Question

Among methanamine, ethanamine. benzenamine, N-methylaniline and N, N-dimethylaniline, the weakest and the strongest base in aqueous phase, respectively are

• A. benzenamine and methanamine
• B. N-methylaniline and ethanamine
• C. N, N-dimethylaniline and ethanamine
• D. benzenamine and ethanamine
• E. N-methylaniline and methanamine

Answer 1

The Correct Option is D

In aqueous solution, the basicity of amines is affected by the electron-donating or electron-withdrawing effects of substituents on the nitrogen atom.

• Methanamine (CH₃NH₂) is a stronger base compared to benzenamine (C₆H₅NH₂) because the methyl group in methanamine is an electron-donating group, which increases the electron density on nitrogen, enhancing its ability to accept a proton.
• Benzenamine (Aniline) has a weaker basicity because the phenyl group (C₆H₅) is electron-withdrawing through resonance, reducing the electron density on nitrogen and making it less available for protonation.
• Ethanamine (C₂H₅NH₂) is a stronger base than methanamine due to the ethyl group being a stronger electron-donating group than the methyl group. 
• N, N-dimethylaniline has reduced basicity compared to ethanamine because the methyl groups on nitrogen withdraw electron density, making it less basic.

Thus, the weakest base is benzenamine and the strongest base is ethanamine.

The correct option is (D) : benzenamine and ethanamine

Answer 2

The basicity of amines in the aqueous phase is influenced by several factors:

• Inductive effect: Alkyl groups are electron-donating (+I effect), which increases the electron density on the nitrogen atom and makes it more basic.
• Solvation: The protonated amine (ammonium ion) is stabilized by solvation (hydrogen bonding with water).
• Steric hindrance: Bulky groups around the nitrogen atom can hinder solvation, decreasing basicity.
• Resonance: In aromatic amines (like aniline), the lone pair of electrons on the nitrogen atom is delocalized into the benzene ring, reducing the availability of the lone pair for protonation and thus decreasing basicity.

Let's analyze the given amines:

• Methanamine (CH₃NH₂): Simple alkylamine, good solvation, less steric hindrance.
• Ethanamine (CH₃CH₂NH₂): Alkylamine, good solvation. A slightly stronger +I effect than methyl, but similar.
• Benzenamine (Aniline, C₆H₅NH₂): Lone pair delocalized into the benzene ring, making it very weak base.
• N-methylaniline (C₆H₅NHCH₃): Still has resonance with the benzene ring, making it less basic than alkylamines. Also some steric hinderance compared to aniline
• N,N-dimethylaniline (C₆H₅N(CH₃)₂): Resonance with the benzene ring, but more steric hindrance due to two methyl groups, which can also affect solvation.

Comparing the basicity:

• Weakest base: Benzenamine (due to resonance).
• Strongest base: Ethanamine.Aliphatic amines like methanamine and ethanamine are stronger bases than aromatic amines due to resonance. Ethanamine is likely slightly more basic than methanamine because it has slightly more inductive effect(more electron density).

The overall order of basicity is approximately: Ethanamine > Methanamine > N-methylaniline > N,N-dimethylaniline > Benzenamine

Therefore, the weakest and the strongest base are benzenamine and ethanamine, respectively.

So the answer is benzenamine and ethanamine.