R/S nomenclature: Priority Rules

RS nomenclature is currently the preferred system for assigning absolute configuration to chiral molecules. The letters R and S come from the Latin words ‘Rectus‘ and ‘Sinister‘ meaning ‘right’ and ‘left’. Molecules that rotate the plane of polarized light to right are referred to as ‘R isomers’ and the molecules that rotate the plane of polarized light to left are referred to ‘S isomers’.

Introduction to R/S nomenclature

R and S nomenclature system is used to assign absolute configurations. This system replaced the old D and L system of nomenclature. R-S system is based upon certain rules devised by R. S. Cahn, Sir Christopher Ingold, and Vladimir Prelog in 1965. They firstly developed a ranking system to deal with the problem of absolute configurations at a chiral center. These rules are also known as Cahn-Ingold-Prelog (CIP) rules or sequencing rules.

Cahn, Ingold, and Prelog (CIP) / Sequencing Rules

According to Cahn-Ingold-Prelog rules, the sequencing criterion should be based on priorities or rankings. A chiral atom always has four different substituents. All of those groups have been assigned priorities for knowing the exact direction of rotation of plane-polarized light, which will happen if the substance is made to go through with the experiment. These priorities are based on the shielding effects and electronic cloud densities, that atoms have, which become the cause for the rotation of planes.

Four substituents on chiral tetrahedral carbon atoms are ranked according to decreasing priority by the sequencing rules.

Rule zero (The ground rule)

A chiral carbon has four different substituents attached. Out of which, the atom or group with the least priority must be on the backside of the plane. It means that the atom or group with the least priority must show a dotted wedge (dashed lines).

Rule no.1 (Atomic numbers)

First of all, there should be an identification of the substituents at the chiral center and ranking them in order of decreasing priority.

Priority is given based on the proton/atomic number (Z) of atoms or groups directly attached with chiral carbon.

For example

R/S nomenclature rule no 1

The chlorine atom (Cl) has an atomic number 17, oxygen (O) has 8, carbon (C) has 6, and hydrogen (H) has 1. So, Cl is ranked the first position, oxygen, the second, with methyl the third and hydrogen the last which is entirely not important.

This molecule becomes an ‘R configuration’ compound.

Rule no.2 (Secondary priority)

If two or more of the atoms or groups are the same, the atomic numbers of the secondary atoms determine the priority order.

For example

R/S nomenclature rule no 2

In the above example, both groups (2) and (3) are carbon atoms. But the carbon atom of group 2 is bonded to another carbon atom and two hydrogen atoms, whereas the group 3 carbon is directly bonded to three hydrogen atoms only. Thus, ethyl (group 2) is preferred over methyl (group 3).

The molecule has the ‘R configuration’.

Rule no.3 (Post-secondary positions)

If the choice of order is not possible based on the secondary atoms too, priority moves to the third, fourth, and further atoms until there is a point of distinction.

R/S nomenclature rule no 3

In the above compound, both groups (2) and (3) have the same first three attachments, but the third carbon of group 2 is bonded with oxygen and two hydrogen atom whereas the third carbon of group 3 is bonded to three hydrogen atoms only. So, group 2 is prior to group 3.

Thus, the above compound is an ‘R configurated’ compound.

Rule no.4 (Isotopic priority)

In the case of isotopes, an isotope with a higher atomic mass will have higher priority over the one with a lower atomic mass.

For example

R/S nomenclature rule no 4

The above compound shows an R configuration in nomenclature because deuterium (1D2) is heavier than hydrogen (1H1).

Rule no.5 (Branches with multiple bonds)

If the branched (secondary) atoms are linked by double or triple bonds, it is counted twice and thrice. The priority order will drive by the number of bonds.

For example

R/S nomenclature rule no 5a

The double bond corresponds to two carbons linked with the branched atom and a triple bond means a link with three such atoms. Obviously, a triple bonded atom will be preferred over a doubly bonded one because it apparently has more atomic numbered atoms joined. So,

R/S nomenclature rule no 5

The above is an ‘R configured’ compound because a triple bond will be prioritized over a double one.

Rule no.6 (Rotation of molecules)

After ranking substituents orient the molecules in such a way that the lowest ranked substituent is pointed away from the viewer if it is not already there as mentioned in rule zero. The sequence of the other three substituents is noted in the order of decreasing priority after the lowest priority group is sent back of the plane with dotted lines (wedged bond).

What must be kept in mind while doing so is that the bonds never break (for nomenclature purposes). The complete molecule will rotate if one of the groups is grasped and rotated to send it backward.

For example

R/S nomenclature rule no 6

Because of this swapping, an apparent ‘R configuration’ compound is exposed to be actually an ‘S configuration’ compound.

Rule no.6 (extended)

In the order of decreasing priority if the other three substituents of a rotated molecule are in a clockwise direction, the absolute configuration is ‘R’. In case if the other three substituents are in an anticlockwise direction, the absolute configuration will be the ‘S’ one.

For example

R/S nomenclature rule no 8

Key Takeaways

  • R refers to the right and S to the left direction of plane rotation for plane polarized light.
  • While rotating molecules in your mind to take the lowest priority group away, always take molecules to be like balls that rotate wholly.

Related Topics

Functional groups priority

Priority order Functional group
1 Iodine (I)
2 Bromine (Br)
3 Chlorine (Cl)
4 Sulphonic acid (SO3H)
5 Thiol (SH)
6 Fluorine (F)
7 Alkoxy (OR)
8 Hydroxy (OH)
9 Nitro (NO2)
10 Amino (NR2)
11 Ester (COOR)
12 Carboxylic acid (COOH)
13 Ketone (RCOR)
14 Aldehyde (RCOH)
15 Alcohol (ROH)
16 Nitrile (CN)
17 Phenyl (Ph)
18 Tritium (T)
19 Deuterium (D)
20 Hydrogen (H)

Concepts berg

What is the R/S system of nomenclature?

R/S system of nomenclature is a naming system used for assigning absolute configuration to chiral molecules. R, Latin ‘Rectus‘ meaning right, and S, Latin ‘Sinister’ meaning left. On a priority basis, branches of a chiral molecule are assigned numbers, and in decreasing trend, molecules are either named R, or S configured.

How do you write R and S configuration?

If a plane polarized light is rotated by a chiral molecule in a clockwise direction, it is an R configurated compound. Whereas, if the plane of that polarized light is rotated anticlockwise, it is S configurated.

How to determine R/S configuration when there is a tie?

When there is a tie between R and S configured compounds in a solution, the plane of polarized light rotated by one is canceled by the other one. It means that there is no shift of plane, which is only possible if the concentration of both types of compounds is exactly equal. This type of tie in solutions is termed racemic mixture.

What is the difference between R and S enantiomers in chemistry?

R enantiomer rotates plane-polarized light in a clockwise direction while S enantiomer rotates plane-polarized light in an anticlockwise direction.

Is clockwise rotation R, or S configuration?

R meaning right by the Latin word ‘rectus’ refers to the right side rotation of plane-polarized light. On the other hand, S means left by the Latin, ‘sinister’ refers to left.

References

  • Organic Chemistry (MasteringChemistry) 9th Edition by Leroy Wade

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