Read the passage given and answer the questions.

Amines constitute an important class of organic compounds derived by replacing one or more hydrogen atoms of ammonia molecule by alkyl/aryl group(s). Like ammonia, nitrogen atom of amines is trivalent and carries an unshared pair of electrons. Nitrogen orbitals in amines are therefore, sp3 hybridised and the geometry of amines is pyramidal. Amines are classified as primary (1°), secondary (2°) and tertiary (3°) depending upon the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule. Due to the electron releasing nature of alkyl group, it (R) pushes electrons towards nitrogen and thus, makes the unshared electron pair more available for sharing with the proton of the acid. Moreover, the substituted ammonium ion formed from the amine gets stabilised due to dispersal of the positive charge by the +I effect of the alkyl group. Hence, alkyl amines are stronger bases than ammonia. Thus, the basic nature of aliphatic amines should increase with increase in the number alkyl groups. The order of basicity of amines of amines in the gaseous phase follows the expected order: 3° > 2° > 1° > NH3. Adrenaline and ephedrine both containing 2° amino group used to increase blood pressure. Well known antihistamine drug Benadryl also contains 3.

Among the following, the weakest base is:

\(NH_3\)

The correct answer is option 4. \(NH_3\).

The key factor influencing the basicity of amines is the stability of the conjugate acid formed after accepting a proton. A more stable conjugate acid signifies a stronger base, as the reaction is more favorable. Let's analyze each option:

1. \(C_2H_5NH_2\):

This is a primary amine with one alkyl group \((C_2H_5)\) attached to the nitrogen atom. The \(C_2H_5\) group exhibits an inductive effect, donating electrons to the nitrogen atom and increasing its electron density. This increased electron density makes the nitrogen more likely to accept a proton, forming the conjugate acid, \(C_2H_5NH_3^+\). However, the lone pair on the nitrogen is still partially exposed and readily available for protonation.

2. \((C_2H_5)_2NH\):

This is a secondary amine with two \(C_2H_5\) groups attached to the nitrogen atom. Here, the inductive effect is even stronger, further increasing the electron density on the nitrogen. Compared to \(C_2H_5NH_2\), the lone pair is slightly shielded by the two bulky \(C_2H_5\) groups, making it slightly less accessible for protonation. However, the overall electron density remains higher, resulting in a stronger base than \(C_2H_5NH_2\).

3. \((C_2H_5)_3N\):

This is a tertiary amine with all three hydrogens on the nitrogen atom replaced by \(C_2H_5\) groups. The inductive effect reaches its maximum, pushing the electron density on the nitrogen to its highest level. Additionally, the bulky \(C_2H_5\) groups create significant steric hindrance around the nitrogen atom, further shielding the lone pair. While the electron density is high, the steric hindrance makes it slightly less accessible for protonation compared to \((C_2H_5)_2NH\).

4. \(NH_3\):

This is ammonia, which lacks any alkyl groups attached to the nitrogen atom. Despite the lone pair being readily available, the electron density on the nitrogen is the lowest among the options. This is because the three hydrogen atoms attached to the nitrogen exert an electron-withdrawing inductive effect, pulling electron density away from the nitrogen. Consequently, \(NH_3\) has the weakest basicity compared to the other amines due to its lower electron density on the nitrogen atom.

Therefore, the order of basicity is:

\((C_2H_5)_2NH\) > \((C_2H_5)_3N\) > \(C_2H_5NH_2\) > \(NH_3\)

In summary, the number of alkyl groups (increasing electron density) positively impacts basicity, while steric hindrance around the nitrogen (shielding the lone pair) slightly reduces it. The electron-withdrawing effect in \(NH_3\) significantly decreases its basicity compared to the other amines.