Problem 108 - A Complete Resource Book in Chemistry for JEE Main

Question

At \(\mathrm{pH}=4\), glycine exists as (a) \(\mathrm{H}_{3} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COO}^{-}\) (b) \(\mathrm{H}_{3} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COOH}\) (c) \(\mathrm{H}_{2} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COOH}\) (d) \(\mathrm{H}_{2} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COO}^{-}\)

Step-by-Step Solution

Answer

At pH 4, glycine exists as \(\mathrm{H}_{3} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COO}^{-}\).

Step 1: Understand the pH Meaning

The pH of the solution is 4, which is slightly acidic. At this pH, we need to determine the dominant form of glycine, an amino acid.

Step 2: Consider Glycine and Its Forms

Glycine can exist in different ionized forms depending on the pH. The relevant forms are the protonated amine form \(\mathrm{H}_3\mathrm{N}^+\) and carboxyl groups \(\mathrm{COOH}\), and their dissociated forms \(\mathrm{H}_2\mathrm{N}\) and \(\mathrm{COO}^-\).

Step 3: Analyze pKa Values of Glycine

Glycine has two relevant pKa values: one around 2.34 for the \(\mathrm{COOH/COO}^-\) transition and one around 9.6 for the \(\mathrm{H}_3\mathrm{N}^+/\mathrm{H}_2\mathrm{N}\) transition. At pH 4, it is above the first pKa and below the second.

Step 4: Determine Charge on Groups

Since pH 4 is above the pKa of the carboxyl group, it will exist in the \(\mathrm{COO}^-\) form. Since pH 4 is below the pKa of the amine group, it will remain in the \(\mathrm{H}_3\mathrm{N}^+\) form.

Step 5: Identify the Zwitterionic Form at pH 4

Given the analyses, glycine at pH 4 will have \(\mathrm{H}_3\mathrm{N}^+\) and \(\mathrm{COO}^-\) groups, corresponding to the zwitterionic structure \(\mathrm{H}_{3} \mathrm{~N}-\mathrm{CH}_{2}-\mathrm{COO}^{-}\).

Key Concepts

Amino Acid Structures

Amino acids are the building blocks of proteins and are essential for various functions in living organisms. Each amino acid consists of a central carbon atom (called the alpha carbon), to which four different groups are attached:

• An amino group (5hnH5hnH3N
• A carboxyl group (5hnCOOH
• A hydrogen atom
• A variable side chain (R group), which differentiates one amino acid from another

Amino acids in solution can exist in different forms depending on the pH of the environment. At a certain pH, an amino acid can be present in its zwitterionic form. This is a state where the molecule contains both a positive charge (5hnH3N^+) and a negative charge (5hnCOO^-5hnCOO^-), making the overall charge neutral. Glycine's side chain is just a hydrogen, making it the simplest amino acid.

pH and pKa Relationship

The pH is a measure of the acidity or basicity of a solution, which correlates with the concentration of hydrogen ions (H⁺) in the solution. Instruments like pKa are used to understand the propensity of chemical groups like the carboxyl group and amino group to donate or accept protons:

• pKa indicates the pH at which the group is 50% ionized.
• If the pH is lower than the pKa, the group tends to stay protonated (e.g., 5hnCOOH rather than 5hnCOO^-).
• If the pH is higher than the pKa, the group tends to deprotonate (e.g., 5hnNH_2 over 5hnNH3^+).

For glycine, the pKa for the carboxyl group is approximately 2.34, while for the amino group, it's about 9.6. At pH 4, which is above the carboxyl group but below the amino group's pKa, the carboxyl group will be deprotonated, while the amino group remains protonated.

Ionization of Amino Acids

Ionization refers to the process in which an atom or a molecule gains or loses an electron, forming ions. In the context of amino acids, this process involves the ionization of specific functional groups:

• Carboxyl group (5hnCOOH): It ionizes to become 5hnCOO^- by losing a proton (H^+).
• Amino group (5hnNH2): It can accept a proton to become 5hnNH3^+.

This behavior is crucial for understanding the existence of amino acids like glycine as zwitterions in certain pH environments. At pH 4, the carboxyl group of glycine exists in its ionized form (5hnCOO^-), while the amino group remains protonated (5hnNH3^+), reflecting the classic characteristics of its zwitterionic state. These ionization steps are fundamental for studying amino acid reactivity, structure interactions, and functions.