201. A student argues that aldehydes and ketones should boil as high as alcohols because all three classes contain oxygen. The most suitable correction is:
ⓐ. Carbonyl oxygen cannot accept hydrogen bonds
ⓑ. Alcohol molecules can both donate and accept hydrogen bonds
ⓒ. Aldehydes and ketones are completely non-polar
ⓓ. Alcohols always have much lower molar masses than carbonyl compounds
Correct Answer: Alcohol molecules can both donate and accept hydrogen bonds
Explanation: The presence of oxygen alone does not guarantee the same intermolecular behaviour. Alcohols contain an \(\mathrm{O-H}\) bond and can both donate and accept hydrogen bonds. Aldehydes and ketones possess oxygen lone pairs but normally cannot donate hydrogen bonds. Their molecules interact mainly through dipole-dipole and dispersion forces. The stronger hydrogen-bond network in alcohols generally produces higher boiling points.
202. Match each property in Column I with its most suitable explanation in Column II.
| Column I | Column II |
|---|
| P. High water solubility of lower carbonyl compounds | 1. Growing non-polar hydrocarbon portion |
| Q. Decreasing water solubility with chain length | 2. Carbonyl oxygen accepts hydrogen bonds from water |
| R. Inability to donate hydrogen bonds | 3. Absence of an oxygen-hydrogen bond |
| S. Solubility in many organic solvents | 4. Compatibility of both polar and non-polar molecular regions with suitable solvents |
ⓐ. P-1, Q-2, R-3, S-4
ⓑ. P-2, Q-1, R-3, S-4
ⓒ. P-2, Q-3, R-1, S-4
ⓓ. P-4, Q-1, R-2, S-3
Correct Answer: P-2, Q-1, R-3, S-4
Explanation: The carbonyl oxygen accepts hydrogen bonds from water, explaining the good solubility of lower members. Increasing chain length enlarges the hydrophobic portion and reduces water solubility. Aldehydes and ketones cannot normally donate hydrogen bonds because they lack an \(\mathrm{O-H}\) bond. Their polar carbonyl group and hydrocarbon skeleton allow useful compatibility with many organic solvents. The matching separates hydrogen-bonding ability from overall molecular polarity.
203. A carbonyl compound is required as a solvent that is completely miscible with water and also dissolves many organic substances. The most suitable choice is:
ⓐ. Cyclohexanone
ⓑ. Acetophenone
ⓒ. Benzophenone
ⓓ. Propanone
Correct Answer: Propanone
Explanation: Propanone contains a strongly polar carbonyl group that accepts hydrogen bonds from water molecules. Its two methyl groups form a comparatively small non-polar portion, so they do not prevent effective interaction with water. Propanone is therefore completely miscible with water and can also dissolve many organic substances. Cyclohexanone and acetophenone contain larger hydrophobic carbon frameworks and have limited water solubility. Benzophenone contains two phenyl rings, making its hydrophobic character still greater. The balance between a polar carbonyl group and a small hydrocarbon portion makes propanone the most suitable solvent among the choices.
204. The following qualitative data are obtained for four carbonyl compounds.
| Compound | Relative carbon-chain size | Water behaviour |
|---|
| P | Small | Completely miscible |
| Q | Moderate | Partly soluble |
| R | Large | Sparingly soluble |
| S | Very large | Highly soluble |
Which entry is least consistent with the normal homologous-series trend?
ⓐ. Compound P
ⓑ. Compound S
ⓒ. Compound Q
ⓓ. Compound R
Correct Answer: Compound S
Explanation: Lower aldehydes and ketones can be highly soluble or miscible with water because their carbonyl oxygen accepts hydrogen bonds. As the carbon chain grows, the hydrophobic part of the molecule becomes increasingly important. Moderate members are therefore expected to be less soluble, and large members may be only sparingly soluble. A very large carbonyl compound described as highly water-soluble contradicts this normal trend unless an additional hydrophilic group is present. No such additional group is indicated for Compound S.
205. Two ethanoic acid molecules form a cyclic dimer. The total number of intermolecular hydrogen bonds in the dimer is:
ⓐ. One
ⓑ. Two
ⓒ. Three
ⓓ. Four
Correct Answer: Two
Explanation: Each ethanoic acid molecule contains one hydroxyl hydrogen that can be donated. It also contains a carbonyl oxygen that can accept a hydrogen bond. In the cyclic arrangement, the first molecule donates to the second, and the second donates back to the first. Two intermolecular hydrogen bonds are therefore formed. These two interactions help stabilise the ring-like dimer.
206. Consider the following statements about carboxylic acids.
Statement I: Lower members are often liquids with pungent odours.
Statement II: Higher members may be solids because intermolecular attractions increase with molecular size.
