For compounds found in organic chemistry, the Cahn Ingold Prelog priority rules are used to determine the orientation of a molecule for purposes of assigning stereochemistry at a stereocenter and for assigning the name of isomers of molecules possessing double bonds such as alkenes. Jump to: navigation, search Organic chemistry is the scientific study of the structure, properties, composition, reactions, and synthesis of organic compounds that by definition contain carbon. ... Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules. ... In chemistry, isomers are molecules with the same chemical formula and often with the same kinds of bonds between atoms, but in which the atoms are arranged differently. ... Covalent bonding is a form of chemical bonding characterized by the sharing of one or more pairs of electrons between atoms, in order to produce a mutual attraction, which holds the resultant molecule together. ... Jump to: navigation, search An alkene in organic chemistry is an unsaturated hydrocarbon containing at least one carbon to carbon double bond. ...

Simply put, any atom attached to a stereocenter or alkene bond carbon (or similar double bond system) has a Cahn-Ingold-Prelog priority corresponding to its atomic number—the higher the atomic number, the higher the priority. Jump to: navigation, search Properties For alternative meanings see atom (disambiguation). ... Jump to: navigation, search A stereocenter is a carbon atom in a chemical compound that has four different types of atoms or groups of atoms attached to each side of it. ... The atomic number (Z) is a term used in chemistry and physics to represent the number of protons found in the nucleus of an atom. ...

If two atoms attached to the stereocenter have the same atomic number, then the atomic number of the atoms bonded to these atoms is compared. The atom of highest atomic number on the first bonded atom is compared to the atom of highest atomic number on the second bonded atom, then the atoms of second highest atomic number are compared, and so on. If the atoms directly bonded to the stereocenter are bonded to exactly the same set of atoms, then the two atoms of highest priority are compared in the same fashion. If these are equivalent the process would continue on the atoms of highest atomic number that are attached to the last evaluated piece. If these prove to be the same through the end of the molecule, the bonds to the atoms of second highest value would be compared next (starting these comparisons at the last point of difference, not the first.) Any double or triple bonds are counted as if the atom was attached to two or three, respectively, of the atom it is bonded to. If the atom contains specific isomers of atoms then these are compared only if everything else is the same. Covalent bonding is a form of chemical bonding characterized by the sharing of one or more pairs of electrons between atoms, in order to produce a mutual attraction, which holds the resultant molecule together. ...

Examples:

CH2OH outranks C(CH3)3

CH(CH3)2 outranks CH2CH2OH

CH=O outranks CH(CH3)OH

CH2CH2CH3 outranks CH(CH3)D, although CH2D outranks CH3

After the substituents of a stereocenter have been assigned their priorities, the molecule is so oriented in space that the group with the lowest priority is pointed away from the observer. If the lowest priority substituent is assigned the number 1, and the highest 4, then the sense of rotation of a route passing through 4, 3 and 2 distinguishes the stereoisomers. A center with a clockwise sense of rotation is an R or rectus center and a center with an anticlockwise sense of rotation is an S or sinister center. The names are derived from the Latin for right and left. In organic chemistry, a substituent is an atom or group of atoms subsituted in place of a hydrogen atom on the parent chain of a hydrocarbon. ... Jump to: navigation, search A stereocenter is a carbon atom in a chemical compound that has four different types of atoms or groups of atoms attached to each side of it. ... Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer. ... Jump to: navigation, search Latin is an Indo-European language originally spoken in the region around Rome called Latium. ...

For alkenes and similar double bonded molecules, the same prioritising process is followed for the substituents. In this case, it is the placing of the two highest priority substituents with respect to the double bond which matters. If both high priority substituents are on the same side of the double bond, ie. in the cis configuration, then the stereoisomer is assigned a Z or Zusammen configuration. If, by contrast they are in a trans configuration, then the stereoisomer is assigned an E or Entgegen configuration. In this case the identifying letters are derived from German. Jump to: navigation, search Cis-2-butene Trans-2-butene In chemistry, geometric isomerism or cis-trans isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. ... Stereoisomerism is the arrangement of atoms in molecules whose connectivity remains the same but their arrangement in space is different in each isomer. ... Jump to: navigation, search Cis-2-butene Trans-2-butene In chemistry, geometric isomerism or cis-trans isomerism is a form of stereoisomerism and describes the orientation of functional groups at the ends of a bond around which no rotation is possible. ...

