optical isomers

Examples of optical isomers in the following topics:

• Isomers in Coordination Compounds

  • As with other compounds, there are several kinds of coordination complex isomers.
  • An example of cis and trans isomers can be seen for platin.
  • When three identical ligands occupy one face of an octahedron, the isomer is said to be facial, or fac.
  • In a fac isomer, any two identical ligands are adjacent or cis to each other.
  • If you shine light through pure solutions of each optical isomer they would rotate the plane of polarized light in opposite directions.

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

  • All the 1,2-dichloro isomers are constitutional isomers of the 1,3-dichloro isomers.
  • In each category (1,2- & 1,3-), the (R,R)-trans isomer and the (S,S)-trans isomer are enantiomers.
  • The cis isomer is a diastereomer of the trans isomers.
  • Since the cis isomer has two centers of chirality (asymmetric carbons) and is optically inactive, it is a meso-compound.
  • These isomers are diastereomers of each other, and are constitutional isomers of the 1,2- and 1,3- isomers.

• 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.
  • How, then, are we to classify these isomers and others like them?
  • Since these two compounds are optically active, each must have an enantiomer.

• Optical Activity

  • This perturbation is unique to chiral molecules, and has been termed optical activity.
  • Compounds that rotate the plane of polarized light are termed optically active.
  • Each enantiomer of a stereoisomeric pair is optically active and has an equal but opposite-in-sign specific rotation.
  • A 50:50 mixture of enantiomers has no observable optical activity.
  • Chiral organic compounds isolated from living organisms are usually optically active, indicating that one of the enantiomers predominates (often it is the only isomer present).

• The Nonclassical Carbocation Hypothesis

  • As shown in the following diagram, the rate of acetolysis of the exo-isomer is substantially faster than that of the endo-isomer, which reacts at a rate similar to the cyclohexyl derivative.
  • The former substitution proceeds with complete retention of configuration and racemization; whereas the endo-isomer is substituted with inversion of configuration and retains a small degree of optical activity.
  • By comparison, the endo-isomer ionizes to a classical 2º-carbocation, which is rapidly converted to the more stable nonclassical ion.
  • Some acetate anion may bond to the 2º-carbocation before it changes, accounting for the residual optical activity in this reaction.
  • Brown pointed out that the norbornyl compounds are better compared with cyclopentyl than with cyclohexyl analogs (eclipsing strain), and in such a comparison the endo isomer is abnormally slow, the exo isomer being only 14 times faster than cyclopentyl.

• Enantiomers and Stereoisomerism in Organic Compounds

  • Stereoisomers are isomers in which bond connectivity is the same, but spatial arrangement of the bonds differs.
  • Stereoisomers, or spatial isomers, are specific types of isomers.
  • Stereoisomers of a compound have not only the same chemical formula as one another (as do all isomers), but the same bond connectivity.
  • Some stereoisomers are not optically active and are actually exactly the same molecule.
  • The (R, R) and (S, S) isomers are enantiomers.

• Achiral Diastereomers (Meso-Compounds)

  • Meso compounds are achiral (optically inactive) diastereomers of chiral stereoisomers.
  • Some physical properties of the isomers of tartaric acid are given in the following table.
  • Fischer projection formulas provide a helpful view of the configurational relationships within the structures of these isomers.

• Isomers

  • Different compounds having the same molecular formula are called isomers, and the prevalence of organic isomers reflects the extraordinary versatility of carbon in forming strong bonds to itself and to other elements.
  • When the group of atoms that make up the molecules of different isomers are bonded together in fundamentally different ways, we refer to such compounds as constitutional isomers.
  • The three C5H12 isomers shown below illustrate these terms.
  • Considering that molecules are much too small to be seen by even the strongest optical microscopes, how is it that the connectivity and relative location of the component atoms can be assigned with confidence?
  • The top row shows isomers in which an electronegative chlorine is attached to a butyl group.

• Stereoisomers

  • When the group of atoms that make up the molecules of different isomers are bonded together in fundamentally different ways, we refer to such compounds as constitutional isomers.
  • For example, in the case of the C4H8 hydrocarbons, most of the isomers are constitutional.
  • Shorthand structures for four of these isomers are shown below with their IUPAC names.
  • A fifth possible isomer of formula C4H8 is CH3CH=CHCH3.
  • In the cis isomer the methyl groups are on the same side; whereas they are on opposite sides in the trans isomer.

• Synthesis of Amino Acids

  • Be careful to distinguish charge symbols, shown in colored circles, from optical rotation signs, shown in parenthesis.
  • Next, an ammonium salt is formed by combining the carboxylic acid with an optically pure amine, such as brucine (a relative of strychnine).
  • Diastereomers may be separated by crystallization, chromatography or other physical manipulation, and in this way one of the isomers may be isolated for further treatment, in this illustration it is the (+):(-) diastereomer.
  • Finally the salt is broken by acid treatment, giving the resolved (+)-amino acid derivative together with the recovered resolving agent (the optically active amine).