Question

Identify the structure of alanine when pH is \(11\).

• A. \({}^{+}\text{NH}_{3}\text{-CH(CH}_{3}\text{)-COOH}\)
• B. \({}^{+}\text{NH}_{3}\text{-CH(CH}_{3}\text{)-COO}^{-}\)
• C. \(\text{NH}_{2}\text{-CH(CH}_{3}\text{)-COO}^{-}\)
• D. \(\text{NH}_{2}\text{-CH(CH}_{3}\text{)-COOH}\)

Hint:

In strong base both the NH3+ and COOH groups lose a proton.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept
An amino acid has an acidic -COOH and a basic -\(\text{NH}_{2}\) group.
Near neutral pH it exists as a zwitterion, \({}^{+}\text{NH}_{3}\text{-CH(CH}_{3}\text{)-COO}^{-}\).

Step 2: Effect of high pH
At pH 11 the solution is strongly basic, so \(\text{OH}^{-}\) removes the proton from the \(\text{NH}_{3}^{+}\) group as well.

Step 3: Resulting structure
Both groups are deprotonated: \(\text{NH}_{2}\text{-CH(CH}_{3}\text{)-COO}^{-}\), an anion.

Step 4: Compare the options
(A) is the cation in acid, (B) is the zwitterion, and (D) is the uncharged form that does not exist in solution.

Final Answer:
At pH 11 alanine exists as the anion with a free amino group and a carboxylate group, option (C). \[ \boxed{\text{(C) }\text{NH}_{2}\text{-CH(CH}_{3}\text{)-COO}^{-}} \]