Question

The order of basic strength of following amines is: (i) \( CH_3NH_2 \) (ii) \( (C_2H_5)_2NH \) (iii) \( C_6H_5NH_2 \) (iv) \( C_6H_5NHCH_3 \)

• A. (ii) < (i) < (iv) < (iii)
• B. (iii) < (iv) < (ii) < (i)
• C. (i) < (iii) < (iv) < (ii)
• D. (i) < (ii) < (iii) < (iv)
• E. (iii) < (iv) < (i) < (ii)

Hint:

Resonance reduces availability of lone pair → lowers basicity.

Answer 1

Answer: (E)

Step 1: Understanding Basicity of Amines:
The basicity of an amine depends on the availability of the lone pair of electrons on the nitrogen atom to donate to a proton. Factors that increase the electron density on the nitrogen atom increase its basicity, while factors that decrease the electron density decrease its basicity.
Step 2: Comparing Aromatic and Aliphatic Amines:
Aliphatic amines (i) and (ii): Alkyl groups (like -CH\(_3\) and -C\(_2\)H\(_5\)) are electron-donating due to the positive inductive effect (+I). They push electron density towards the nitrogen atom, making the lone pair more available.
Aromatic amines (iii) and (iv): The lone pair of electrons on the nitrogen atom is delocalized into the benzene ring through resonance (-R effect). This makes the lone pair less available for donation to a proton.
Therefore, aliphatic amines are significantly stronger bases than aromatic amines.
Step 3: Comparing within Aromatic and Aliphatic Groups:
Aromatic Amines:
(iii) Aniline (C\(_6\)H\(_5\)NH\(_2\)): The lone pair is delocalized.
(iv) N-Methylaniline (C\(_6\)H\(_5\)NHCH\(_3\)): The lone pair is also delocalized, but the methyl group has a weak +I effect which slightly increases the electron density on nitrogen compared to aniline.
Thus, basicity: Aniline (iii)< N-Methylaniline (iv).
Aliphatic Amines (in aqueous solution):
(i) Methanamine (CH\(_3\)NH\(_2\)): A primary (1\(^\circ\)) amine with one +I group.
(ii) Diethylamine ((C\(_2\)H\(_5\))\(_2\)NH): A secondary (2\(^\circ\)) amine with two +I groups.
The basicity of aliphatic amines in aqueous solution is a combined effect of +I effect, steric hindrance, and solvation. For methyl and ethyl groups, the general order of basicity in water is: Secondary>Primary>Tertiary. The two ethyl groups in diethylamine provide a strong +I effect, making it a stronger base than methylamine.
Thus, basicity: Methanamine (i)< Diethylamine (ii).
Step 4: Combining the Orders:
Combining all the comparisons, we get the final order of increasing basic strength:
Aniline (iii)< N-Methylaniline (iv)< Methanamine (i)< Diethylamine (ii).