Cahn–Ingold–Prelog priority rules

The Cahn -Ingold -Prelog convention (short: CIP convention or ( RS)- System) is used to uniquely describe the spatial arrangement of the different substituents on atoms or double bonds. The CIP convention was proposed in 1966 by Robert Sidney Cahn, Christopher Kelk Ingold and Swiss Nobel laureate Vladimir Prelog and revised in 1982 by Vladimir Prelog and Günter Helmchen.

The purpose of the CIP nomenclature is:

• The determination of the absolute configuration [(R ) - or ( S)- descriptor ] of the substituents at the stereocenter of a molecule with chiral centers
• The determination of the geometrical arrangement of [ (E ) - or ( Z)- notation ] - and (E) - or (Z) Descriptor - the substituents on the double bond of a cis -trans- isomer
• The determination of the arrangement [ (RA ) - or ( SA) descriptor ] to cumulated double bonds (see allenes )

Complex molecules with multiple stereogenic centers and / or more double bonds with cis -trans isomerism can be clearly identified in their geometric structure through the systematic IUPAC name prefixed CIP descriptors.

• 2.1 software to determine the (E ) - or ( Z)- descriptor

Procedure for determining the stereodescriptors to chirality and pseudochirality

Identification of the chirality

First, the chirality of the molecule can be identified. A is a chiral atom bearing four different substituents. To stereocenters, on most molecules of carbon atoms. However, they can also occur on the nitrogen, sulfur, silicon or phosphorus atoms. Suitable substituents include atoms, groups of atoms or lone pairs. It marks the stereocenters in the structural formula by asterisks. Each chiral center is considered individually.

Prioritization of substituents

There are considered the substituents at the chiral center. The aim is to map the four different substituents, the priorities 1 to 4.

Determination of the descriptor

The substituent with the lowest priority 4 is placed under the image plane. Then you count circle around the active site of the substituent with priority 1 to priority 3 Runs this circular motion clockwise, ie clockwise, then there is an (R )-configuration, it runs in reverse ( counterclockwise), there is a (S) -configuration. (R ) is the abbreviation of the Latin rectus (right) and (S) from the Latin sinister (left).

Special

Double bonds, and conjugated systems

Double and triple bonds are treated as if the corresponding atom or the particular group would be twice or three times exists (duplicate atoms). Duplicate atoms have by convention no substituents in the next sphere. It should be noted that double bonds between hetero atoms with at least one element from the third period, by convention, considered as single bonds ( for example, P = O and P -O interpreted ). In conjugated systems ( such as aromatics ) is a fictitious duplicate atom whose atomic number is the average of the atomic numbers of the atoms, which double bonds in resonance structures can be drawn, using, instead of the duplicate atom.

(Carbo ) cycles

On chirality of carbocycles each branch of the ring in all spheres will be considered until the starting point is reached, it will be considered only as a duplicate atom.

Some examples of the priority of the substituents are shown here in descending priority: - I> -Cl > -S- CH3 > -SH > -F > -O- CH3 > -OH > -N3 > -N ( CH3 ) 2> - NH- C6H5 > - NH2 > -COOH > - CONH 2 > -CHO > CH2OH - > CD3 > CD2H > CDH2 > - CH3 > -D > H> free electron pair

The CIP rules can also be used to uniquely determine the configuration of molecules with chiral axes, or helical chirality structures. When a molecule has multiple chiral centers, then each individual according characterizes the rules above and listed in the systematic name.

Software to determine the absolute configuration

A number of commercial software packages support the determination of the configuration of organic chemical molecules. Among other things, this is supported by the chemical drawing programs ChemDraw or Symyx Draw.

Related to the angle of rotation α

In the clockwise direction, resulting in counting down of the priorities of the substituents for the determination of the configuration (R or S) can not automatically α to the angle of rotation or the direction of rotation [( ) or (-)] of the polarization plane of the linearly polarized light can be closed. Examples:

• (S) -alanine, has a rotational angle of α 13.0 ° (c = 2 in 5 N hydrochloric acid)
• (R) -cysteine ​​has a rotation angle α of ± 7.9 ° (c = 2 in 5 N hydrochloric acid)

Molecules with multiple stereogenic centers

When a molecule contains more stereogenic centers, the configuration of each individual stereogenic center is indicated and preceded by the position number in the molecule the stereodescriptor [(R) or (S) ]. If all stereogenic centers have the same configuration, it is either the name of the compound " ( all-R ) -" or "( all-S ) -" ahead.

Double bonds (E ) - or ( Z)- notation

Alkenes and other double bonds in the molecules is used in the same process of establishing the CIP priorities for all substituents on the double bond. Then, it is checked as are the two substituents with the highest CIP priority relative to each other on the two atoms of the double bond. When the two substituents with highest CIP priority on the same side ( = arranged eclipsed ) are at the two adjacent atoms of the double bond, this stereoisomer the CIP descriptor (Z) is allocated by combination. Are, however, the two substituents with highest CIP priority relative to each other to the two atoms of the double bond to the opposite side (= anti - periplanar ) stereoisomer of this CIP descriptor (E) is allocated by contrary.

Often - but not always! - Cis isomers are also (Z)- isomers and trans-isomers at the same time (E)- isomers. In the case of disubstituted alkenes, the cis isomer is always classified as (Z )-isomer and the trans isomer as a (E )-isomer.

Software for determining the (E ) - or ( Z)- descriptor

A number of commercial software packages support the determination of the (E ) - or ( Z)- descriptor of alkenes and other groups of substances with similar double bonds, such as the chemical drawing program ChemDraw.