Statement III: Carboxylic acids cannot form intermolecular hydrogen bonds.
ⓐ. Statements I and II only
ⓑ. Statements II and III only
ⓒ. Statements I and III only
ⓓ. Statements I, II, and III
Correct Answer: Statements I and II only
Explanation: Lower carboxylic acids are commonly pungent liquids. Increasing chain length strengthens dispersion forces and raises melting and boiling tendencies. Higher homologues may consequently occur as solids. Statement III is false because carboxylic acids form particularly strong hydrogen-bonded associations. Their ability to form cyclic dimers is a major physical-property feature.
207. Use the arrangement described below: two carboxylic acid molecules are positioned head-to-head so that the hydroxyl hydrogen of each molecule points toward the carbonyl oxygen of the other. This arrangement represents:
ⓐ. A covalent polymer
ⓑ. An ionic salt
ⓒ. A cyclic hydrogen-bonded dimer
ⓓ. A nucleophilic substitution intermediate
Correct Answer: A cyclic hydrogen-bonded dimer
Explanation: Each acid molecule acts simultaneously as a hydrogen-bond donor and acceptor. The two reciprocal hydrogen bonds close the arrangement into a ring. The molecules remain separate covalent units and are not joined by new carbon-carbon bonds. No complete proton transfer is required. This dimeric association is especially important in the liquid phase, vapour phase, and suitable non-polar solvents.
208. For compounds of broadly comparable molar mass, the most reasonable increasing order of boiling point is:
ⓐ. Hydrocarbon \(\lt\) carbonyl compound \(\lt\) alcohol \(\lt\) carboxylic acid
ⓑ. Carboxylic acid \(\lt\) alcohol \(\lt\) carbonyl compound \(\lt\) hydrocarbon
ⓒ. Alcohol \(\lt\) hydrocarbon \(\lt\) carboxylic acid \(\lt\) carbonyl compound
ⓓ. Carbonyl compound \(\lt\) carboxylic acid \(\lt\) hydrocarbon \(\lt\) alcohol
Correct Answer: Hydrocarbon \(\lt\) carbonyl compound \(\lt\) alcohol \(\lt\) carboxylic acid
Explanation: Hydrocarbons rely mainly on dispersion forces. Aldehydes and ketones additionally possess dipole-dipole attractions. Alcohols form intermolecular hydrogen bonds and usually boil at still higher temperatures. Carboxylic acids can form strongly associated cyclic dimers containing two hydrogen bonds. Their intermolecular association commonly gives them the highest boiling points in this comparison.
209. The row that correctly relates a carboxylic acid property to its explanation is:
| Row | Property | Explanation |
|---|
| P | High boiling point | Complete ionic dissociation in the pure liquid |
| Q | Decreasing water solubility with chain length | The carboxyl group becomes more polar |
| R | Dimer formation | Each molecule can donate and accept hydrogen bonds |
| S | Higher members becoming solids | Dispersion forces disappear as molecular size increases |
ⓐ. Row P
ⓑ. Row Q
ⓒ. Row S
ⓓ. Row R
Correct Answer: Row R
Explanation: A carboxylic acid molecule contains an oxygen-hydrogen donor and carbonyl-oxygen acceptor. Two molecules can therefore form a cyclic dimer through reciprocal hydrogen bonds. Row P incorrectly attributes the high boiling point to complete ionic dissociation. Row Q reverses the reason for decreasing solubility, while Row S incorrectly states that dispersion forces disappear. Row R alone gives a valid property-explanation pair.
210. A chemist compares ethanoic acid and hexanoic acid. Ethanoic acid mixes readily with water, whereas hexanoic acid has much lower water solubility. Both compounds contain one carboxyl group. Which conclusion best explains the observation?
ⓐ. Hexanoic acid cannot form hydrogen bonds with water
ⓑ. Ethanoic acid is ionic whereas hexanoic acid is covalent
ⓒ. Its larger hydrocarbon chain opposes hydration more strongly
ⓓ. Hexanoic acid contains fewer carbon atoms than ethanoic acid
Correct Answer: Its larger hydrocarbon chain opposes hydration more strongly
Explanation: Both acids can form hydrogen bonds with water through their carboxyl groups. Ethanoic acid has only a small methyl group, so its polar carboxyl group dominates its solution behaviour. Hexanoic acid contains a much larger non-polar hydrocarbon chain. Water must organise around this hydrophobic region, making dissolution less favourable. The difference arises from the relative size of the hydrocarbon portion, not from loss of hydrogen-bonding ability.
211. Assertion: Nucleophilic addition to a carbonyl group involves movement of the carbon-oxygen pi electrons toward oxygen.
