201. A water-insoluble amine dissolves after addition of dilute hydrochloric acid. What is the best explanation?
ⓐ. The amine is converted into an alcohol.
ⓑ. The amine is oxidised to a nitro compound.
ⓒ. The amine forms an ionic ammonium salt.
ⓓ. The amine loses nitrogen as \(N_2\) gas.
Correct Answer: The amine forms an ionic ammonium salt.
Explanation: Amines can react with acids by protonation at nitrogen. The product is an ammonium salt, which is ionic in nature. Ionic salts generally interact more strongly with water than the neutral amine, so dissolution can occur. This behaviour is often used to recognize the basic character of amines.
202. Which reagent can liberate the free amine from an alkylammonium chloride salt?
ⓐ. A strong base such as \(NaOH\)
ⓑ. A neutral hydrocarbon such as hexane
ⓒ. A weak oxidant such as dilute \(Br_2\)
ⓓ. A dry salt such as \(NaCl\)
Correct Answer: A strong base such as \(NaOH\)
Explanation: An alkylammonium chloride salt contains a protonated amine ion such as \(RNH_3^+\). A strong base such as \(NaOH\) can remove a proton from \(RNH_3^+\). This regenerates the neutral amine, \(RNH_2\). Thus salt formation by amines is reversible under basic conditions.
203. Which equation represents liberation of a free primary amine from its ammonium salt?
ⓐ. \(RNH_3^+Cl^- + HCl \rightarrow RNH_2 + Cl_2\)
ⓑ. \(RNH_3^+Cl^- + H_2O \rightarrow RNO_2 + NH_4Cl\)
ⓒ. \(RNH_3^+Cl^- + NaCl \rightarrow RNH_2 + NaCl + HCl + H_2O\)
ⓓ. \(RNH_3^+Cl^- + NaOH \rightarrow RNH_2 + NaCl + H_2O\)
Correct Answer: \(RNH_3^+Cl^- + NaOH \rightarrow RNH_2 + NaCl + H_2O\)
Explanation: \(\textbf{Starting species:}\)
\(RNH_3^+Cl^-\) is a protonated primary amine salt.
\(\textbf{Role of }NaOH\textbf{:}\)
\(NaOH\) supplies \(OH^-\), which removes a proton from \(RNH_3^+\).
\(\textbf{Free amine formed:}\)
Loss of \(H^+\) from \(RNH_3^+\) gives \(RNH_2\).
\(\textbf{By-products:}\)
\(Na^+\) combines with \(Cl^-\), and \(H^+\) combines with \(OH^-\) to form \(H_2O\).
\(\textbf{Final Answer:}\)
\[\boxed{RNH_3^+Cl^- + NaOH \rightarrow RNH_2 + NaCl + H_2O}\]
204. Which statement best describes the reversibility of amine salt formation?
ⓐ. Amines form salts with acids, and the salts cannot react further.
ⓑ. Ammonium salts can be converted back to amines by strong bases.
ⓒ. Amines form salts only with \(NaOH\), not with acids.
ⓓ. Ammonium salts convert into nitriles on adding water.
Correct Answer: Ammonium salts can be converted back to amines by strong bases.
Explanation: Amines accept protons from acids to form ammonium salts. When a strong base is added, it removes the proton from the ammonium ion. This regenerates the neutral amine. Therefore, acid-base interconversion between amines and ammonium salts is a useful reversible process.
205. Which observation supports the basic nature of an amine?
ⓐ. It forms a neutral hydrocarbon with \(HCl\).
ⓑ. It forms a salt with dilute mineral acid.
ⓒ. It releases \(CO_2\) with carbonate only.
ⓓ. It turns directly into an ester in acid.
Correct Answer: It forms a salt with dilute mineral acid.
Explanation: A basic compound accepts a proton from an acid. Amines do this through the lone pair on nitrogen and form ammonium salts with mineral acids. For example, \(RNH_2\) forms \(RNH_3^+Cl^-\) with \(HCl\). Salt formation is therefore strong evidence that the amine has basic character.
