Relative & Absolute Configuration

DR. R. SRINIVASANDEPT OF PHARMACEUTICAL CHEMISTRYPALLAVAN PHARMACY COLLEGE

ConfigurationArrangement of atoms or gps of atoms around the asymmetric centre

3 difference conventions are: D&L, d&l and +&- 2 are same used to indicate the sign of rotation of 2 enantiomers

Relative configuration(D & L) compares the arrangement of atoms in space of one compound with those of another.

Absolute configuration is the precise arrangement of atoms in space.There is no significance difference between sign of rotation &

configuration of an enantiomers because of a compound and derivative have differences in sign of rotations eg. Lactic acid

Relative configuration compares the arrangement ofatoms in space of one compound with those of anotheruntil the 1950s, all configurations were relative

Absolute configuration(actual arrangement of atom inspace) is the precise arrangement of atoms in space.

we can now determine the absolute configurationof almost any compound

Configuration

Relative configurationAny compd prep from

or converted into D glyceraldehyde and prep or converted into Lglyceraldehyde are called relative configuration eg.

D-glyceraldehyde to D Glyceric acid

No bonds are made or broken at the chiral carbonin this experiment. Therefore, when (+) d-3-buten-2-ol and (+) d -2-butanol have the same sign of rotation, the arrangement of atoms in space at the chiral carbon atom is analogous. The twohave the same relative configuration.

CH3CHCH2CH3

OH

Pd

[a] + 33.2° [a] + 13.5°

Relative configuration

CH3CHCH

OH

CH2

Not all compounds that have the same relativeconfiguration have the same sign of rotation. No bondsare made or broken at the chiral carbon in thereaction shown, so the relative positions of the atoms are the same. Yet the sign of rotation can change.

CH3CH2CHCH2Br

CH3

HBr

[a] -5.8° [a] + 4.0°

Relative configuration

CH3CH2CHCH2OH

CH3

Absolute configuration

Clears the basic defects of D&L(relative configuration)were over come when we study in structure of more than one asymmetric C atom.

However it is overcome by Cahn Ingold & Prelog (1956, 1964) which is based on three dimensional formula

Three imp features of absolute configuration (R&S system Nomenclature) are:

1. Four different group attached to the asymmetric carbon atom or atoms are arranged in a priority sequence in accordance to the sequences rulesAfter assigning the priorities of the 4 group or atoms attached to chiral carbon the molecules is imagined to be a position where the atom or group of lowest i.e., arrangement towards clockwise direction is specified as R & anticlockwise direction is R

2. R –rectus(latin) for right S-sinister – left used as D & L series

3. When the molecule is more than one asymmetric centre the procedure is applied to each for explain the of compound towards clockwise R configuration and anticlockwise S configuration

Naming from the Perspective Formula

1

23

4

1. Rank the groups bonded to the asymmetric carbon

2. If the group (or atom) with the lowest priority is bonded by hatched wedge,

3. If necessary, rotate the molecule so that the lowest priority group (or atom) is bonded by a hatched wedge

4.

HH3C

H

H

chiral carbon in a ring

R

—CH2C=C > —CH2CH2 > —CH3 > —H

Rules for Fischer projections

Arrange the molecule so that horizontal bonds at chiral carbon point toward you and vertical bonds point away from you.

Br Cl

F

H

Rules for Fischer projections

Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.

Br Cl

F

H

Rules for Fischer projections

Projection of molecule on page is a cross. When represented this way it is understood that horizontal bonds project outward, vertical bonds are back.

Br Cl

F

H

SEQUENCE RULE: 4 RULES ARE GOVERN THE ARRANGMENT OF 4 DIFFERENT GROUP AROUND AN ASYMMETRIC C ATOM

RULE 1: In case all the four atoms directly attached to the asymmetric carbon atom are different from one another, sequence of priorities is determined by atomic numbersThe atoms of higher molecular weight higher priority and lower molecular weight the last priority

Rule 2: If the two atoms directly attached to asymmetric centre having same atomic number

the priority may be determined by comparing the next atom and so on

Rule 3: A double or triple bonded atom X is equivalent to two or less such atom thus the priority of order is aldehyde or CooH gp or OH gp

Rule 4: In case of geometrical stereo isomeric group the cis gp precedes trans gp similarly isomer R precedes S gp In case of pseudosymmetric centre the symbol R & S replaced r & s