Nomenclature of Optical Isomers

Nomenclature of Optical Isomers

Session Objectives

By the end of this session, students will be able to:

• Explain D & L system

• Explain the rules for assigning R & S Configuration

Enantiomers can be described as (+) or (-)

• The direction in which light is rotated is not dependent on whether a stereogenic centre is R or S

• An (R) compound is equally as likely to be (+) as (–)—of course, if it is (+) then its (S) enantiomer must be (–)

• The labels (+) and (–) were more useful before the days of X-ray crystallography, when chemists did not know the actual configuration of the molecules they studied, and

• Could distinguish two enantiomers only by the signs of their specific rotations

• We have already seen that a (+) or (-) sign indicates the optical activity of an enantiomer

• Optical activity does not tell us the actual configuration of an enantiomer

• It only gives us the information whether an enantiomer rotates the plane-polarized light clockwise or anti-clockwise

How to designate configuration of enantiomer?

• Two systems to designate configuration of enantiomers: D and L system

• R and S system (also known as the Cahn–Ingold–Prelog system)

D and L system

• Emil Fischer used glyceraldehyde as a standard for the D and L system of designating configuration

• He arbitrarily took the (+)-glyceraldehyde enantiomer and assigned this as D-glyceraldehyde

• Other enantiomer is the (-)-glyceraldehyde and this was assigned as L-glyceraldehyde

• Only difference in the following structures, which is the orientation of the hydroxyl group at the chiral center

• In the case of D-glyceraldehyde the –OH group on the chiral carbon is in on the right hand side, whereas in L-glyceraldehyde it is on the left

• No correlation between D and L configurations, and (+) and (-) rotations

• It can be D(+) or D(-) and L(+) or L(-)

• D and L system is common in biology/biochemistry especially with sugars and amino acids

• For example, D-glucose, L-rhamnose and L-alanine

Describing R & S Configuration

• Set of rules to assign a letter R or S, to describe the configuration of groups at chiral center in the molecule

• R- From the Latin, rectus, straight, correct; to show that the order of priority of groups on a chiral center is clockwise

• S- From the Latin, sinister, left; to show that the order of priority of groups on a chiral center is counterclockwise

• For a given sample of a pure enantiomer, the absolute configuration must be determined experimentally by X-ray analysis of a derivative that has a chiral center with known configuration

• A system for designating the absolute configuration of a chiral center was devised in the late 1950s by R. S. Cahn and C. K. Ingold in England and V. Prelog in Switzerland and is named after them with CIP rules

• It has been incorporated in IUPAC rules of nomenclature also

Priority rules

Rule 1: Each atom bonded to the chiral center is assigned a priority based on atomic number; the higher the atomic number, the higher the priority

Rule 2: If priority cannot be assigned on the basis of the atoms bonded directly to the chiral center look at the next set of atoms and continue until a priority can be assigned.

Rule 3: Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar “phantom” atoms by single bonds.

• That is, atoms of the double bond are duplicated, and atoms of a triple bond are triplicated.

Note: Priority assignment is made at the first point of difference between groups.

• A common mistake is to assume that larger groups must always have higher priority, but this might not necessarily be the case. For example, a –CH2Cl group has priority over a -CH2CH2CH2CH3 group because the Cl atom is the first point of difference

Rule 4: Having decided on the priority of the four groups, one has to arrange (rotate) the molecule in such a way that group 4, i.e. the lowest priority, is pointing away from the viewer

• Then an arrow from group 1 to 2 to 3 is to be drawn. If the direction is clockwise, it is called an (R)-isomer. If it is anti-clockwise, it is called an (S)-isomer

• R & S enantiomers of 2,3-dihydropropanoic acid

• When there is more than one stereocentre (chiral carbon) present in a molecule, it is possible to have more than two stereoisomers.

• It is then necessary to designate all these stereoisomers using the (R) and (S) system. In 2,3,4-trihydroxybutanal, there are two chiral carbons at C-2 & C-3

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