206. Which pair correctly matches the amine with its protonated ion?
ⓐ. \((CH_3)_2NH\) — \((CH_3)_2N^-\)
ⓑ. \(CH_3NH_2\) — \(CH_3NH^-\)
ⓒ. \((CH_3)_3N\) — \((CH_3)_3N^-\)
ⓓ. \((CH_3)_3N\) — \((CH_3)_3NH^+\)
Correct Answer: \((CH_3)_3N\) — \((CH_3)_3NH^+\)
Explanation: A tertiary amine such as \((CH_3)_3N\) has a nitrogen lone pair and can accept one proton. After protonation, nitrogen becomes bonded to three methyl groups and one hydrogen, giving \((CH_3)_3NH^+\). Protonation adds \(H^+\); it does not create a negatively charged nitrogen species. The positive charge appears because nitrogen has formed an additional bond using its lone pair.
207. Which equation best represents protonation of dimethylamine?
ⓐ. \((CH_3)_2NH + H^+ \rightarrow CH_3CN + NH_4^+\)
ⓑ. \((CH_3)_2NH + H^+ \rightarrow (CH_3)_2N^-\)
ⓒ. \((CH_3)_2NH + H^+ \rightarrow CH_3NH_2 + CH_4\)
ⓓ. \((CH_3)_2NH + H^+ \rightarrow (CH_3)_2NH_2^+\)
Correct Answer: \((CH_3)_2NH + H^+ \rightarrow (CH_3)_2NH_2^+\)
Explanation: \(\textbf{Starting amine:}\)
Dimethylamine is \((CH_3)_2NH\).
\(\textbf{Available site:}\)
The nitrogen has a lone pair that can bond with \(H^+\).
\(\textbf{After protonation:}\)
Nitrogen gains one additional hydrogen and carries a positive charge.
\(\textbf{Product:}\)
The protonated ion is \((CH_3)_2NH_2^+\).
\(\textbf{Final Answer:}\)
\[\boxed{(CH_3)_2NH + H^+ \rightarrow (CH_3)_2NH_2^+}\]
208. Which statement correctly describes the charge change when a neutral amine is protonated?
ⓐ. The amine loses an electron and becomes negatively charged.
ⓑ. The amine gains \(Cl^-\) directly on carbon and remains neutral.
ⓒ. Nitrogen forms an extra bond and becomes positive.
ⓓ. Nitrogen loses all bonds and the compound becomes a nitrile.
Correct Answer: Nitrogen forms an extra bond and becomes positive.
Explanation: A neutral amine has three bonds around nitrogen and one lone pair. During protonation, the lone pair forms a new bond to \(H^+\). Nitrogen now has four bonds and carries a positive charge in the ammonium ion. This explains why amine salts contain positively charged ammonium-type species paired with anions.
209. Which factor mainly increases the basicity of aliphatic amines in the gaseous phase compared with \(NH_3\)?
ⓐ. Alkyl groups increase electron density on nitrogen.
ⓑ. Alkyl groups remove the nitrogen lone pair completely.
ⓒ. Alkyl groups convert the amine into a nitro compound.
ⓓ. Electron-withdrawing alkyl groups reduce proton attraction.
Correct Answer: Alkyl groups increase electron density on nitrogen.
Explanation: In the gaseous phase, solvation effects are absent, so electron donation becomes the dominant factor. Alkyl groups show a \(+I\) effect and push electron density toward nitrogen. Greater electron density on nitrogen makes the lone pair more available for accepting \(H^+\). Therefore, aliphatic amines are generally more basic than \(NH_3\) in the gaseous phase.
210. Which order is commonly observed for basicity of simple methyl amines in the gaseous phase?
ⓐ. \(NH_3 > CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N\)
ⓑ. \(CH_3NH_2 > NH_3 > (CH_3)_3N > (CH_3)_2NH\)
ⓒ. \((CH_3)_3N > (CH_3)_2NH > CH_3NH_2 > NH_3\)
ⓓ. \((CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3\)
Correct Answer: \((CH_3)_3N > (CH_3)_2NH > CH_3NH_2 > NH_3\)
Explanation: In the gaseous phase, the \(+I\) effect of alkyl groups strongly controls basicity. More methyl groups donate more electron density to nitrogen, making the lone pair more available for protonation. Trimethylamine has three electron-releasing methyl groups, dimethylamine has two, and methylamine has one. Ammonia lacks an alkyl group, so it is the least basic in this comparison.
