Which of the following `alpha`-amino acids is not optically active?

Options

A. Alanine
B. Glycine
C. Phenylalanine
D. All are optically active

Answer

To determine which of the following alpha-amino acids is not optically active, we need to identify the presence of a chiral center in each amino acid. A chiral center is a carbon atom that is bonded to four different groups. If an amino acid does not have a chiral center, it will not be optically active.

### Step-by-Step Solution:

1. **Understand the General Structure of Alpha-Amino Acids**:
- Alpha-amino acids have a central carbon (Cα) that is attached to:
- An amino group (NH2)
- A carboxyl group (COOH)
- A hydrogen atom (H)
- A variable side chain (R group), which differs among amino acids.

2. **Identify the Amino Acids Given in the Question**:
- The question likely provides a list of amino acids to analyze. We will consider common amino acids: Glycine, Alanine, and Phenylalanine.

3. **Analyze Each Amino Acid for Chirality**:
- **Glycine (R = H)**:
- Structure: NH2-CH(COOH)-H
- Since the R group is hydrogen, Glycine has two hydrogen atoms attached to the central carbon. Therefore, it does not have four different groups and is not chiral. **Glycine is not optically active.**

- **Alanine (R = CH3)**:
- Structure: NH2-CH(COOH)-CH3
- The central carbon is attached to NH2, COOH, H, and CH3. All four groups are different, making Alanine chiral. **Alanine is optically active.**

- **Phenylalanine (R = CH2-C6H5)**:
- Structure: NH2-CH(COOH)-CH2-C6H5
- The central carbon is attached to NH2, COOH, H, and CH2-C6H5. All four groups are different, making Phenylalanine chiral. **Phenylalanine is optically active.**

4. **Conclusion**:
- Among the amino acids analyzed, **Glycine is the only amino acid that is not optically active** because it lacks a chiral center.

### Final Answer:
**Glycine is the alpha-amino acid that is not optically active.**

---