The Cahn-Ingold-Prelog ( CIP ) sequence rule , named for organic chemist Robert Sidney Cahn, Christopher Kelk Ingold, and Vladimir Prelog - or called CIP priority rules , the system , or convention - is a standard process used in organic chemistry for completely and without a stereoisomeric name of a molecule. The purpose of the CIP system is to assign R or S descriptor for each stereocenter and E or Z descriptor for each double bond so that the configuration of all molecules can be uniquely determined by entering descriptor in its systematic name. A molecule can contain a number of stereofer and a number of double bonds, and each usually gives rise to two possible isomers. A molecule with integers that explains the number of stereogenic centers will usually have stereoisomers, diastereomers each have a pair of related enantiomers. The CIP sequence rules contribute to the exact naming of each stereoisomer of each organic and organometallic molecule with all ligand atoms less than 4 (but including ligancy 6 as well, this term refers to the "number of neighboring atoms" attached to the center).
The key article establishing the CIP sequence rules was published in 1966, and followed by further improvements, before being incorporated into the International Union of Pure and Applied Chemistry (IUPAC) rules, the official body defining organic nomenclature. The IUPAC presentation of the rules is the official official standard for its use, and it notes that "this method has been developed to cover all compounds with ligands up to 4... and... [extended for the case] ligancy 6... [also] for all configurations and conformation of such compounds. "Nevertheless, although the IUPAC documentation presents a comprehensive introduction, it includes a warning that" it is important to study original papers, especially paper 1966, before using sequence rules for other than simple cases. "
Video Cahn-Ingold-Prelog priority rules
Steps to name
The steps to naming molecules using a CIP system are often presented as:
- Identification of stereocenters and double bonds;
- Priority assignments to groups bound to each stereocenter or double bond; and
- Appointment of R/S and E/Z descriptors
Priority setting
The explanations of R/S and E/Z are assigned by using the system to prioritize the ranking of the groups attached to each stereocenter. This procedure, often known as sequence rule , is the heart of the CIP system.
- Compare atomic number (Z) atom connected directly to stereocenter; groups that have higher atomic atomic numbers receive a higher priority.
- If there is a tie, we should consider the atoms at a distance of 2 from the stereocenter - as the list is made for each group of atoms attached to atoms directly attached to the stereocenter. Each list is arranged in order of decreasing atomic number. Then the list compares atomic by atom; at the earliest discrepancy, a group containing higher atomic atomic numbers receives a higher priority.
- If there is still a bond, every atom in each of the two lists is replaced with a sub-list of other atoms attached to it (at a distance of 3 from the stereocenter), the sub-list is arranged in the order of the atomic number, and the whole structure again than atoms with atoms. This process is repeated, each time with atoms one bond farther from the stereocenter, until the bond is broken.
Isotope
If two groups differ only in isotopes, the atomic mass is used at each step to break the bond in the atomic number.
Double and triple bonds
If an atom A binds to an atom B , A is treated as a single bond on two atoms: B and an atomic ghost which is a duplicate of B (has the same atomic number) but is not attached to anything but A . When B is replaced with the list of attached atoms, A itself is excluded in accordance with the general principle of not doubling the recently attended bundle. Triple bonds are handled in the same way except that both A and B have duplicated the 'ghost' atom.
Geometric Isomers
If two substituents on the atom are geometric isomers, the Z isomer has a higher priority than the E-isomer.
Cycles
To handle a molecule containing one or more cycles, one must first develop it into a tree (called hierarchical digraph ) by traversing the bonds in all possible paths starting at the stereocenter. When the traversal meets an atom passed by the current path has passed, the ghost atom is generated to keep the tree remains limited. A single atom of the original molecule can appear in many places (some as ghosts, some not) in the tree.
Specify descriptors
Stereocenters: R / S
Once the substituent of the stereocenter has been set priority, the molecule is oriented to space so that the group with the lowest priority is shown away from the observer. If the substituent is numbered from 1 (highest priority) to 4 (lowest priority), then the sense of rotation of curves passing through 1, 2 and 3 distinguishes stereoisomers. A center with rotation of rotation clockwise is the center of R or rectus and the center with a sense of rotation opposite the clock is S or sinister center. The names are from Latin for right and left, respectively.
