Question

At pH = 4, glycine exists as:
A. $N{{H}_{3}}^{+}-C{{H}_{2}}CO{{O}^{-}}$
B. $N{{H}_{3}}^{+}-C{{H}_{2}}COOH$
C. $N{{H}_{2}}C{{H}_{2}}COOH$
D. $N{{H}_{2}}C{{H}_{2}}CO{{O}^{-}}$

Answer 1

Hint: pH is used to measure the acidity or basicity of an aqueous solution. In pH p stands for potential of hydrogen atom or power of hydrogen atom and H stands for hydrogen. If the pH value of any solution is less than 7 then it is acidic in nature and if the value is greater than 7 then the solution given is basic in nature while at 7 solution is neutral in nature.

Complete Solution :
- Glycine is the simplest amino acid represented by the symbol Gly or G. It is an amino acid that has a single hydrogen atom in its side chain. The chemical formula of glycine is $N{{H}_{2}}C{{H}_{2}}COOH$. Glycine generally exists as colorless crystalline solid. Out of all the amino acids glycine is the only achiral proteinogenic amino acid and it can be fit into hydrophilic or hydrophobic environments due to its chain of only one hydrogen atom.
- Now in the given question glycine at pH 4 will be exist as, at pH 4 glycine is in acidic form and the zwitterion of glycine is shown by:
- $N{{H}_{3}}^{+}-C{{H}_{2}}CO{{O}^{-}}$ when it reacts with ${{H}^{+}}$ ion then the reaction can be described in the following manner:
\[N{{H}_{3}}^{+}-C{{H}_{2}}CO{{O}^{-}}\to N{{H}_{3}}^{+}-C{{H}_{2}}COOH\]

- Hence from the above equation we can say that at pH 4 glycine exists in $N{{H}_{3}}^{+}-C{{H}_{2}}COOH$
So, the correct answer is “Option B”.

Note: Zwitterion is also known by the other name called inner salt. It is a molecule that contains an equal number of positively and negatively charged functional groups. When positive charge is located on quaternary ammonium ions then it will not isomerize.