It is important to note that there can be more than one of each type of system requiring assignment in a particular molecule. For example, ephedrine exists in both 1-(R), 2-(S) and 1-(S), 2-(R) forms. A compound with the same formula also exists in 1-(R), 2-(R) and 1-(S), 2-(S). Said stereoisomers are not ephedrine, but pseudoephedrine. They are chemically distinct from ephedrine, with only the three dimensional configuration in space, as notated by the Cahn-Ingold-Prelog rules to distinguish them in systematic nomenclature (both are 2-methylamino-1-phenyl-1-propanol in systematic nomenclature). The ephedrine enantiomers are referred to as being diastereoisomers of the pseudoephedrine enantiomers. In general where there are n stereocenters, there will be 2ⁿ stereoisomers possible. However, often there are situations where some of these stereoisomers are superimposable, reducing the number of different molecules which actually exist. Ephedrine (EPH) is a sympathomimetic amine commonly used as a decongestant. ... Jump to: navigation, search Pseudoephedrine is a sympathomimetic amine commonly used as a decongestant. ... In chemistry two stereoisomers are said to be enantiomers if they are mirror images of each other. ... Diastereomers are stereoisomers that are not enantiomers or mirror images of each other. ... In chemistry two stereoisomers are said to be enantiomers if they are mirror images of each other. ...

It should also be noted that a common misnomer is to label tetrahedral atoms with four distinct substituents as chiral centers. This is incorrect, since merely because a center has such a structure does not mean that it is part of a chiral molecule. It may be part of a molecule which is an optically inactive diastereoisomer. They should correctly be labelled as stereocenters or prochiral centers. Jump to: navigation, search A stereocenter is a carbon atom in a chemical compound that has four different types of atoms or groups of atoms attached to each side of it. ...

Stereochemistry also plays a role assigning faces to trigonal molecules such as ketones. A nucleophile in a nucleophilic addition can approach the carbonyl group from two opposite sides or faces. When an achiral nucleophile attacks acetone, both faces are identical and there is only one reaction product. When the nucleophile attacks butanone, the faces are not identical (enantiotopic) and a racemic product results. When the nucleophile is a chiral molecule diastereoisomers are formed. When one face of a molecule is shielded by substituents or geometric constraints compared to the other face the faces are called diastereotopic. The same rules that determine the stereochemistry of a stereocenter (R or S) also apply when assigning the face of a molecular group. The faces are then called the Re face or the Si face. Jump to: navigation, search A ketone is either the functional group characterized by a carbonyl group linked to two other carbon atoms or a compound that contains this functional group. ... In chemistry, a nucleophile (literally nucleus lover) is a reagent which is attracted to centres of positive charge. ... In organic chemistry, a nucleophilic addition reaction is an addition reaction where in a chemical compound a pi bond is removed by the creation of two new covalent bonds by the addition of a nucleophile. ... In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom. ... Jump to: navigation, search For an alternate use of acetone, see Acetone (music). ... 2-Butanone is a manufactured organic chemical but it is also present in the environment from natural sources. ... Chirality refers to several phenomena, all having to do with objects that differ from their mirror image. ... Diastereomers are stereoisomers that are not enantiomers or mirror images of each other. ...

Vladimir Prelog won the Nobel Prize in chemistry in 1975. Jump to: navigation, search Vladimir Prelog (July 23, 1906 – January 7, 1998) was a renowned chemist from Croatia who worked in Prague, Zagreb and Zurich and who won the Nobel Prize in chemistry in 1975. ... Sir Edward Appletons medal Photographs of Nobel Prize Medals. ... Chemistry (in Greek: χημεία) is the science of matter that deals with the composition, structure, and properties of substances and with the transformations that they undergo. ... Jump to: navigation, search 1975 was a common year starting on Wednesday (the link is to a full 1975 calendar). ...

References

1. ^  J.March Advanced Organic Chemistry 3Ed. ISBN 0471854727