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.

Which one amongst the following will have the lowest \(pK_b\)?

Ethanamine

The correct answer is option 1. Ethanamine.

The \(pK_b\) values represent the negative logarithm (base 10) of the base dissociation constant \((K_b)\) for each compound. A lower \(pK_b\) value indicates a stronger base.

Let us break down the explanation for each compound:

1. Ethanamine (ethylamine): \(pK_b\) = 3.29
Ethylamine is a primary amine with the formula \(C_2H_5NH_2\). It has a relatively low \(pK_b\), indicating that it is a stronger base compared to the other compounds listed. The lone pair on the nitrogen atom is more available for donation.

2. Methanamine (methylamine): \(pK_b\) = 3.38
Methylamine has a slightly higher pKb compared to ethylamine. While it is still a strong base, the presence of a smaller alkyl group may make the lone pair on nitrogen a bit less available compared to ethylamine.

3. N, N-dimethylmethanamine (dimethylamine): \(pK_b\) = 4.22
Dimethylamine has two methyl groups attached to the nitrogen atom, making it a secondary amine. The presence of additional alkyl groups reduces the basicity, resulting in a higher pKb compared to ethylamine and methylamine.

4. Phenyl Methanamine: \(pK_b\) = 4.70
Aniline has a phenyl (aromatic) ring attached to the nitrogen atom. The aromatic ring withdraws electron density from the nitrogen lone pair, making it less available for donation. Consequently, aniline has the highest pKb among the listed compounds, indicating the weakest base.

In summary, the trend in \(pK_b\) values reflects the decreasing basicity in the order: Ethanamine > Methanamine > N, N-dimethylmethanamine > Phenyl Methanamine. Ethylamine is the strongest base among the given compounds, while Phenyl methanamine is the weakest base due to the electron-withdrawing effect of the phenyl group.