Q.Assertion (A): All naturally occurring $\alpha$-amino acids except glycine are optically active.
Reason (R): Most naturally occurring amino acids have L-configuration.

The key idea is that optical activity in $\alpha$-amino acids arises from the presence of a chiral carbon (the $\alpha$-carbon) bonded to four different groups. Glycine has two hydrogen atoms on the $\alpha$-carbon, making it achiral and thus optically inactive — so Assertion (A) is true. …

1. Understanding optical activity and chirality

   A molecule is optically active if it rotates plane-polarised light. This happens when the molecule is chiral — it has a non-superimposable mirror image. In organic chemistry, the most common source of chirality is a carbon atom bonded to four different groups (a chiral centre).

   For $\alpha$-amino acids, the general structure is $H_2N-CHR-COOH$, where $R$ is a side chain. The central $\alpha$-carbon is bonded to an amino group ($-NH_2$), a carboxyl group ($-COOH$), a hydrogen atom ($-H$), and the side chain $R$.

   If $R \neq H$, all four groups are different, so the $\alpha$-carbon is chiral — the molecule exists as two non-superimposable mirror images (enantiomers), and each is optically active.

   If $R = H$, the molecule is glycine: $H_2N-CH_2-COOH$. Here, the $\alpha$-carbon has two hydrogen atoms (the side chain is just H), so it is bonded to only three different groups. This makes glycine achiral — it has no mirror image that is different from itself — and therefore optically inactive.

2. Why Assertion (A) is true

   All naturally occurring $\alpha$-amino acids except glycine have $R \neq H$, so they possess a chiral $\alpha$-carbon and are optically active. Glycine, with $R = H$, is the sole exception. Thus, Assertion (A) is correct.

3. Why Reason (R) is true

   It is a well-established fact in biochemistry that almost all naturally occurring amino acids (in proteins) have the L-configuration at the $\alpha$-carbon. This refers to the absolute stereochemistry (based on the Fischer projection, with the amino group on the left). So Reason (R) is also true.

4. Does (R) correctly explain (A)? …

Method: Assertion–Reason Analysis (Conceptual Verification)

This is a standard Assertion–Reason question from organic chemistry (biomolecules). The method is to verify each statement independently, then check if the Reason correctly explains the Assertion.

Steps

1. Verify Assertion (A)

   • All naturally occurring $\alpha$-amino acids (except glycine) have a chiral $\alpha$-carbon (four different groups attached).
   • Glycine has two H atoms on $\alpha$-carbon → achiral, hence not optically active.
   • All others are chiral and exist in nature predominantly as L-isomers, which are optically active.
   • ✓ Assertion is true.

2. Verify Reason (R)

   • Naturally occurring $\alpha$-amino acids (except glycine) indeed have L-configuration at the $\alpha$-carbon (based on Fischer projection, with $-\text{NH}_2$ on left).
   • ✓ Reason is true.

3. Check if R explains A

   • Optical activity arises from chirality, not directly from L-configuration. …

Common Mistakes on This Question (Optical Activity of Amino Acids)

Mistake 1: Confusing Optical Activity with L/D Configuration

The error: Students think "L-configuration" automatically means "optically active."

Why it's wrong: Optical activity depends on the presence of a chiral carbon (four different groups attached), not on the L/D label. L-configuration is a naming convention based on glyceraldehyde, not a guarantee of chirality.

How to avoid: Remember — glycine has two hydrogen atoms on the $\alpha$-carbon, so it has no chiral centre. All other $\alpha$-amino acids have four different groups ($\text{NH}_2$, $\text{COOH}$, $\text{H}$, and a variable R group), making them chiral and optically active.

Mistake 2: Thinking "All L-amino acids are optically active"

The error: Assuming every amino acid with L-configuration rotates plane-polarised light.

Why it's wrong: L-configuration is a relative stereochemical descriptor. Optical activity is an absolute physical property. While most L-amino acids are indeed optically active, the reason is chirality, not the L-label.

How to avoid: Separate the two ideas:

• Chirality → optical activity (caused by asymmetric carbon)
• L/D → a naming system (based on Fischer projection of glyceraldehyde)

Mistake 3: Misreading the Assertion-Reason Link

The error: Choosing option (A) — "Both true, R is correct explanation of A."

Why it's wrong: The Assertion says "all except glycine are optically active" (true). The Reason says "most have L-configuration" (also true). But L-configuration does not explain optical activity — chirality does. The Reason is a separate fact, not a causal explanation.

How to avoid: Ask yourself: Does the Reason directly cause the Assertion? Here, L-configuration is a consequence of stereochemistry, not the cause of optical rotation.

Mistake 4: Forgetting Glycine is the Exception

The error: Claiming "all $\alpha$-amino acids are optically active" or "none are." …