201. For comparable methyl-substituted amines in the gas phase, the broad basicity order is:
ⓐ. trimethylamine \(>\) dimethylamine \(>\) methylamine \(>\) ammonia
ⓑ. ammonia \(>\) methylamine \(>\) dimethylamine \(>\) trimethylamine
ⓒ. methylamine \(>\) ammonia \(>\) trimethylamine \(>\) dimethylamine
ⓓ. dimethylamine \(>\) trimethylamine \(>\) ammonia \(>\) methylamine
Correct Answer: trimethylamine \(>\) dimethylamine \(>\) methylamine \(>\) ammonia
Explanation: Alkyl groups generally release electron density toward nitrogen through the positive inductive effect. Increasing the number of methyl groups therefore raises the electron density associated with the nitrogen lone pair. In the gas phase, there is no aqueous hydration competition to offset this electronic effect. The proton affinity consequently rises broadly from ammonia to primary, secondary, and tertiary methylamines. The order should not be transferred automatically to aqueous solution because solvation changes the balance.
202. Assertion: The gas-phase basicity order of methylamines should not be used automatically as their aqueous basicity order.
Reason: In water, solvation and steric accessibility of the conjugate acids compete with the electron-releasing effect of alkyl groups.
ⓐ. 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
ⓓ. Both Assertion and Reason are true, and Reason explains Assertion
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Gas-phase basicity largely reflects the intrinsic tendency of the nitrogen centre to bind a proton. Additional alkyl groups generally enhance this tendency through electron release. In water, the protonated product must also be stabilised by hydration. Increasing substitution can make the conjugate acid bulkier and less accessible to solvent molecules. Steric effects may also hinder proton approach. These competing aqueous factors explain why the gas-phase order cannot be transferred without modification.
203. The standard aqueous basicity order for methyl-substituted amines is:
ⓐ. dimethylamine \(>\) methylamine \(>\) trimethylamine \(>\) ammonia
ⓑ. methylamine \(>\) dimethylamine \(>\) ammonia \(>\) trimethylamine
ⓒ. trimethylamine \(>\) dimethylamine \(>\) methylamine \(>\) ammonia
ⓓ. ammonia \(>\) trimethylamine \(>\) methylamine \(>\) dimethylamine
Correct Answer: dimethylamine \(>\) methylamine \(>\) trimethylamine \(>\) ammonia
Explanation: Alkyl groups release electron density toward nitrogen and generally favour proton acceptance. If this were the only effect, increasing methyl substitution would continuously increase basicity. In water, however, the protonated amines must also be stabilised by hydration. The dimethylammonium ion combines strong electron donation with reasonably effective solvation. Trimethylammonium ion is more crowded and less effectively hydrated, so trimethylamine falls below methylamine in the standard aqueous order.
204. The conjugate acid that is generally most strongly hydrated among the methylammonium series is:
ⓐ. \(\mathrm{(CH_3)_3NH^+}\)
ⓑ. \(\mathrm{(CH_3)_2NH_2^+}\)
ⓒ. \(\mathrm{CH_3NH_3^+}\)
ⓓ. \(\mathrm{NH_4^+}\)
Correct Answer: \(\mathrm{NH_4^+}\)
Explanation: Ammonium ion is the smallest and least sterically shielded member of the series. Its positive charge is readily accessible to surrounding water molecules. It also contains four nitrogen-bonded hydrogens capable of participating in strong hydration interactions. Progressive replacement of hydrogens by methyl groups reduces accessibility and the number of such hydrogen-bonding sites. Strong hydration of \(\mathrm{NH_4^+}\) helps stabilise it, although ammonia remains the weakest base because it lacks alkyl electron donation.
205. Use the following passage.
A student argues that trimethylamine must be the strongest methylamine in water because it has three electron-releasing methyl groups. Another student points out that aqueous basicity depends on the stability of both the neutral base and its protonated form.
Which observation most directly supports the second student?
ⓐ. Trimethylamine has the greatest molar mass of the series
ⓑ. Trimethylammonium is less hydrated than dimethylammonium
ⓒ. Dimethylamine has a lower boiling point than water
ⓓ. Ammonia contains no carbon atoms
Correct Answer: Trimethylammonium is less hydrated than dimethylammonium
Explanation: The first argument considers only the electron-releasing effect of methyl groups. Aqueous equilibrium also depends on stabilisation of the conjugate acid. Trimethylammonium ion is bulkier and its charged nitrogen is more shielded from water. Dimethylammonium ion is less crowded and can be hydrated more effectively. This solvation difference explains why increasing substitution does not produce a simple monotonic rise in aqueous basicity.
