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Question: For the following compound, number of chiral centers and stereoisomers are; ...

For the following compound, number of chiral centers and stereoisomers are;

A

1,2

B

2, 4

C

2, 3

D

3,8

Answer

B

Explanation

Solution

To determine the number of chiral centers and stereoisomers for the given compound, let's first identify the carbon atoms and their substituents. The compound is:

    CH2-COOH
    |
    CH-COOH
    |
    HO-CH-COOH

Let's analyze each carbon atom to identify potential chiral centers. A chiral center is a carbon atom bonded to four different groups.

  1. Carbon 1 (CH2-COOH): This carbon atom is bonded to two hydrogen atoms, one -COOH group, and one -CH(COOH)-CH(OH)-COOH chain. Since it is bonded to two identical hydrogen atoms, it is not a chiral center.

  2. Carbon 2 (CH-COOH): This carbon atom is bonded to:

    • A hydrogen atom (-H)
    • A carboxylic acid group (-COOH)
    • A -CH2COOH group (the top part of the molecule)
    • A -CH(OH)COOH group (the bottom part of the molecule)

All four groups attached to Carbon 2 are different. Therefore, Carbon 2 is a chiral center.

  1. Carbon 3 (HO-CH-COOH): This carbon atom is bonded to:
    • A hydroxyl group (-OH)
    • A hydrogen atom (-H)
    • A carboxylic acid group (-COOH)
    • A -CH(COOH)CH2COOH group (the upper part of the molecule, including Carbon 2 and Carbon 1)

All four groups attached to Carbon 3 are different. Therefore, Carbon 3 is a chiral center.

So, the total number of chiral centers in the compound is 2.

Now, let's determine the number of stereoisomers. For a molecule with 'n' chiral centers, the maximum number of possible stereoisomers is 2n2^n. In this case, n = 2, so the maximum number of stereoisomers is 22=42^2 = 4.

We need to check if any of these stereoisomers are meso compounds. A meso compound is an achiral compound that has chiral centers, typically due to an internal plane of symmetry or a center of inversion. Meso compounds occur when the molecule is symmetrical and has equivalent chiral centers.

Let's examine the two chiral centers (Carbon 2 and Carbon 3). The groups attached to Carbon 2 are: H, COOH, -CH2COOH, and -CH(OH)COOH. The groups attached to Carbon 3 are: H, OH, COOH, and -CH(COOH)CH2COOH.

Since the set of groups attached to Carbon 2 is different from the set of groups attached to Carbon 3, the two chiral centers are not equivalent. The molecule is unsymmetrical. For unsymmetrical molecules with 'n' chiral centers, all 2n2^n stereoisomers will be distinct and optically active (no meso forms).

Therefore, the number of stereoisomers is 22=42^2 = 4.

The compound has 2 chiral centers and 4 stereoisomers.