Reason: Carbon cannot normally retain the original pi bond while also forming an additional bond to the attacking nucleophile without exceeding its usual valency.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Both Assertion and Reason are true, and Reason explains Assertion
ⓒ. Assertion is true, but Reason is false
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: The nucleophile donates an electron pair to the carbonyl carbon and forms a new carbon-nucleophile bond. To maintain the normal valency of carbon, the carbon-oxygen pi bond must be broken during this step. Its electron pair moves onto oxygen, producing an alkoxide-type intermediate. The carbon-oxygen sigma bond remains intact. The Reason therefore explains the electron movement described in the Assertion.
212. The general order of reactivity toward nucleophilic addition is:
ⓐ. Ketones \(\gt\) aldehydes \(\gt\) methanal
ⓑ. Aldehydes \(\gt\) methanal \(\gt\) ketones
ⓒ. Ketones \(\gt\) methanal \(\gt\) aldehydes
ⓓ. Methanal \(\gt\) aldehydes \(\gt\) ketones
Correct Answer: Methanal \(\gt\) aldehydes \(\gt\) ketones
Explanation: Methanal has no alkyl group attached to its carbonyl carbon. It therefore experiences minimal steric crowding and minimal electron donation from alkyl groups. Other aldehydes contain one alkyl or aryl group and one hydrogen. Ketones contain two carbon groups, which generally increase both crowding and electron donation. These steric and electronic effects make methanal the most reactive and ketones generally the least reactive.
213. Which sequence correctly describes the central events in a typical nucleophilic addition to a neutral carbonyl compound followed by work-up?
ⓐ. Nucleophilic attack \(\rightarrow\) pi-electron shift to oxygen \(\rightarrow\) alkoxide \(\rightarrow\) protonation
ⓑ. Alpha-proton removal \(\rightarrow\) enolate formation \(\rightarrow\) ring closure \(\rightarrow\) carbonyl cleavage
ⓒ. Attack at oxygen \(\rightarrow\) carbonyl-carbon loss \(\rightarrow\) carbocation formation \(\rightarrow\) deprotonation
ⓓ. Carbon-carbon cleavage \(\rightarrow\) nucleophile protonation \(\rightarrow\) alkene formation \(\rightarrow\) carbonyl regeneration
Correct Answer: Nucleophilic attack \(\rightarrow\) pi-electron shift to oxygen \(\rightarrow\) alkoxide \(\rightarrow\) protonation
Explanation: The electron-rich nucleophile approaches the electron-deficient carbonyl carbon. Its attack creates a new sigma bond while the carbon-oxygen pi electrons shift toward oxygen. This gives a tetrahedral alkoxide-type intermediate. Protonation during work-up converts the negatively charged oxygen into a neutral hydroxyl group. The sequence preserves the carbon skeleton and represents the characteristic pattern of nucleophilic addition.
214. Assertion: Methanal is generally more reactive than ethanal toward nucleophilic addition.
Reason: Methanal lacks an electron-donating alkyl group attached to the carbonyl carbon.
ⓐ. Both Assertion and Reason are true, and Reason explains Assertion
ⓑ. Both Assertion and Reason 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 Assertion and Reason are true, and Reason explains Assertion
Explanation: Methanal has two hydrogen atoms attached to its carbonyl carbon. Ethanal has one hydrogen and one methyl group. The methyl group releases electron density and lowers the electrophilicity of the ethanal carbonyl carbon. Methanal also has less steric crowding around the reaction centre. The absence of an alkyl group therefore contributes directly to its greater reactivity.
215. A graph has nucleophilic-addition reactivity on the vertical axis and increasing electron donation by carbonyl substituents on the horizontal axis. The expected overall trend is:
ⓐ. A steep upward trend
ⓑ. A horizontal line
ⓒ. A downward trend
ⓓ. An alternating rise and fall
Correct Answer: A downward trend
Explanation: Greater electron donation increases electron density around the carbonyl carbon. Its partial positive character then becomes smaller. Nucleophiles are less strongly attracted to the reaction centre. The rate or tendency of nucleophilic addition therefore generally decreases. The graph should show an overall downward trend as substituent donation increases.
216. Benzaldehyde is generally less reactive than an aliphatic aldehyde such as ethanal toward nucleophilic addition mainly because:
ⓐ. Benzaldehyde has no oxygen atom in its carbonyl group
ⓑ. The aromatic ring converts the aldehyde into an ionic species
ⓒ. Ethanal has two electron-withdrawing alkyl groups
ⓓ. Ring conjugation lowers carbonyl-carbon electrophilicity
Correct Answer: Ring conjugation lowers carbonyl-carbon electrophilicity
Explanation: In benzaldehyde, the carbonyl group is conjugated with the aromatic ring. Electron delocalisation involving the ring reduces the local positive character of the carbonyl carbon. The phenyl group is also larger than the hydrogen or small alkyl groups present in simple aliphatic aldehydes. Nucleophilic attack is therefore electronically and sterically less favourable. Benzaldehyde still undergoes addition reactions, but generally less readily than comparable aliphatic aldehydes.
