Chemistry Multiple Choice Questions on “Amines Chemical Reactions – 2”.
1. If p, q and r are the pKb values of methylamine, N-methylamine and N,N-dimethylamine respectively, what is the correct order of p, q and r?
a) p > q > r
b) r > q > p
c) q > p > r
d) r > p > q
Answer: d
Clarification: This peculiar order of basic strength is due to a combination of inductive, solvation and stearic effects of the alkyl groups in the amines. In methyl substituted amines, the solvation effect superimposes the inductive effect to make CH3NH2 more basic than (CH3)3N. The 2° compound is the most basic because of its higher inductive effect than primary amine and higher solvation than tertiary amine.
2. What is the correct order of basic strength of the following ethyl substituted amines in aqueous solution? (R=ethyl group)
a) RNH2 > R2NH > R3N
b) R2NH > R3N > RNH2
c) R3N > R2NH > RNH2
d) R2NH > RNH2 > R3N
Answer: b
Clarification: There is a subtle interplay of inductive effect, solvation effect and stearic hinderance of alkyl groups to determine the order of basicity of alkylamines in aqueous solutions. For ethyl substituted amines, it is 2°>3°>1°
3. If the pKb value of N,N-diethylethanamine is 3.25, predict the pKb value of ethanamine.
a) 3.00
b) 3.29
c) 4.75
d) 9.38
Answer: b
Clarification: In ethyl substituted amines (aqueous), the inductive effect has a bigger role than the solvation effect (because the size of ethyl is larger than methyl group), and as a result the basic strength of N,N-diethylethanamine will be higher than that of ethanamine. Thus, the pKb value of the latter will be very slightly higher than the former.
4. Which of the following aromatic amines has lower pKb value than ammonia?
a) Benzylamine
b) Benzenamine
c) N-Methylbenzenamine
d) N,N-Dimethylbenzenamine
Answer: a
Clarification: In aryl amines, the N atom is directly attached to the benzene ring. This makes the lone pair of N to be in conjugation with the benzene ring, thus making it less available for protonation. Whereas, benzylamine is a primary arylalkyl amine and has higher basic strength than ammonia.
5. Which of the following amines will be most reactive when treated with HCl?
a) N-Methylmethanamine
b) N,N-Dimethylmethanamine
c) N-Ethylethanamine
d) N,N-Diethylethanamine
Answer: c
Clarification: Between methyl substituted and ethyl substituted aqueous amines, the latter will have relatively higher Kb values due to the ethyl group being larger than methyl group. Of the ethyl substituted amines, the secondary amine will be the most basic because of the combined inductive and solvation effect of alkyl group. Thus, being the most basic, it will react faster with HCl.
6. How many more resonating structures does aniline have than anilinium ion?
a) 2
b) 3
c) 4
d) 5
Answer: b
Clarification: Aniline has five resonating structures, out of which three of them have a positive charge on nitrogen. This results in unavailability of electron pair for protonation. When aniline accepts a proton, it forms anilinium ion which has only two resonating structures and is less stable and more basic than aniline.
7. Which of the following groups when present at para position increases the basic strength of aniline?
a) NO2
b) Br
c) NH2
d) COOH
Answer: c
Clarification: Electron donating groups at para position release electrons towards the nitrogen atom, stabilizes the anilinium cation and hence increase the basic strength. Since NH2 is an electron donating group, p-phenylenediamine will be more basic than aniline.
8. Identify X if the shown compound has a higher pKb value than aniline.
a) OH
b) CH3
c) NH2
d) Cl