Configuration notational systems

Isomeric alkenes may be either constitutional isomers or stereoisomers There is a sizable barrier to rotation about a carbon-carbon double bond which corresponds to the energy required to break the rr component of the double bond Stereoisomeric alkenes are configurationally stable under normal conditions The configurations of stereoisomeric alkenes are described according to two notational systems One system adds the prefix CIS to the name of the alkene when similar substituents are on the same side of the double bond and the prefix trans when they are on opposite sides The other ranks substituents according to a system of rules based on atomic number The prefix Z is used for alkenes that have higher ranked substituents on the same side of the double bond the prefix E is used when higher ranked substituents are on opposite sides... 

Absolute Configuration According to the Cahn-Ingold-Prelog Notational System... 

Glycine is the simplest ammo acid and the only one m Table 27 1 that is achiral The a carbon atom is a chirality center m all the others Configurations m ammo acids are normally specified by the d l notational system All the chiral ammo acids obtained from proteins have the l configuration at their a carbon atom meaning that the amine group IS at the left when a Fischer projection is arranged so the carboxyl group is at the top... 

The CHEMiCAL ABSTRACTS stereochemical notation system is composed of four parts the site symmetry term, the configuration number, the chirality label, and the ligand stereochemical label. 

Biological reduction of pyruvic acid, catalyzed by the enzyme lactate dehydrogenase, gives (+)-lactic acid, represented by the Fischer projection shown. What is the configuration of (+)-lactic acid according to the Cahn-Ingold-Prelog R-S notational system ... 

Proposals modifying the Cahn-Ingold-Prelog system of configurational notation have been put forward by V. Prelog and G. Helmchen Angew. Chem., Int. Ed. Engl., 1982, 21, 567). 

In modern chemistry texts the cisHrans nomenclatnre has been replaced by a notational system known as E-Z where corresponds to cis and Z to trans. This system is less ambiguous as it is based on a more precise atomic number criterion for ranking substituents. In this system, when the higher atomic number atoms are on opposite sides of the double bond the configuration is (Carey 2000). 

The same system of notation can be extended further by focusing attention on the backbone substituents rather than on the methylenes. Consider bracketing a center substituent with a pair of monomers in which the substituents have either the same or opposite configurations as the central substituent. Thus the two bracketing units are either m or r with respect to the central unit and the probabilities of the resulting triads are obtained from the probabilities of the respective m or r additions. The following possibilities exist ... 

Cahn-Ingold-Prelog notation (Section 7.6) System for specifying absolute configuration as / or S on the basis of the order in which atoms or groups are attached to a chirality center. Groups are ranked in order of precedence according to rules based on atomic number. 

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