206. A graph plots aqueous basicity on the vertical axis and the number of methyl groups attached to nitrogen on the horizontal axis, beginning with ammonia. Which shape best represents the standard trend?
ⓐ. A steady increase from ammonia to trimethylamine
ⓑ. A steady decrease from ammonia to trimethylamine
ⓒ. A rise to dimethylamine followed by a decline
ⓓ. A horizontal line because all species contain nitrogen
Correct Answer: A rise to dimethylamine followed by a decline
Explanation: Ammonia begins as the weakest member because it lacks alkyl electron donation. Addition of one methyl group increases basicity, and addition of a second gives a further increase. The dimethylamine point is therefore the maximum in the standard aqueous order. Addition of the third methyl group increases steric crowding and reduces effective hydration of the protonated form. The curve consequently falls at trimethylamine rather than continuing upward.
207. Match each factor in Column I with its principal effect in Column II.
| Column I | Column II |
|---|
| P. Positive inductive effect | 1. Can reduce hydration of a bulky conjugate acid |
| Q. Hydration | 2. Raises electron density at nitrogen |
| R. Steric crowding | 3. Stabilises the protonated amine in water |
| S. Gas phase | 4. Removes solvent stabilisation from the comparison |
ⓐ. P-3, Q-2, R-4, S-1
ⓑ. P-2, Q-3, R-1, S-4
ⓒ. P-1, Q-4, R-3, S-2
ⓓ. P-4, Q-1, R-2, S-3
Correct Answer: P-2, Q-3, R-1, S-4
Explanation: The positive inductive effect pushes electron density toward nitrogen and matches item \(2\). Hydration stabilises the positively charged conjugate acid and matches item \(3\). Steric crowding can shield the charged centre and reduce hydration, corresponding to item \(1\). In the gas phase, solvent stabilisation is absent, matching item \(4\). The combined matching shows why gas-phase and aqueous basicity trends cannot be analysed using a single factor.
208. The aqueous basicity order of ethyl-substituted amines is commonly represented as:
ⓐ. triethylamine \(>\) diethylamine \(>\) ethylamine \(>\) ammonia
ⓑ. ammonia \(>\) ethylamine \(>\) diethylamine \(>\) triethylamine
ⓒ. diethylamine \(>\) triethylamine \(>\) ethylamine \(>\) ammonia
ⓓ. ethylamine \(>\) diethylamine \(>\) ammonia \(>\) triethylamine
Correct Answer: diethylamine \(>\) triethylamine \(>\) ethylamine \(>\) ammonia
Explanation: Ethyl groups release electron density toward nitrogen and favour proton acceptance. Diethylamine benefits from two electron-releasing groups while its conjugate acid can still be solvated reasonably well. Triethylamine has stronger alkyl donation but suffers from increased steric crowding and reduced hydration. In the commonly used aqueous data set, it nevertheless remains above ethylamine. The order differs from that of methylamines, demonstrating that one fixed primary–secondary–tertiary sequence cannot be applied universally.
209. Comparing the usual aqueous orders of methylamines and ethylamines shows that:
ⓐ. tertiary amines are always the weakest alkylamines
ⓑ. secondary amines are always weaker than primary amines
ⓒ. the tertiary-amine rank varies with alkyl-group size
ⓓ. alkyl-group identity has no influence on basicity
Correct Answer: the tertiary-amine rank varies with alkyl-group size
Explanation: Trimethylamine lies below methylamine in the standard methylamine order. Triethylamine commonly lies above ethylamine in the corresponding ethylamine order. The difference arises because methyl and ethyl groups do not produce identical balances of electron donation, steric crowding, and conjugate-acid hydration. A universal rule based only on amine degree would miss this variation. Basicity comparisons must therefore specify the actual compounds and the phase.
210. Assertion: One universal aqueous order such as secondary \(>\) primary \(>\) tertiary cannot safely be applied to every alkylamine set.
Reason: Changing the alkyl group changes both electron donation and solvation or steric effects.
ⓐ. 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: Aqueous basicity is determined by several competing contributions. Alkyl groups differ in their electron-releasing strength and in the steric bulk they create around nitrogen. Their protonated forms also differ in how effectively water can solvate the positive charge. Changing from methyl to ethyl substituents can therefore alter the relative position of a tertiary amine. The Reason directly explains why a single universal degree-based order is unreliable.
