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DL and RS System of Nomenclature of Optical Isomers and Sequence Rules

A molecule is named by relating it to glyceraldehyde by means of the dexter/leaf system ("right" and "left" respectively).

DL System of Nomenclature of Optimal Isomers

Around 1900, Fischer and Rosanoff developed the d/l system. Since the OH group on the C2 is attached to the right side of the molecule, (+) glyceraldehyde was arbitrarily considered D. (-) Glyceraldehyde was classified as L due to an OH group on the left.

A molecule is named by relating it to glyceraldehyde by means of the dexter/leaf system ("right" and "left" respectively). In this nomenclature, amino acids and carbohydrates are distinguished by their enantiomers. Such arbitrary configurations are known as "Relative Configurations".

  • In Fischer projection, carbonyl groups (aldehyde, ketone, carboxylic acid) will be positioned on top, and CH2OH will be positioned on the bottom.
  • For carbon 2, the OH or NH2 is used as the D descriptor when it is on the right side, while the L descriptor is used when it is on the left side. Thus, it is possible to determine the stereochemistry of all other stereocenters in a molecule by the stereochemistry of its second carbon from the bottom.
  • Dextrorotatory and levorotatory enantiomers are not specified by the d/l nomenclature. A similarity can be found between its stereochemistry and that of the levo or dextro enantiomer of glyceraldehyde. Levorotatory substances, such as d-fructose, are examples. Thus, all natural amino acids have the L configuration, whereas all natural carbohydrates have the D configuration. In this sense, (+) glucose has a D-configuration, while (+) ribose has an L-configuration.

Limitations of the Sugar Convention

  • There are multiple chirality centers in sugar molecules. The configuration of the chirality center with the highest number is shown, but that of the other centers is not shown (hidden in the names).
  • In the conversion of compounds from one series to another, another complication occurs. D-configuration is assigned to saccharic acid, which is obtained from D-glucose through oxidation. The carbon with the highest number of chiral configurations is D. The highest chiral configuration then is L, which is formula 25. However, there is a serious problem with this approach, in that the same molecule can possess both D- and L- configurations.
  • It may be necessary to designate (+)-tartaric acid and (-)-threonine as D or L depending on whether the reference compound is glyceraldehyde (the highest number of chiral carbons) or hydroxyl or amino acid (the lowest number of chiral carbons). Additionally, this system is limited to sugars, amino acids, and hydroxy acids and is therefore of limited use.
  • Hydroxy acid or amino acid convention
A chirality center is indicated by the D- and L- prefixes when the lowest numbered α-hydroxy or α-amino acid of the Fischer projection formula is used. In the case of groups α-OH or α-NH2, the prefix D- is used if they are on the right side, while in the case of groups on the left, the prefix L- is used.

RS System of Nomenclature of Optimal Isomers

Robert Sidney Cahn, Christopher Ingold, and Vladimir Prelog, developing a nomenclature system in 1956, led to the assignment of chirality centers in molecules to their absolute configurations. It provides accurate and unambiguous names for chiral molecules regardless of the presence of multiple asymmetric centers when the RS nomenclature system is added to the IUPAC nomenclature system.

As light passes through a solution containing chiral molecules, they usually rotate plane-polarized light. Rotation of plane-polarized light does not reveal the RS configuration of chiral compounds; thus, it should be emphasized that this is not true. It is also possible to describe the structure of chiral molecules using the Fischer-Rosanoff convention. This system labels the entire molecule rather than each chirality center, and often returns ambiguous results when describing molecules with more than one chirality center.

Priority Rules of RS System

There is a curved arrow to indicate whether the chirality center is R or S, depending on the priority sequence assigned to the group attached to it.

First Rule

On the basis of the atomic number of the atom connected directly to the chiral center, priority sequence is assigned to each group.
  • Atoms with the highest atomic number are given priority.
  • Atoms with the lowest atomic number are given the lowest priority.
Accordingly, an oxygen atom, O, with an atomic number of 8, a carbon atom, C, with an atomic number of 6, a chlorine atom, Cl, with an atomic number of 17, and a bromine atom, Br, with an atomic number of 35 are attached to the chiral center in this manner: Br > Cl > O > C. Isotopes are ranked according to atoms with the highest atomic mass.

Second Rule

In order to determine the priority sequence, the atomic number of the next atom bound to the chirality center is used, and this number is increased from the center until the first point of difference is reached. In other words, different groups attached to the chirality center by the same atoms are assigned different priorities based on the atomic number. A chiral center consists of three groups attached to the same atom: -CH3, -CH2CH3, and -CH2OH. When we examine the next atom bound, we find:

As oxygen has a higher atomic number than carbon, and carbon is higher than hydrogen, oxygen comes first in the priority order

CH2OH > –CH2CH3 > –CH3

Some groups are listed in priority order as follows:

–I > –Br > –Cl > –SH > –OR > –OH > –NHR > –NH2 > –COOR > –COOH > –CHO > –CH2OH > –C6H5 > –CH3 > –2H > –1H

A chirality center cannot be chiral if the groups attached to it have similar priority rankings. In order to align molecules in space properly, the lowest priority group must face away from the viewer, then be situated behind the chiral center. Make a circle from the highest to the lowest priority group by tracing a curved arrow.
  • A clockwise rotation yields R as the configuration of the chiral center, from the Latin rectus, which means right.
  • The chiral center of a circle is shaped like an S, based on the Latin sinister, which means "left".

As indicated in rule three of the RS system, when chirality centers have double or triple bonds in their groups, we can determine the configuration of the chirality center. Assigning priorities to atoms with double and triple bonds requires the consideration of duplicates and triples.

Double bonds are formed when one Y atom is attached to one carbon atom, and the other way around. A carbon atom is connected to three Y atoms at the same time a Y atom is connected to a carbon atom. This is known as a C ≡ Y triple bond.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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