211. In the gaseous phase, why is \((CH_3)_3N\) generally more basic than \(CH_3NH_2\)?
ⓐ. \((CH_3)_3N\) has more \(N-H\) bonds.
ⓑ. Methyl groups donate electron density.
ⓒ. \(CH_3NH_2\) has no nitrogen lone pair.
ⓓ. \((CH_3)_3N\) is an amide in the gaseous phase.
Correct Answer: Methyl groups donate electron density.
Explanation: \((CH_3)_3N\) has three methyl groups attached to nitrogen. These methyl groups release electron density through the \(+I\) effect, increasing the availability of the nitrogen lone pair. \(CH_3NH_2\) has only one methyl group, so the electron-releasing effect is smaller. In the gaseous phase, this electron-donating effect is not countered by solvent effects.
212. Which statement is most accurate for gas-phase basicity of aliphatic amines?
ⓐ. Basicity depends only on how many \(N-H\) bonds are present.
ⓑ. Ammonia is always more basic than all alkylamines.
ⓒ. Solvation of ammonium ions decides the entire order.
ⓓ. Alkyl groups raise basicity through the \(+I\) effect.
Correct Answer: Alkyl groups raise basicity through the \(+I\) effect.
Explanation: In the gaseous phase, there is no water or solvent to stabilize the protonated amine. The main factor is how easily nitrogen donates its lone pair to \(H^+\). Alkyl groups release electron density toward nitrogen through the \(+I\) effect. This generally makes alkylamines more basic than ammonia under gas-phase conditions.
213. Which compound is expected to be most basic in the gaseous phase?
ⓐ. \(NH_3\)
ⓑ. \(CH_3NH_2\)
ⓒ. \((CH_3)_3N\)
ⓓ. \(C_6H_5NH_2\)
Correct Answer: \((CH_3)_3N\)
Explanation: \((CH_3)_3N\) has three methyl groups that donate electron density to nitrogen through the \(+I\) effect. In the gaseous phase, this electron donation strongly increases the availability of the nitrogen lone pair. \(NH_3\) has no alkyl group, while \(CH_3NH_2\) has only one. Aniline, \(C_6H_5NH_2\), has its nitrogen lone pair partly delocalised into the benzene ring, which lowers its basicity.
214. Which factor is ignored if one predicts aqueous basicity of amines only from the \(+I\) effect?
ⓐ. Solvation of the protonated ion
ⓑ. Presence of carbon-nitrogen bonds
ⓒ. Formation of covalent \(N-H\) bonds
ⓓ. Availability of a nitrogen lone pair
Correct Answer: Solvation of the protonated ion
Explanation: In aqueous solution, basicity depends not only on electron donation to nitrogen but also on stabilization of the protonated amine. Water can solvate ammonium ions through hydrogen bonding. A protonated amine that is better solvated becomes more stabilized, which can increase the apparent basicity of the original amine. Therefore, using only the \(+I\) effect can give an incomplete or even wrong aqueous basicity order.
215. Which order is commonly used for basicity of simple methyl amines in aqueous solution?
ⓐ. \((CH_3)_3N > (CH_3)_2NH > CH_3NH_2 > NH_3\)
ⓑ. \(NH_3 > CH_3NH_2 > (CH_3)_2NH > (CH_3)_3N\)
ⓒ. \(CH_3NH_2 > (CH_3)_3N > (CH_3)_2NH > NH_3\)
ⓓ. \((CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3\)
Correct Answer: \((CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3\)
Explanation: In aqueous solution, both the \(+I\) effect and solvation of the conjugate acid influence basicity. Secondary methylamine has strong electron donation and its protonated ion is still well solvated. Tertiary methylamine has a strong \(+I\) effect but its conjugate acid is less effectively solvated because bulky methyl groups hinder hydration. The combined result commonly gives \((CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3\).