211. A bulky trialkylamine has high electron density at nitrogen but shows lower aqueous basicity than expected. The most suitable explanation is that:
ⓐ. the nitrogen atom cannot accept a proton
ⓑ. the alkyl groups withdraw electrons by resonance
ⓒ. steric crowding weakens protonation and hydration
ⓓ. the amine becomes an aromatic compound in water
Correct Answer: steric crowding weakens protonation and hydration
Explanation: Electron donation from alkyl groups can make the nitrogen lone pair intrinsically favourable for proton acceptance. Bulky groups, however, may obstruct the approaching proton or solvent molecules. After protonation, shielding around the positive nitrogen centre can reduce stabilisation by water. The observed aqueous basicity reflects the net balance of these opposing effects. High electron density alone is therefore insufficient for predicting the final equilibrium order.
212. Which structural change is most likely to lower aqueous basicity when its electron-releasing effect is small but its steric effect near nitrogen is large?
ⓐ. Introducing a bulky group close to the nitrogen atom
ⓑ. Replacing an alkyl group by a smaller alkyl group
ⓒ. Increasing accessibility of the protonated nitrogen to water
ⓓ. Strengthening hydration of the conjugate acid
Correct Answer: Introducing a bulky group close to the nitrogen atom
Explanation: A bulky group close to nitrogen can obstruct the approach of a proton to the lone pair. After protonation, it may also shield the positive nitrogen centre from water molecules. Reduced hydration makes the conjugate acid less stable in aqueous solution. If the substituent provides little additional electron donation, this steric and solvation penalty may dominate. Increasing accessibility or hydration would instead favour protonation and raise aqueous basicity.
213. A hypothetical graph compares two predicted aqueous-basicity trends as the number of alkyl groups on nitrogen increases. Curve P rises continuously because it considers only the positive inductive effect. Curve Q rises initially but then levels off or falls for highly substituted amines. Curve Q differs from P mainly because Q includes:
ⓐ. complete loss of the nitrogen lone pair in every tertiary amine
ⓑ. conversion of all amines into aromatic compounds
ⓒ. removal of alkyl groups during protonation
ⓓ. poorer solvation and greater steric crowding
Correct Answer: poorer solvation and greater steric crowding
Explanation: \( \textbf{Curve P:} \)
It treats alkyl groups only as electron-releasing substituents.
More alkyl groups then increase electron density at nitrogen.
This model predicts a continuous rise in proton-accepting tendency.
\( \textbf{Additional aqueous factor:} \)
Protonated amines must be stabilised by surrounding water molecules.
\( \textbf{Effect of crowding:} \)
Highly substituted ammonium ions have a more shielded positive centre.
Water may approach and hydrate them less effectively.
Bulky groups may also hinder proton approach to the neutral amine.
\( \textbf{Interpretation of Curve Q:} \)
Curve Q includes the competition between induction and solvation or steric effects.
The levelling or decline at high substitution therefore reflects the aqueous environment rather than disappearance of the lone pair.
214. Examine the qualitative data below.
| Amine | Relative electron donation | Hydration of conjugate acid | Observed aqueous basicity |
|---|
| P | Moderate | Strong | High |
| Q | High | Poor | Moderate |
| R | Low | Strong | Lowest possible in every case |
| S | High | Strong | Necessarily zero |
Which row gives a chemically reasonable balance of the stated factors?
ⓐ. R only
ⓑ. S only
ⓒ. P and Q only
ⓓ. Q and S only
Correct Answer: P and Q only
Explanation: Amine P can show high aqueous basicity because moderate electron donation and strong conjugate-acid hydration both favour protonation. Amine Q has strong electron donation but poor hydration, so a moderate net basicity is plausible. Row R incorrectly claims that low electron donation must always produce the lowest possible basicity regardless of all other factors. Row S is also impossible because strong electron donation and strong hydration would not force basicity to zero. Rows P and Q correctly illustrate that aqueous basicity is the result of competing contributions.
215. A bulky tertiary amine is found to be a stronger base in the gas phase than a less substituted amine, but a weaker base in water. Which explanation best reconciles the observations?
ⓐ. Water removes the alkyl groups from the tertiary amine
ⓑ. The tertiary amine becomes an amide only in water
ⓒ. Its nitrogen lone pair is available in the gas phase but becomes completely unavailable in water
ⓓ. Induction favours gas-phase basicity, whereas hydration controls aqueous basicity
Correct Answer: Induction favours gas-phase basicity, whereas hydration controls aqueous basicity
Explanation: In the gas phase, solvent stabilisation is absent and the electron-releasing effect of alkyl groups is prominent. The tertiary amine can therefore have a high intrinsic proton affinity. In water, protonation creates a bulky trialkylammonium ion. Steric shielding may prevent close and effective interaction with solvent molecules. The resulting loss of hydration stabilisation can outweigh the inductive advantage and reverse the observed order.
