Question

The amino acid with R configuration is 

• A. (A)
• B. (B)
• C. (C)
• D. (D)

Hint:

To determine the R or S configuration, apply the Cahn-Ingold-Prelog priority rules and arrange the substituents accordingly.

Answer 1

The Correct Option is B

To determine which amino acid has an R configuration, we must first understand the concept of R and S configurations in stereochemistry. The R (from Latin "Rectus" meaning right) and S (from Latin "Sinister" meaning left) configurations depend on the priority of the groups attached to a chiral center according to the Cahn-Ingold-Prelog priority rules.

The priority rules are as follows:

1. Assign priorities to the substituents based on atomic number; the higher the atomic number, the higher the priority.
2. If two atoms attached to the chiral center are identical, proceed along the chain of each substituent until a point of difference is found.
3. Orient the molecule so that the group with the lowest priority (4) is pointing away from you.
4. If the path from highest priority (1) to third (3) is clockwise, the configuration is R. If counterclockwise, it is S.

Let's apply these rules to the given structures:

1. Identify the chiral center: It's typically the carbon attached to four different substituents.
2. For each amino acid, assign priorities:
   • (A) Priorities: \(CO_2H \gt NH_2 \gt CH_2CH_3 \gt H\)
   • (B) Priorities: \(CO_2H \gt NH_2 \gt CH_2SH \gt H\)
   • (C) Priorities: \(CO_2H \gt NH_2 \gt CH_2Ph \gt H\)
   • (D) Priorities: \(CO_2H \gt NH_2 \gt CH_2CH_2SCH_3 \gt H\)
3. Orient the molecule so the hydrogen (lowest priority) is away, and observe the direction from 1 → 2 → 3.

In option (B), the path from CO₂H → NH₂ → CH₂SH is clockwise when the hydrogen is in the back, indicating R configuration.

Therefore, the amino acid with the R configuration is:

(B)