Chapter 21, Problem 21.31P

The amino acid alanine is a solid at room temperature and has a melting point of $315°\text{C}$ whereas pyruvic acid $\left({\text{CH}}_{\text{3}}{\text{COCO}}_{\text{2}}\text{H}\right)$ has a similar molecular weight but is a liquid at room temperature with a boiling point of $\text{165}\text{°C}\text{.}$ Account for the difference.

Interpretation Introduction

Interpretation:

The reason for alanine which is solid to have a melting point of $315°\text{C}$ and pyruvic acid which is liquid to have a boiling point of $165°\text{C}$ needs to be explained.

Concept introduction:

Structure of alanine is as follows:

  

Alanine is a hydrophobic amino acid as it has only one ${\text{CH}}_{\text{3}}$ (methyl) group as its side chain which is non-polar.

Structure of pyruvic acid is as follows:

  

Answer to Problem 21.31P

Due to the strong electrostatic force of attraction alanine is solid and has a high melting point of $315°\text{C}$ whereas because of the weaker intermolecular force of attraction, pyruvic acid is liquid at room temperature and has a boiling point of $165°\text{C}$.

Explanation of Solution

Alanine is a hydrophobic amino acid also an ionic salt with strong electrostatic forces. Therefore, alanine has a high melting point of $315°\text{C}$ . As alanine has a very high melting point so, it is solid at room temperature. Pyruvic acid is an organic acid. When it loses a hydrogen atom, it acquires a negative charge. The intermolecular force of attraction is very much weaker. So, pyruvic acid is liquid at room temperature and has a boiling point of $165°\text{C}$.