What will be the increasing order of basic strength of the following com pounds?

C₂H₅NH₂, (C₂H₅)₂NH, (C₂H₅)₃N, C₆H₅NH₂

The correct option representing the increasing order of basic strength is:
C₆H₅NH₂ < C₂H₅NH₂ < (C₂H₅)₃N < (C₂H₅)₂NH

Explanation:
The basic strength of amines depends on their ability to donate a lone pair of electrons on the nitrogen atom to an electron-deficient species (proton). This is determined by several factors, including the inductive effect, steric hindrance, and solvation (hydration) energy in aqueous medium.

1. Aniline (C₆H₅NH₂) vs. Aliphatic Amines:
In aniline (C₆H₅NH₂), the lone pair of electrons on the nitrogen atom is delocalized over the benzene ring via resonance. As a result, this lone pair is less available for donation to a proton. On the other hand, in aliphatic amines, the alkyl groups exhibit an electron-donating inductive effect (+I effect) that increases the electron density on the nitrogen atom, making it more basic. Therefore, aniline is the weakest base among the given compounds.

2. Comparison among Aliphatic Ethyl Amines in Aqueous Solution:
For aliphatic amines in an aqueous medium, three main factors influence basicity:
• Inductive effect (+I effect): Electron-donating alkyl groups increase the availability of the lone pair. Based on this effect alone, the order of basicity should be: tertiary (3°) > secondary (2°) > primary (1°).
• Solvation effect: The conjugate acid formed after accepting a proton can form hydrogen bonds with water molecules. More hydrogen atoms on the nitrogen of the conjugate acid mean stronger hydration and greater stability. The order of stability of the ammonium cation due to solvation is: 1° > 2° > 3°.
• Steric hindrance: Bulky alkyl groups block the attack of a proton on the nitrogen atom. Steric hindrance increases with the number of alkyl groups: 1° < 2° < 3° (hence, basicity decreases in the direction: 1° > 2° > 3°).

When ethyl groups (-C₂H₅) are present, the combined influence of these three opposing factors results in a net basicity order of:
Secondary (2°) > Tertiary (3°) > Primary (1°)
Specifically: (C₂H₅)₂NH > (C₂H₅)₃N > C₂H₅NH₂

Combining all the observations, the overall increasing order of basic strength is:
C₆H₅NH₂ < C₂H₅NH₂ < (C₂H₅)₃N < (C₂H₅)₂NH