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Optical Isomers (Worksheet)

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    Work in groups on these problems. You should try to answer the questions without referring to your textbook. If you get stuck, try asking another group for help.

    Optical isomers are molecules that differ three-dimensionally by the placement of substituents around one or more atoms in a molecule. Optical isomers were given their name because they were first able to be distinguished by how they rotated plane-polarized light. These molecules are not necessarily locked into their positions, but cannot be converted into one another, even by a rotation around a single bond.

    Chirality

    A molecule is chiral if it is not superimposable on its mirror image. Most chiral molecules can be identified by their lack of a plane of symmetry or a center of symmetry. Your hand is a chiral object, as it does not have either of these types of symmetry. Pure samples of enantiomers have identical physical properties (e.g., boiling point, density, freezing point). Chiral molecules and ions have different chemical properties only when they are in chiral environments.


    The molecule on the left has a plane of symmetry through the center carbon. This is a mirror plane; in other words, one half of the molecule is a perfect reflection of the other half of the molecule. This molecule is not chiral because of its mirror plane.

    The molecule on the right has a center of symmetry, or an inversion center. An inversion center is a point in the molecule - not necessarily on an atom - through which all other atoms can be reflected 180 degrees into another, identical, atom. (In more accurate symmetry terms, an inversion through a center is equivalent to rotating groups by 180 degrees and then reflecting the groups through a plane perpendicular to the rotation axis.) This type of symmetry is rare in organic molecules, and is more common in inorganic molecules. The inversion center is represented by the blue circle in the above example. The inversion center is in the center of the middle carbon-carbon bond. This molecule is not chiral because of its inversion center.

    Optical isomerism arises in complexes that are mirror images of one another. Such molecules are often described as “left-handed” or “right-handed.” Optically active molecules are also referred to as enantiomers. In the case of octahedral complexes, any complex ion containing three bidentate ligands will be optically active. Therefore, the compounds \([Co(CO_3)_3]^{3-}\), \([Co(C_2O_4)_3]^{3-}\), and \([Co(en)_3]^{3+}\) re all optically active. Show below are two enantiomers of the \([Co(CO_3)_3]^{3-}\) ion. Try to rotate and superimpose thesein your mind and you will see that they are indeed different molecules.

    Keep in mind that a compound can exhibit geometric as well as optical isomerism. For example, consider the complex ion \([Co(CO_3)(CN_2)(NH_3)_2]^-\). Three geometric isomers are possible for this compound; one in which the \(CN^-\) ligands are trans, one in which the \(NH_3\) ligands are trans, and one in which each \(CN^-\) ligand is trans to an \(NH_3\) ligand. Only the last is optically active and has a nonsuperimposible mirror image.

    Stereocenters

    In chemical terms, the most common cause of chirality in a molecule is an atom that is bonded to four different groups. This atom with four different groups is called a stereocenter (or stereogenic center). For example, consider the following molecule.


    This molecule has no plane or center of symmetry, so it is a chiral molecule. The third and fourth carbons from the left are stereocenters, because they are each bonded to four different groups. The hydrogens have been drawn in here to make it easier to determine that there are four different groups; this will not be done in future sections.

    Achiral

    A molecule is achiral if it is superimposable on its mirror image. Most achiral molecules do have a plane of symmetry or a center of symmetry. The molecules shown below are achiral because they possess either a plane of symmetry or a center of symmetry.


    The molecule on the left has a plane of symmetry through the center carbon. This is a mirror plane; in other words, one half of the molecule is a perfect reflection of the other half of the molecule. This molecule is achiral because of its mirror plane.

    The molecule on the right has a center of symmetry, or an inversion center. An inversion center is a point in the molecule - not necessarily on an atom - through which all other atoms can be reflected 180 degrees into another, identical, atom. (In more accurate symmetry terms, an inversion through a center is equivalent to rotating groups by 180 degrees and then reflecting the groups through a plane perpendicular to the rotation axis.) This type of symmetry is rare in organic molecules, and is more common in inorganic molecules. The inversion center is represented by the blue circle in the above example. The same molecule is shown three-dimensionally below. The inversion center is in the center of the middle carbon-carbon bond. This molecule is achiral because of its inversion center.