216. Why is the aqueous basicity order of methyl amines different from the gaseous-phase order?
ⓐ. Water solvates protonated amines differently.
ⓑ. Nitrogen loses its lone pair only in aqueous solution.
ⓒ. Alkyl groups stop showing any electron effect in water.
ⓓ. All amines become neutral salts in dry air.
Correct Answer: Water solvates protonated amines differently.
Explanation: In the gaseous phase, the \(+I\) effect mainly controls basicity. In water, the protonated amines formed after proton acceptance are solvated by water molecules. The extent of solvation differs for primary, secondary, and tertiary ammonium ions because their structures and steric crowding differ. This additional solvation factor changes the order from the simple gas-phase electron-donation trend.
217. Which protonated species is generally less effectively solvated than expected from electron donation alone because of crowding around nitrogen?
ⓐ. \(NH_4^+\)
ⓑ. \(CH_3NH_3^+\)
ⓒ. \((CH_3)_2NH_2^+\)
ⓓ. \((CH_3)_3NH^+\)
Correct Answer: \((CH_3)_3NH^+\)
Explanation: \((CH_3)_3NH^+\) is the conjugate acid of trimethylamine. It has three methyl groups around the positively charged nitrogen, which reduces effective interaction with surrounding water molecules. Although methyl groups donate electron density and help protonation, the bulky environment reduces solvation stabilization. This is why trimethylamine is not the strongest among simple methyl amines in aqueous solution.
218. Which amine best balances electron donation and solvation in water among simple methyl amines?
ⓐ. Methylamine
ⓑ. Dimethylamine
ⓒ. Trimethylamine
ⓓ. Ammonia
Correct Answer: Dimethylamine
Explanation: Dimethylamine has two methyl groups that donate electron density to nitrogen, increasing the tendency to accept a proton. After protonation, \((CH_3)_2NH_2^+\) can still be reasonably well solvated by water. Trimethylamine has stronger electron donation, but its conjugate acid is less effectively solvated because of steric crowding. This balance commonly makes dimethylamine the strongest base among simple methyl amines in aqueous solution.
219. Consider the statements:
Statement I: In the gaseous phase, tertiary aliphatic amines are often more basic than primary aliphatic amines.
Statement II: In the gaseous phase, electron-releasing alkyl groups increase electron density on nitrogen.
Which choice is correct?
ⓐ. Both statements are true, and Statement II explains Statement I.
ⓑ. Both statements are true, but Statement II does not explain Statement I.
ⓒ. Statement I is true, but Statement II is false.
ⓓ. Statement I is false, but Statement II is true.
Correct Answer: Both statements are true, and Statement II explains Statement I.
Explanation: In the gaseous phase, solvent effects do not influence the stability of the protonated amine. Alkyl groups donate electron density to nitrogen through the \(+I\) effect. A tertiary amine has more alkyl groups than a primary amine, so its nitrogen lone pair is generally more electron-rich. Statement II therefore gives the reason for the trend described in Statement I.
220. Which comparison is most suitable for aqueous basicity of \(CH_3NH_2\) and \((CH_3)_3N\)?
ⓐ. \((CH_3)_3N\) is always stronger because it has the maximum \(+I\) effect only.
ⓑ. \(CH_3NH_2\) is stronger because it has no lone pair delocalisation in water.
ⓒ. Both are equally basic because both have one nitrogen atom.
ⓓ. \(CH_3NH_2\) can exceed \((CH_3)_3N\).
Correct Answer: \(CH_3NH_2\) can exceed \((CH_3)_3N\).
Explanation: Trimethylamine has three electron-releasing methyl groups, which favours protonation. However, after protonation, \((CH_3)_3NH^+\) is less effectively solvated because the methyl groups crowd the positively charged nitrogen. Methylammonium ion, \(CH_3NH_3^+\), has more \(N-H\) bonds and is more easily solvated by water. This solvation advantage helps \(CH_3NH_2\) rank above \((CH_3)_3N\) in the common aqueous methylamine basicity order.
