Cahn-Ingold-Prelog Convention

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Example for the application of the CIP convention: the substituents are sorted according to their priority, the substituent with the lowest priority is rotated below the image plane, the direction of the circular movement along the substituents defines the absolute configuration.

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

The purpose of the CIP nomenclature is:

Complex molecules with several stereocenters and / or several double bonds with cis - trans isomerism can be clearly identified in their geometrical structure by the CIP descriptors preceding the systematic IUPAC name.

Procedure for determining the stereo descriptors at centers of chirality and centers of pseudochirality

Identification of the centers of chirality

First, the chiral centers of the molecule are identified. A center of chirality is an atom that has four different substituents . Most molecules have stereocenters on carbon atoms . But they can also occur on nitrogen , sulfur , silicon or phosphorus atoms . Atoms, groups of atoms or free electron pairs count as substituents . The stereocenters in the structural formula are marked with asterisks. Each center of chirality is considered individually.

Prioritization of the substituents

The first atoms of the substituents directly at the center of chirality are considered. The aim is to assign priorities 1 to 4 to the 4 different substituents.

  1. The atoms that are bound directly to the chiral center (these are called atoms of the first sphere) are ordered according to their atomic or atomic number , with free electron pairs receiving the fictitious atomic number 0 and thus the lowest priority. The other priorities are assigned from high to low ordinal number (priority 1: highest ordinal number, priority 2: second highest ordinal number, etc.). If two atoms are different isotopes of the same element (e.g. normal hydrogen, deuterium and tritium ), the isotope with the greater mass also has the higher priority.
  2. If two or more atoms are identical, these individual atoms are replaced by a list of all the atoms bound to them in the second sphere, again in the order of the atomic number . The lists are compared with one another, whereby the first different atom is decisive. Again, the priorities are assigned to the substituents of sphere 1 according to the ordinal number (this time of the first different atom). (Example: the side chain −CH (CH 3 ) 2 takes precedence over the side chain −CH 2 CH 2 CH 3 )
  3. If the lists of atoms in the second sphere are identical, the individual atoms in this sphere are replaced by a list of those in the third sphere, in the order of their ordinal numbers. Proceed as in point 2 again.
  4. Point 3 is repeated in the next sphere until a distinction is made.
  5. If no distinction is possible even when looking at the last sphere (the end of the molecule or, in the case of cycles, the starting atom (see below)), further distinguishing criteria must be examined in the following order:
    1. If double bonds with different configurations are present in the molecule, the ( Z ) isomer has higher priority than the ( E ) isomer in the first different position .
    2. Identical pairs of descriptors in the substituting atomic groups have priority over different ones [for example ( SS ) before ( RS )].
    3. If it is a pseudochirality center , ( R ) -configured atom groups have priority over ( S ) -configured ones.

Determination of the descriptor

Distribution of priorities "clockwise" or "counterclockwise"

The substituent with the lowest priority 4 is placed behind the image plane. Then count in a circle around the active center from the substituent with priority 1 to priority 3.If this circular movement runs to the right, i.e. clockwise, there is an ( R ) configuration ; if it runs to the left (counterclockwise), there is one ( S ) configuration. ( R ) is the abbreviation of Latin rectus (straight) and ( S ) from Latin sinister (left).

From the clockwise direction, which in the counting of the priorities of the substituents for determining the configuration ( R or S results), can not automatically on the rotation angle α or the direction of rotation [(+) or (-)] of the plane of polarization of the linearly polarized light to be closed . Examples:

  • ( S ) - Alanine has an angle of rotation α of + 13.0 ° (c = 2 in 5 N hydrochloric acid)
  • ( R ) - Cysteine has an angle of rotation α of + 7.9 ° (c = 2 in 5 N hydrochloric acid)

particularities

Double bonds and conjugated systems

Double and triple bonds are treated as if the respective atom or the respective group were double or triple (duplicate atoms). By convention, duplicate atoms have no substituents in the next sphere. It should be noted that double bonds between heteroatoms with at least one element from the third period onwards are, according to convention, regarded as single bonds (for example, P = O is interpreted as P – O). In conjugated systems (such as aromatics ), instead of the duplicate atom, a fictitious duplicate atom is used, the atomic number of which corresponds to the mean value of the atomic numbers of the atoms to which double bonds can be drawn in mesomeric boundary structures.

(Carbo) cycles

At centers of chirality on carbocycles , each branch of the ring is considered in all spheres until the starting point is reached, which is only considered as a duplicate atom.

The CIP rules can also be used to clearly determine the configuration of molecules with chiral axes , chiral planes or helical structures. If a molecule has several centers of chirality, each one is characterized according to the rules mentioned above and listed in the systematic name.

Molecules with multiple stereogenic centers

If a molecule contains several stereogenic centers, the configuration of each individual stereogenic center is given and the position number in the molecule is preceded by the stereo descriptor [( R ) or ( S )]. If all stereocenters have the same configuration, either “(all- R ) -” or “(all- S ) -” is prefixed to the name of the compound .

Software for determining the absolute configuration

A number of commercial software packages support the determination of the configuration of organic chemical molecules. This is supported by the chemical drawing programs ChemDraw or Symyx Draw , among others .

Double bonds: ( E ) or ( Z ) notation

( EZ ) nomenclature for alkenes. The CIP priority of the four substituents is
a > b and c > d .
(E) -Alkenes V.1.svg
(Z) -Alkenes V.1.svg

For alkenes and similar double bonds in molecules, one uses the same process of setting the CIP priorities for all substituents on the double bond. It is then checked how the two substituents with the highest CIP priority are relative to one another on the two atoms of the double bond. If the two substituents with the highest CIP priority are on the same side (= ecliptically arranged) on the two adjacent atoms of the double bond, the CIP descriptor ( Z ) of together is assigned to this stereoisomer . On the other hand, the two substituents with the highest CIP priority relative to each other at the two atoms of the double bond on the opposite side (= anti -periplanar) stereoisomer of this CIP descriptor (will E ) of Contrary allocated.

Often - but not always! - cis isomers are also ( Z ) isomers and trans isomers are also ( E ) isomers. In the case of disubstituted alkenes, the cis isomer must always be classified as the ( Z ) isomer and the trans isomer as the ( 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, e.g. B. the chemical drawing program ChemDraw .

Web links

See also

Individual evidence

  1. RS Cahn, Christopher Ingold, V. Prelog: Specification of the molecular chirality . In: Angewandte Chemie . tape 78 , no. 8 , 1966, pp. 413-447 , doi : 10.1002 / anie.19660780803 .
  2. Vladimir Prelog, Günter Helmchen: Basics of the CIP system and suggestions for a revision . In: Angewandte Chemie . tape 94 , no. 8 , 1982, pp. 614-631 , doi : 10.1002 / anie.19820940805 .
  3. a b c d e f Karl-Heinz Hellwich: Stereochemistry: Basic Concepts . Springer-Verlag, 2007, ISBN 978-3-540-71707-2 ( preview in Google book search).
  4. ^ A b Hans-Dieter Jakubke, Hans Jeschkeit: Amino acids, peptides, proteins . Verlag Chemie, Weinheim 1982, ISBN 3-527-25892-2 , p. 30.
  5. Albert Gossauer: Structure and reactivity of biomolecules . Verlag Helvetica Chimica Acta, Zurich, 2006, ISBN 978-3-906390-29-1 , p. 102.
  6. Jonathan Clayden, Nick Greeves, Stuart Warren, Peter Wothers: Organic Chemistry . Oxford University Press, 2001, ISBN 978-0-19-850346-0 , p. 487.