stereoisomers

Examples of stereoisomers in the following topics:

• Configurational Stereoisomers of Cycloalkanes

  • Stereoisomers are also observed in certain disubstituted (and higher substituted) cyclic compounds.
  • As in the case of the 2-butene stereoisomers, disubstituted cycloalkane stereoisomers may be designated by nomenclature prefixes such as cis and trans.
  • This is similar to the substitution pattern that gives rise to stereoisomers in alkenes; indeed, one might view a double bond as a two-membered ring.

• Compounds Having Two or More Chiral Centers

  • Both compounds are stereoisomers of 2-methylamino-1-phenyl-1-propanol, and both are optically active, one being levorotatory and the other dextrorotatory.
  • Although these missing stereoisomers were not present in the natural source, they have been prepared synthetically and have the expected identical physical properties and opposite-sign specific rotations with those listed above.
  • Depending on the overall symmetry of the molecular structure, some of these combinations may be identical, but in the absence of such identity, we would expect to find 2n stereoisomers.
  • Some of these stereoisomers will have enantiomeric relationships, but enantiomers come in pairs, and non-enantiomeric stereoisomers will therefore be common.
  • We refer to such stereoisomers as diastereomers.

• Enantiomers and Stereoisomerism in Organic Compounds

  • Stereoisomers, or spatial isomers, are specific types of isomers.
  • What differs between stereoisomers of a compound is the spatial arrangement of atoms.
  • Stereoisomers can be divided into enantiomers, diastereomers and meso compounds.
  • Diastereomers are stereoisomers that are not enantiomers.
  • When multiple stereoisomers of the same compound have different configurations of one or more (but not all) of the related stereoisomers, they are not mirror images.

• Nomenclature of Alkene Stereoisomers

  • Configurational stereoisomers of the kind shown above need an additional nomenclature prefix added to the IUPAC name, in order to specify the spatial orientations of the groups attached to the double bond.
  • Thus far, the prefixes cis- and trans- have served to distinguish stereoisomers; however, it is not always clear which isomer should be called cis and which trans.
  • Both compound A (1-bromo-1-chloropropene) and compound B ( 1-cyclobutyl-2-ethyl-3-methyl-1-butene) can exist as a pair of configurational stereoisomers (one is shown below).
  • How are we to name these stereoisomers so that the configuration of each is unambiguously specified?
  • In the isomers illustrated under Configurational Stereoisomers of Alkenes, for which cis-trans notation was adequate, Z is equivalent to cis and E is equivalent to trans.

• Configurational Stereoisomers of Alkenes

  • As a result, certain disubstituted alkenes may exist as a pair of configurational stereoisomers, often designated cis and trans.

• Conformational Stereoisomers

  • Here too it was noted that nomenclature prefixes must be used when naming specific stereoisomers.

• Stereoisomers

  • Isomers that differ only in the spatial orientation of their component atoms are called stereoisomers.
  • Stereoisomers always require that an additional nomenclature prefix be added to the IUPAC name in order to indicate their spatial orientation, for example, cis (Latin, meaning on this side) and trans (Latin, meaning across) in the 2-butene case.

• Fischer Projection Formulas

  • Using the Fischer projection notation, the stereoisomers of 2-methylamino-1-phenylpropanol are drawn in the following manner.
  • There are eight stereoisomers of 2,3,4,5-tetrahydroxypentanal, a group of compounds referred to as the aldopentoses.
  • Since there are three chiral centers in this constitution, we should expect a maximum of 23 stereoisomers.
  • These eight stereoisomers consist of four sets of enantiomers.
  • Each of these compounds has an enantiomer, which is a member of the "L"-family so, as expected, there are eight stereoisomers in all.

• General Summary of Isomerism and Molecular DescriptorsInclude Relationship of Constitutional and Stereoisomers and Relationships of Stereoisomers

  • The distinction between configurational stereoisomers and the conformers they may assume is well-illustrated by the disubstituted cyclohexanes.
  • The 1,4-dichlorocyclohexanes may exist as cis or trans stereoisomers.

• Achiral Diastereomers (Meso-Compounds)

  • As a result, two of the four possible stereoisomers of this compound are identical due to a plane of symmetry, so there are only three stereoisomeric tartaric acids.
  • Two of these stereoisomers are enantiomers and the third is an achiral diastereomer, called a meso compound.
  • Meso compounds are achiral (optically inactive) diastereomers of chiral stereoisomers.