216. Stronger hydration of an ammonium ion can increase the aqueous basicity of its parent amine because hydration:
ⓐ. destabilises the protonated product and shifts equilibrium toward the neutral amine
ⓑ. stabilises the protonated product and shifts proton transfer toward ammonium-ion formation
ⓒ. removes the nitrogen lone pair from the neutral amine before proton transfer occurs
ⓓ. affects only the neutral amine and leaves the conjugate acid unstabilised
Correct Answer: stabilises the protonated product and shifts proton transfer toward ammonium-ion formation
Explanation: The parent amine behaves as a base by accepting a proton and forming an ammonium ion. Hydration lowers the energy of this charged product in aqueous solution. Greater product stabilisation favours the forward proton-transfer equilibrium. The corresponding \(K_b\) can therefore increase when the conjugate acid is more effectively solvated. This is why aqueous basicity depends on solvation as well as on electron density at nitrogen.
217. Which order correctly represents the basic strength of a typical lower alkylamine, ammonia, and aniline in water?
ⓐ. aniline \(>\) ammonia \(>\) lower alkylamine
ⓑ. lower alkylamine \(>\) ammonia \(>\) aniline
ⓒ. ammonia \(>\) aniline \(>\) lower alkylamine
ⓓ. aniline \(>\) lower alkylamine \(>\) ammonia
Correct Answer: lower alkylamine \(>\) ammonia \(>\) aniline
Explanation: A lower alkylamine receives electron density from its alkyl group through the positive inductive effect. Its nitrogen lone pair is therefore generally more available than that of ammonia. Ammonia has a localised lone pair but lacks alkyl electron donation. In aniline, the lone pair is delocalised into the aromatic ring and becomes less available for protonation. These factors give the broad order lower alkylamine \(>\) ammonia \(>\) aniline.
218. The conjugate acid of aniline does not receive the same lone-pair resonance stabilisation as neutral aniline because, after protonation:
ⓐ. the lone pair bonds to the added proton
ⓑ. the benzene ring is destroyed completely
ⓒ. nitrogen becomes negatively charged
ⓓ. the phenyl group is removed from nitrogen
Correct Answer: the lone pair bonds to the added proton
Explanation: Neutral aniline can donate its nitrogen lone pair into the aromatic ring. Protonation uses this lone pair to form a bond with \(\mathrm{H^+}\). The resulting anilinium ion has four sigma bonds around nitrogen and no corresponding free lone pair for donation into the ring. The resonance stabilisation present in the neutral base is therefore lost on protonation. This loss makes formation of the conjugate acid less favourable and contributes to the weak basicity of aniline.
219. Assertion: Delocalisation of the nitrogen lone pair lowers the basicity of aniline.
Reason: A lone pair involved in resonance is less available for bonding to a proton.
ⓐ. 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
ⓓ. Both Assertion and Reason are true, and Reason explains Assertion
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: Basicity depends on the ability of nitrogen to use its lone pair for proton acceptance. In aniline, the lone pair participates in conjugation with the benzene ring. This delocalisation stabilises the neutral molecule and reduces electron-pair localisation at nitrogen. Protonation would interrupt that donation because the pair becomes part of a nitrogen–hydrogen bond. The Reason therefore directly explains why aniline is a weaker base.
220. Cyclohexylamine is much more basic than aniline even though both compounds contain a six-membered carbon ring. The decisive difference is that:
ⓐ. cyclohexylamine has no nitrogen atom
ⓑ. cyclohexylamine lacks lone-pair conjugation with an aromatic ring
ⓒ. aniline cannot form salts with acids
ⓓ. cyclohexylamine gains stronger resonance donation from its saturated ring
Correct Answer: cyclohexylamine lacks lone-pair conjugation with an aromatic ring
Explanation: In aniline, nitrogen is attached directly to an aromatic carbon, allowing the lone pair to overlap with the benzene-ring system. This resonance interaction reduces the availability of the pair for protonation. Cyclohexylamine contains a saturated ring without an aromatic conjugated system. Its nitrogen lone pair remains comparatively localised, and the cyclohexyl group also exerts an electron-releasing inductive effect. The presence of a ring alone is therefore not responsible for weak basicity; direct aromatic conjugation is the crucial feature.
