isomer

Examples of isomer in the following topics:

• 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.
  • This isomer is also a meso compound.
  • These isomers are diastereomers of each other, and are constitutional isomers of the 1,2- and 1,3- isomers.

• 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.

• 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 these three ligands and the metal ion are in one plane, the isomer is said to be meridional, or mer.

• Organic Isomers

  • Molecules that share the same chemical formula but differ in the placement (structure) of their atoms and/or chemical bonds are known as isomers.
  • Structural isomers (such as butane and isobutane ) differ in the placement of their covalent bonds.
  • Molecules that have the same number and type of atoms arranged differently are called isomers.
  • (a) Structural isomers have a different covalent arrangement of atoms.
  • (b) Geometric isomers have a different arrangement of atoms around a double bond.

• 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.
  • The (R, R) and (S, S) isomers are enantiomers.

• Organic Enantiomers

  • Stereoisomers are a type of isomer where the order of the atoms in the two molecules is the same but their arrangement in space is different.
  • Optical isomers are stereoisomers formed when asymmetric centers are present; for example, a carbon with four different groups bonded to it.
  • Enantiomers are two optical isomers (i.e. isomers that are reflections of each other).
  • Every stereocenter in one isomer has the opposite configuration in the other.
  • Compounds that are enantiomers of each other have the same physical properties except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds.

• Nomenclature of Alkene Stereoisomers

  • 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.
  • Assignment of a cis or trans prefix to any of these isomers can only be done in an arbitrary manner, so a more rigorous method is needed.
  • 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.
  • Applying these rules to the isomers of compounds A and B shown above, we assign the configuration of the 1-bromo-1-chloropropene isomer as E (Br has higher priority than Cl, and CH3 a higher priority than H).
  • The configuration of the 1-cyclobutyl-2-ethyl-3-methyl-1-butene isomer is determined to be Z (C4H7 has higher priority than H, and the isopropyl group has higher priority than an ethyl group).

• Alkene Isomerization

  • The molar absorptivity of the cis-isomer is less than that of the trans-isomer because steric crowding of the ortho sites causes the phenyl groups to twist slightly out of coplanarity.
  • A small proportion (6%) of the trans-S1 state fluoresces back to the trans-isomer, but there is less than 0.1% fluorescence from the cis-S1 state.
  • A few of their findings are presented in the table below, the same photostationary cis:trans ratio being observed from either isomer as the starting point.
  • The unexpected change in steady state isomer distribution with the triplet energy of the sensitizer could not be rationalized as a single classical energy transfer.
  • These competing excitations and subsequent decay to cis and trans ground states lead to remarkable variations in isomer ratios.

• Stereogenic Nitrogen

  • A close examination of the ephedrine and pseudoephedrine isomers suggests that another stereogenic center, the nitrogen, is present.
  • If other chiral centers are present, as in the ephedrin isomers, a mixture of diastereomers will result.

• Group Frequencies

  • To illustrate the usefulness of infrared absorption spectra, examples for five C4H8O isomers are presented below their corresponding structural formulas.
  • Try to associate each spectrum (A - E) with one of the isomers in the row above it.