217. The expected decreasing order of nucleophilic-addition reactivity is:
ⓐ. \(4\)-Methoxybenzaldehyde \(\gt\) benzaldehyde \(\gt\) \(4\)-nitrobenzaldehyde
ⓑ. Benzaldehyde \(\gt\) \(4\)-methoxybenzaldehyde \(\gt\) \(4\)-nitrobenzaldehyde
ⓒ. \(4\)-Nitrobenzaldehyde \(\gt\) benzaldehyde \(\gt\) \(4\)-methoxybenzaldehyde
ⓓ. Benzaldehyde \(\gt\) \(4\)-nitrobenzaldehyde \(\gt\) \(4\)-methoxybenzaldehyde
Correct Answer: \(4\)-Nitrobenzaldehyde \(\gt\) benzaldehyde \(\gt\) \(4\)-methoxybenzaldehyde
Explanation: The nitro group withdraws electron density and increases carbonyl electrophilicity. Unsubstituted benzaldehyde provides the intermediate reference case. The methoxy group donates electron density through resonance into the aromatic system. This donation reduces the electron deficiency of the carbonyl carbon. The resulting reactivity order is nitro-substituted, unsubstituted, and then methoxy-substituted benzaldehyde.
218. Use the arrangements described below.
Structure P has a carbonyl carbon bonded to two hydrogen atoms.
Structure Q has a carbonyl carbon bonded to one hydrogen and one methyl group.
Structure R has a carbonyl carbon bonded to two methyl groups.
Which order represents increasing steric hindrance to nucleophile approach?
ⓐ. \(\mathrm{P\lt Q\lt R}\)
ⓑ. \(\mathrm{R\lt Q\lt P}\)
ⓒ. \(\mathrm{Q\lt P\lt R}\)
ⓓ. \(\mathrm{P\lt R\lt Q}\)
Correct Answer: \(\mathrm{P\lt Q\lt R}\)
Explanation: Structure P represents the environment in methanal and has only hydrogen atoms around the carbonyl carbon. Structure Q represents a simple aldehyde with one methyl group and one hydrogen. Structure R represents propanone with two methyl groups. Methyl groups occupy more space than hydrogen atoms. Steric hindrance therefore increases from P to Q to R.
219. Four carbonyl compounds are compared under the same nucleophilic-addition conditions.
Compound P is methanal.
Compound Q is ethanal.
Compound R is propanone.
Compound S is benzophenone.
Which decreasing reactivity order is most reasonable?
ⓐ. \(\mathrm{S\gt R\gt Q\gt P}\)
ⓑ. \(\mathrm{Q\gt P\gt S\gt R}\)
ⓒ. \(\mathrm{R\gt Q\gt P\gt S}\)
ⓓ. \(\mathrm{P\gt Q\gt R\gt S}\)
Correct Answer: \(\mathrm{P\gt Q\gt R\gt S}\)
Explanation: Methanal has no carbon groups and offers the least steric and electronic resistance to attack. Ethanal has one methyl group, so it is less reactive than methanal. Propanone has two methyl groups that donate electron density and increase crowding. Benzophenone contains two bulky phenyl groups and has substantial conjugative stabilisation of the carbonyl system. The combined electronic and steric effects give the order \(\mathrm{P\gt Q\gt R\gt S}\).
220. During acid-catalysed addition of a weak neutral nucleophile to an aldehyde, the step that directly activates the carbonyl group is:
ⓐ. Deprotonation of carbonyl oxygen to increase carbon electrophilicity
ⓑ. Complete cleavage of the carbon-oxygen bond before nucleophile attack
ⓒ. Protonation of carbonyl oxygen to increase carbon electrophilicity
ⓓ. Protonation of the nucleophile to increase its electron density
Correct Answer: Protonation of carbonyl oxygen to increase carbon electrophilicity
Explanation: Protonation occurs at the electron-rich carbonyl oxygen. This produces a protonated carbonyl species in which the carbonyl carbon bears greater electron deficiency than in the unprotonated molecule. A weak neutral nucleophile can therefore attack the carbonyl carbon more readily. The carbon-oxygen bond is not completely cleaved during this activation step; its \(\pi\)-electron density shifts toward oxygen as the new bond forms. Protonating the nucleophile would generally reduce, rather than increase, its electron-pair availability. The acid is regenerated after the subsequent proton-transfer steps, so it functions as a catalyst.
