therefore carboxylic acids often have much higher boiling points than aldehydes, ketones, ethers, or even many alcohols of similar molecular mass.
218. Which structural feature prevents aldehydes and ketones from forming strong hydrogen-bonded dimers with themselves?
ⓐ. Presence of a polar \(C=O\) bond
ⓑ. Absence of an \(O-H\) hydrogen
ⓒ. Presence of carbon atoms in the chain
ⓓ. Presence of lone pairs on oxygen
Correct Answer: Absence of an \(O-H\) hydrogen
Explanation: A hydrogen bond donor usually requires hydrogen attached to a highly electronegative atom such as oxygen. Aldehydes and ketones have carbonyl oxygen, but they do not contain an \(O-H\) bond. Their oxygen can accept hydrogen bonds from other substances such as water, but they cannot donate \(O-H\)-type hydrogen bonds to one another. This is why their self-association is weaker than that of alcohols and carboxylic acids.
219. Which compound is expected to show the strongest intermolecular association among the following?
ⓐ. \(CH_3COOH\)
ⓑ. \(CH_3CHO\)
ⓒ. \(CH_3COCH_3\)
ⓓ. \(CH_3OCH_3\)
Correct Answer: \(CH_3COOH\)
Explanation: \(CH_3COOH\) is a carboxylic acid and contains the \(COOH\) group. It can form strong intermolecular hydrogen bonds and commonly associates as dimers. \(CH_3CHO\) and \(CH_3COCH_3\) have polar \(C=O\) groups but lack \(O-H\) bonds for self-hydrogen bonding. \(CH_3OCH_3\) is an ether and has weaker intermolecular attraction than a carboxylic acid.
220. Which statement correctly compares the boiling point behaviour of \(CH_3CHO\) and \(CH_3CH_2CH_3\)?
ⓐ. \(CH_3CH_2CH_3\) boils higher because it has no oxygen.
ⓑ. Both boil at the same temperature because both have three carbons.
ⓒ. \(CH_3CHO\) boils lower because \(C=O\) removes all attractions.
ⓓ. \(CH_3CHO\) boils higher because it has a polar \(C=O\) group.
Correct Answer: \(CH_3CHO\) boils higher because it has a polar \(C=O\) group.
Explanation: \(CH_3CHO\) contains the polar carbonyl group \(C=O\). This produces dipole-dipole attraction between ethanal molecules. \(CH_3CH_2CH_3\) is a hydrocarbon and mainly has weak dispersion forces. Therefore ethanal generally has a higher boiling point than propane.
