Q18.145 CP - Chemistry: Molecular Nature Of Matter And Change

Question

Pantothenic acid is shown below. This biologically active molecule is an optical isomer that behaves like a monoprotic Bronsted-Lowry acid in water.

(a) Use the molecular formula to write the equation for the reaction of pantothenic acid with water and the Ka expression. 

(b) Which C atom is the chiral center?

Step-by-Step Solution

Answer

(a) ${\text{K}}_{\text{a}}\text{ = }\frac{\left[{\text{C}}_{\text{8}}{\text{H}}_{\text{16}}{\text{NO}}_{\text{3}}{\text{COO}}^{\text{ - }}\right]\left[{\text{H}}_{\text{3}}{\text{O}}^{\text{ + }}\right]}{\left[{\text{C}}_{\text{8}}{\text{H}}_{\text{16}}{\text{NO}}_{\text{3}}\text{COOH}\right]}$

(b) The chiral carbon was drawn.

Step 1: To write the equation for the reaction

(a)

The chemical structure of a Pantothenic acid $\left({\text{C}}_{\text{9}}{\text{H}}_{\text{17}}{\text{NO}}_{\text{5}}\right)$ is provided in the book. As observed, it has a carboxylic acid end $(\left.-\text{COOH}\right)$ and that is where the acid reaction occurs.

$$\begin{gathered} {\text{C}}_{\text{8}}{\text{H}}_{\text{16}}{\text{NO}}_{\text{3}}{\text{COOH + H}}_{\text{2}}\text{O}\rightleftharpoons {\text{C}}_{\text{8}}{\text{H}}_{\text{16}}{\text{NO}}_{\text{3}}{\text{COO}}^{\text{ - }}{\text{ + H}}_{\text{3}}{\text{O}}^{\text{ + }} \\ {\text{K}}_{\text{a}}\text{ = }\frac{\text{[ product ]}}{\text{[ reactant ]}} \\ \text{ = }\frac{\left[{\text{C}}_{\text{8}}{\text{H}}_{\text{16}}{\text{NO}}_{\text{3}}{\text{COO}}^{\text{ - }}\right]\left[{\text{H}}_{\text{3}}{\text{O}}^{\text{ + }}\right]}{\left.{\left[{\text{C}}_{\text{8}}{\text{H}}_{\text{16}}{\text{NO}}_{\text{3}}\text{COOH}\right]}^{\text{OO}}\right]} \end{gathered}$$

Hence, the equation is:

 ${\text{K}}_{\text{a}}\text{ = }\frac{\left[{\text{C}}_{\text{8}}{\text{H}}_{\text{16}}{\text{NO}}_{\text{3}}{\text{COO}}^{\text{ - }}\right]\left[{\text{H}}_{\text{3}}{\text{O}}^{\text{ + }}\right]}{\left[{\text{C}}_{\text{8}}{\text{H}}_{\text{16}}{\text{NO}}_{\text{3}}\text{COOH}\right]}$

 

 

 

Step 2: Find the C atom center

(b)

A chiral atom is an atom attached to 4 UNIQUE TYPES of atoms or group of atoms. In a Pantothenic acid, there is only one chiral carbon, and it is the carbon atom in a red box. The four different atoms and groups of atoms are enclosed in boxes of other colors.

 

Hence, the chiral carbon is as shown in the above image.