Question

The amine with the highest $pK_{b}$ value is ________.

• A. Methanamine
• B. N-methylmethanamine
• C. Benzeneamine
• D. N-Methylaniline
• E. Ethanamine

Hint:

Lower $pK_b$ = Stronger base; Resonance with ring = Weaker base.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept
The basicity of an amine is quantified by its base dissociation constant, \(K_b\). The pK\(_b\) is the negative logarithm of \(K_b\) (\(pK_b = -\log K_b\)). This means that pK\(_b\) and basicity are inversely related: a stronger base has a larger \(K_b\) and a smaller pK\(_b\), while a weaker base has a smaller \(K_b\) and a higher pK\(_b\). The question is asking for the weakest base among the options.
Step 2: Key Formula or Approach
The basicity of an amine depends on the availability of the nitrogen's lone pair of electrons for donation. - Alkyl groups are electron-donating (+I effect), which increases electron density on the nitrogen, making the amine more basic (lower pK\(_b\)). - Aryl (phenyl) groups are electron-withdrawing by resonance (-R effect), which delocalizes the nitrogen's lone pair into the benzene ring, making it less available for donation. This makes the amine much less basic (higher pK\(_b\)).
Step 3: Detailed Explanation
Let's classify and compare the amines.
- (A) Methanamine (\(CH_3NH_2\)) and (E) Ethanamine (\(CH_3CH_2NH_2\)) are aliphatic primary amines. The alkyl groups donate electron density, making them relatively strong bases (pK\(_b\) \(\approx\) 3.3).
- (B) N-methylmethanamine (\((CH_3)_2NH\)), a secondary aliphatic amine, has two electron-donating methyl groups. It is generally a stronger base than primary aliphatic amines (pK\(_b\) \(\approx\) 3.2).
- (C) Benzeneamine (Aniline, \(C_6H_5NH_2\)), an aromatic primary amine. The lone pair on the nitrogen is delocalized into the \(\pi\)-system of the benzene ring through resonance. This significantly reduces the availability of the lone pair for accepting a proton, making aniline a very weak base. Its pK\(_b\) is high (approx. 9.4).
- (D) N-Methylaniline (\(C_6H_5NHCH_3\)), an aromatic secondary amine. Similar to aniline, the lone pair is delocalized into the ring. The additional methyl group has a weak +I effect, which makes N-methylaniline slightly more basic than aniline (pK\(_b\) \(\approx\) 9.1).
Comparison:
The general order of basicity is: Aliphatic amines >> Aromatic amines. Within the aromatic amines, aniline is weaker than N-methylaniline. Therefore, aniline (Benzeneamine) is the weakest base in the list. The weakest base has the highest pK\(_b\) value.
Step 4: Final Answer
The amine with the highest pK\(_b\) value is Benzeneamine (Aniline).