Question

Consider the following compounds: \[ (C_2H_5)_2NH,\quad C_6H_5NH_2,\quad C_6H_5CH_2NH_2,\quad NH_3,\quad (C_2H_5)_3N \] The correct increasing order of the above bases on the basis of their basic strength is:

• A. C\(_6\)H\(_5\)NH\(_2\)<NH\(_3\)<C\(_6\)H\(_5\)CH\(_2\)NH\(_2\)<(C\(_2\)H\(_5\))\(_3\)N<(C\(_2\)H\(_5\))\(_2\)NH
• B. NH\(_3\)<C\(_6\)H\(_5\)NH\(_2\)<C\(_6\)H\(_5\)CH\(_2\)NH\(_2\)<(C\(_2\)H\(_5\))\(_3\)N<(C\(_2\)H\(_5\))\(_2\)NH
• C. C\(_6\)H\(_5\)NH\(_2\)<(C\(_2\)H\(_5\))\(_2\)NH<NH\(_3\)<C\(_6\)H\(_5\)CH\(_2\)NH\(_2\)<(C\(_2\)H\(_5\))\(_3\)N
• D. C\(_6\)H\(_5\)NH\(_2\)<C\(_6\)H\(_5\)CH\(_2\)NH\(_2\)<NH\(_3\)<(C\(_2\)H\(_5\))\(_2\)NH<(C\(_2\)H\(_5\))\(_3\)N

Hint:

In aqueous solution: \[ \text{Secondary amine}>\text{Tertiary amine}>\text{Primary amine}>NH_3 \] Aniline is least basic due to resonance.

Answer 1

The Correct Option is A

To determine the increasing order of basic strength among the given compounds, we must consider the electron-releasing and withdrawing effects, as well as the steric and inductive effects present in the molecules. The compounds given are:

• \((C_2H_5)_2NH\) (Diethylamine)
• \(C_6H_5NH_2\) (Aniline)
• \(C_6H_5CH_2NH_2\) (Benzylamine)
• \(NH_3\) (Ammonia)
• \((C_2H_5)_3N\) (Triethylamine)

Basic strength is primarily determined by the availability of the lone pair of electrons on the nitrogen atom to accept protons (H⁺). Let's analyze each compound:

1. Aniline (C₆H₅NH₂): 
   The lone pair on nitrogen in aniline is delocalized due to resonance with the aromatic ring, decreasing its availability to accept a proton. Thus, aniline is the least basic.
2. Ammonia (NH₃): 
   Compared to aniline, ammonia has a lone pair that is not associated with any resonance structures, making it more available to accept protons than aniline.
3. Benzylamine (C₆H₅CH₂NH₂): 
   The lone pair on the nitrogen atom is not involved in resonance with the aromatic ring, making benzylamine more basic than ammonia. The benzyl group provides some electron-donating resonance effect but not as significantly as alkyl groups.
4. Triethylamine ((C₂H₅)₃N): 
   The presence of three ethyl groups increases the electron density on the nitrogen due to the +I (inductive) effect, enhancing its basicity. However, steric hindrance slightly reduces its ability to coordinate with protons compared to secondary amines.
5. Diethylamine ((C₂H₅)₂NH): 
   It has two ethyl groups, providing a strong +I effect with less steric hindrance than triethylamine, making it the most basic among the given compounds.

Based on the analysis above, the correct increasing order of basic strength is:

\(C_6H_5NH_2 < NH_3 < C_6H_5CH_2NH_2 < (C_2H_5)_3N < (C_2H_5)_2NH\)