The practical method for determining whether an enantiomer is R or S is by using the right hand rule: one wraps the molecule with the radius in the direction of 1-> 2-> 3. If the mother finger pointing toward the 4th substituent, the enantiomer is R . Otherwise it's S .
It is possible in the rare case that the two substituents in the atom differ only in their absolute configuration ( R or S ). If the relative priority of this substituent needs to be set, R takes priority over S . When this happens, the descriptor of the stereocenter is lowercase ( r or s ) instead of the commonly used capital letter.
Double bond: E/Z
For alkene and similar double bond molecules, the same preemptive process is followed for substituents. In this case, the placement of the two highest priority substituents concerns an important double bond. If the two high priority substituents are on the same side of the double bond, ie in cis configuration, the stereoisomers are given Z or Zusammen configuration . If, instead they are in a trans configuration, the stereoisomer is given E or Entgegen configuration . In this case the letters of identification are derived from German for 'joint' and 'contrary to', respectively.
Example
The following is an example of applying nomenclature.
Maps Cahn-Ingold-Prelog priority rules
Describes some centers
If a compound has more than one stereocenter, each center is denoted by R or S. For example, ephedrine exists with configurations (1R, 2S) and (1S, 2R), known as enantiomers. This compound also exists with configurations (1R, 2R) and (1S, 2S). The last two stereoisomers are not ephedrine, but pseudoephedrine. All isomers are 2-methylamino-1-phenyl-1-propanol in a systematic nomenclature. Pseudoephedrine is chemically distinct from ephedrine only with a three-dimensional configuration in space, as denoted by the Cahn-Ingold-Prelog rule. Both compounds, ephedrine and pseudoephedrine, are diastereomers, or non-enantiomeric stereoisomers. They have different names because, as diastereomers, they have different chemical properties.
In pairs of enantiomers, all descriptors are opposite: R, R and S, S or R, S and S, R. Diastereomers have one of the same descriptors: R, S and R, R or S, R and S, S. This applies to compounds with more than two stereocenters; if at least one descriptor is the same in both pairs, then the compound is a diastereomer. If all stereocenters are opposite, they are enantiomers.
Relative configuration
The relative configuration of two stereoisomers can be represented by descriptors R and S with asterisks (*). "R *, R *" means two centers having identical configurations (R, R or S, S); "R *, S *" means two centers having the opposite configuration (R, S or S, R). To begin, the lowest number (according to IUPAC systematic numbering) of the stereogenic center is given R * descriptor.
To designate two relative anomers stereodescriptors alpha (?) And beta (?) Used. In? anomer anomeric carbon atom and reference atom have the opposite configuration (R, S or S, R), whereas in? their anomers are the same (both R or both S).
Face
Stereochemistry also plays the role of assigning faces to trigonal molecules such as ketones. The nucleophile in the nucleophilic addition can approach the carbonyl group from two sides or the opposite face. When the achiral nucleophile strikes acetone, both faces are identical and there is only one reaction product. When the nucleophile attacks the butanone, the face is not identical ( enantiotopic ) and the result of a racemic product. When a nucleophile is a crucial molecule a diastereoisomer is formed. When one face of a molecule is shielded by substituents or geometric boundaries compared to other faces, the face is called diastereotopic. The same rule that determines stereocenter stereochemistry (R or S) also applies when assigning faces to molecular groups. The faces are then called faces-red and the-face . In the example shown on the right, the acetophenone compound is seen from the back face. The addition of a hydride as in the reduction process from this side will form the S-enantiomer and the attack of the opposite Si will give R-enantiomer. However, one should note that adding a chemical group to the proletal center of the back face will not necessarily lead to S stereosenter, because the chemicals group priority should be taken into account. That is, the absolute stereochemistry of a product is determined by itself and not taking into account where it was attacked. In the above-mentioned example, if chloride ( Cl - ) is added to the projective center of the back face , this will produce R-enantiomer.
References
Source of the article : Wikipedia
0 Comments