In which of the following molecules carbon atom marked with asterisk (*) is a stereocentre or chiral centre?

Question

Prepp Team · Accepted Answer

Understanding Stereocenters and Chiral Centers

A stereocenter, also known as a chiral center, is typically a carbon atom that is bonded to four different atoms or groups of atoms. The presence of a stereocenter usually leads to the molecule being chiral, meaning it is non-superimposable on its mirror image.

To identify a stereocenter in a molecule, we need to examine each carbon atom and check if it is bonded to four distinct substituents. The question asks us to find the molecule where the carbon atom marked with an asterisk (*) is a stereocenter.

Analyzing Each Option for Stereocenters

Option 1: Molecule with *C bonded to H, Br, Cl, OH

In this molecule, the carbon atom marked with * is bonded to:

A Hydrogen atom (H)
A Bromine atom (Br)
A Chlorine atom (Cl)
A Hydroxyl group (OH)

These four substituents (H, Br, Cl, OH) are all different from each other. Therefore, the carbon atom marked with * in this molecule is bonded to four different groups.

Based on the definition, this carbon atom is a stereocenter or chiral center.

Option 2: Molecule with *C bonded to H, CH₃, H, C₂H₅

In this molecule, the carbon atom marked with * is bonded to:

A Hydrogen atom (H)
A Methyl group (CH₃)
A Hydrogen atom (H)
An Ethyl group (C₂H₅)

The carbon atom is bonded to two Hydrogen atoms, which are identical substituents. Since it is not bonded to four different groups, this carbon atom is not a stereocenter.

Option 3: Molecule with *C bonded to OH, CH₃, H, CH₃

In this molecule, the carbon atom marked with * is bonded to:

A Hydroxyl group (OH)
A Methyl group (CH₃)
A Hydrogen atom (H)
A Methyl group (CH₃)

The carbon atom is bonded to two Methyl groups (CH₃), which are identical substituents. Since it is not bonded to four different groups, this carbon atom is not a stereocenter.

Option 4: Molecule with *C bonded to H, Cl, H, F

In this molecule, the carbon atom marked with * is bonded to:

A Hydrogen atom (H)
A Chlorine atom (Cl)
A Hydrogen atom (H)
A Fluorine atom (F)

The carbon atom is bonded to two Hydrogen atoms, which are identical substituents. Since it is not bonded to four different groups, this carbon atom is not a stereocenter.

Conclusion

Comparing the analysis of all options, only the carbon atom marked with an asterisk (*) in the first molecule is bonded to four distinct substituents (H, Br, Cl, and OH). Therefore, the first molecule contains a stereocenter at the indicated position.

Revision Table: Identifying Stereocenters

Option	Substituents on *C	Are all substituents different?	Is *C a Stereocenter?
1	H, Br, Cl, OH	Yes	Yes
2	H, CH₃, H, C₂H₅	No (Two H)	No
3	OH, CH₃, H, CH₃	No (Two CH₃)	No
4	H, Cl, H, F	No (Two H)	No

Additional Information on Stereochemistry Concepts

Understanding stereocenters is fundamental in stereochemistry. Here are some related concepts:

Chirality: A molecule is chiral if it contains at least one stereocenter and does not have a plane of symmetry or a center of inversion. Chiral molecules are non-superimposable on their mirror images.
Enantiomers: These are stereoisomers that are non-superimposable mirror images of each other. Molecules with a single stereocenter exist as a pair of enantiomers.
Diastereomers: These are stereoisomers that are not mirror images of each other. Molecules with more than one stereocenter can exist as diastereomers.
Meso Compounds: These are achiral molecules that contain stereocenters. They have an internal plane of symmetry, making them superimposable on their mirror images despite having chiral centers.

Identifying stereocenters is the first step in determining the possible stereoisomers of a molecule and understanding their properties.