Question

The increasing order ${\mathrm{pK}}_{\mathrm{b}}$ for the following compounds will be :

(i) ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}=\mathrm{NH}$

(ii)

(iii) ${\mathrm{CH}}_3{\mathrm{NHCH}}_3$

• A. $\mathrm{ii}<\mathrm{iii}<\mathrm{i}$
• B. $\mathrm{iii}<\mathrm{i}<\mathrm{ii}$
• C. $\mathrm{i}<\mathrm{ii}<\mathrm{iii}$
• D. $\mathrm{ii}<\mathrm{i}<\mathrm{iii}$

Answer 1

The correct option is D

$\mathrm{ii}<\mathrm{i}<\mathrm{iii}$

The Explanation for the correct options:

(D)${\mathrm{pK}}_{\mathrm{b}}$ value depends on-

${\mathrm{pK}}_{\mathrm{b}}\propto \frac{1}{\mathrm{Basicity}}\propto \mathrm{Acidity}$

Basicity depends on the number of nitrogen atoms. More the number of nitrogen atoms higher the basicity.

• Explanation: The stronger the base, the weaker the conjugate acid. It has a high tendency to accept a proton, resulting in a very stable conjugate acid. A number of aromatic molecules have common names; for example, benzene with a ${\mathrm{CH}}_3$ group attached is known as toluene, while benzene with a ${\mathrm{NH}}_2$group attached is known as aniline.
• $\left(\mathrm{ii}\right)$ has a lower value of basicity than ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}=\mathrm{NH}$. ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}=\mathrm{NH}$ is more basic than of ${\mathrm{CH}}_3{\mathrm{NHCH}}_3$.

The Explanation for the incorrect options:

(A)

• Explanation: As the base becomes stronger, the conjugate acid decreases. It has a strong proclivity to accept a proton, resulting in a chemical conjugate acid that is stable and has alkalinity about equal to that of ammonia
• There will be a low tendency of $\left(\mathrm{ii}\right)$ being less basic than ${\mathrm{CH}}_3{\mathrm{NHCH}}_3$ less than of ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}=\mathrm{NH}$.

(B)

• Explanation: The conjugate acid weakens as the base becomes stronger. It readily accepts a proton, resulting in a very stable conjugate acid. which has about the same alkalinity as ammonia
• It has a low proclivity to ${\mathrm{CH}}_3{\mathrm{NHCH}}_3$ less than or ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}=\mathrm{NH}$ less than of ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}=\mathrm{NH}$ will be less than$\left(\mathrm{ii}\right)$.

(C)

• Explanation: As the base becomes stronger, the conjugate acid becomes weaker. It readily accepts a proton. It readily accepts a proton, resulting in a very stable conjugate acid. which has about the same alkalinity as ammonia
• It has a low proclivity to be ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}=\mathrm{NH}$. ${\mathrm{H}}_2\mathrm{N}-\mathrm{CH}=\mathrm{NH}$ less than the $\left(\mathrm{ii}\right)$. $\left(\mathrm{ii}\right)$ less than the ${\mathrm{CH}}_3{\mathrm{NHCH}}_3$.

Hence, the correct option is (D): It has a high tendency of accepting a proton, giving rise to a very stable conjugate acid.