Question

How many stereoisomers are possible in

Answer 1

4

Answer 2

The given molecule is 2,3-dichloropentane. The structure is CH3-CH(Cl)-CH(Cl)-CH2-CH3.

We need to identify the chiral centers. A chiral center is a carbon atom bonded to four different groups.

Carbon 2 is bonded to CH3, H, Cl, and -CH(Cl)CH2CH3. These are four different groups. So, carbon 2 is a chiral center.

Carbon 3 is bonded to -CH(Cl)CH3, H, Cl, and -CH2CH3. These are four different groups. So, carbon 3 is a chiral center.

Carbon 1, 4, and 5 are not chiral centers.

The molecule has two chiral centers, C2 and C3. Let's denote them as n=2.

The maximum number of stereoisomers is 2^n = 2^2 = 4.

The possible configurations for the two chiral centers are (R,R), (S,S), (R,S), and (S,R).

We need to check for the existence of a meso compound. A meso compound is a stereoisomer that is achiral despite having chiral centers, due to the presence of an internal plane of symmetry.

To check for a meso compound, we need to see if any stereoisomer is superimposable on its mirror image. This often occurs when there is a plane of symmetry in a specific conformation. For molecules with two chiral centers, a meso compound exists if the molecule has a plane of symmetry when the configurations are (R,S) or (S,R), provided the two chiral centers are symmetrically substituted.

In 2,3-dichloropentane, the two chiral centers are C2 and C3. The groups attached to C2 are CH3, H, Cl, and -CH(Cl)CH2CH3. The groups attached to C3 are -CH(Cl)CH3, H, Cl, and -CH2CH3. The molecule is not symmetric with respect to the substituents on the chiral centers. Specifically, the groups attached to the ends of the chain containing the chiral centers are different (CH3 and CH2CH3).

Therefore, there is no plane of symmetry that can make the (R,S) or (S,R) forms superimposable on their mirror images.

Let's consider the (R,S) and (S,R) forms. The (R,S) form is the mirror image of the (S,R) form. Since there is no internal plane of symmetry, the (R,S) and (S,R) forms are enantiomers and are chiral.

Similarly, the (R,R) form is the mirror image of the (S,S) form, and they are enantiomers and are chiral.

Since there is no meso compound, all 4 possible stereoisomers are distinct. These are (R,R), (S,S), (R,S), and (S,R).

The pair (R,R) and (S,S) are enantiomers. The pair (R,S) and (S,R) are enantiomers.

(R,R) and (R,S) are diastereomers. (R,R) and (S,R) are diastereomers. (S,S) and (R,S) are diastereomers. (S,S) and (S,R) are diastereomers.

All four stereoisomers are distinct and are not superimposable on each other.

Therefore, there are 4 possible stereoisomers.