Question

Indicate whether the asymmetric atom in each of the following compounds has the R or S configuration.

Indicate whether the asymmetric atom in each of the following compounds has the R or S configuration.

Answer 1

Concepts and reason

Asymmetric carbon (chiral center) can be identified, when the carbon atom is bonded with four different groups or atoms.

Assign the R and S configurations to the asymmetric carbon atoms.

Fundamentals

Chirality center: An atom (usually carbon) which is bonded to four different atoms or groups is known as chiral center. In general, it is represented with an asterisk (*).

An asymmetric carbon atom is carbon atom that is attached to four different types of atoms or groups. This leads to a chiral center.

A stereocenter is a point in a molecule bearing groups, such that it produces two stereoisomers on interchanging any two groups. A chiral carbon may also be a stereocenter.

R and S configuration can be assign to the compound by using the following CIP (Cahn-Ingold-Prelog) sequence rules:

Rule 1: Select the chiral center in the compound and assign priorities to the atoms based on its atomic number. The atom which is having highest atomic number gets the first priority and atom which is having least atomic number (usually hydrogen) gets fourth priority.

Rule 2: If the isotopes of same atom are attached to the chiral center, then then atom with the higher atomic mass receives higher priority.

Rule 3: If the first atom of each substituent is same then give priority to the second atom in the each substituent.

Rule 4: If the substituents have multiple bonds, then the multiple bonded atoms are considered as same number of single boned atoms.

If the rotation of the numbering is in clockwise direction, then the configuration of the isomer is R. Similarly, if the rotation of the numbering is in anticlockwise direction, then the configuration of the isomer is S.

The structure of alanine is as follows:

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The rotation of the numbering is as follows:

The configuration is S

In the structure of malic acid, assign the groups in priority order as follows:

The rotation of the numbering is as follows:

Ans:

The configuration for the asymmetric carbon atom in alanine is S.

The configuration for the asymmetric carbon atom in malic acid is S.