Molecular Geometry

Molecular Geometry Types

 Elmhurst College
Alkanes Alkynes  Optical or Chiral  Chemistry Department
Alkenes Cis / Trans Alkenes Rings  Virtual ChemBook



Introduction to Chiral or Optical Isomers:

Some chemical compounds have optical activity in the sense that these compounds have the ability to rotate the plane of polarized light. Polarized light has light waves all traveling parallel to each other. Ordinary light has light waves traveling in all directions. When polarized light is passed through a solution of an optically active compound, the plane of polarization is rotated to the right or the left. The angle of rotation can be measured in a polarimeter.

An optically active organic compound can be identified by finding a chiral carbon. A chiral carbon is one that has four different "groups" attached to it. The groups can be anything from a single H to functional groups to one or more other carbons. See bromochloroiodomethane on the left - it has 3 halogens and one hydrogen.

In relatively complicated compounds, each carbon must be examined carefully to determine whether it is chiral. Some compounds may have two or more chiral carbon centers such as in carbohydrates.

See glyceraldehyde on the left. Carbon # 1 has only three groups attached. Carbon # 3 has two hydrogens which count as 2 of the same groups. Finally, carbon # 2 has four different groups attached.

Quiz: For the structures in the left graphic:

Identify which carbons are chiral. Click on the box to check your answers.


 carbon # 1
carbon # 2


 carbon # 1
carbon # 2
carbon # 3

 ketose sugar

 carbon # 1
carbon # 2
carbon # 3
carbon # 4
carbon # 5

Optical Isomers

Optically active compounds exist in two isomeric forms. The isomer that rotates the plane of polarized light to the left (counterclockwise) is called levorotatory (l). The other isomer that rotates the light to the right (clockwise) is called dextrorotatory (d).

The optical isomers are mirror images of each other. The isomers result from the tetrahedral geometry around the chiral carbon center. To draw mirror images, write the structure of the first isomer, draw a plane (dotted line) to represent the mirror, and finally draw the mirror image behind the plane.

See the graphic on the left.

Optical isomers are also designated with the letters (D) and (L) to indicate the absolute configuration. In an absolute configuration the position of the groups is compared to a commonly agreed upon standard molecule. In amino acids, this molecule is serine and for carbohydrates, it is glyceraldehyde. The capital (D) and (L) designations do not necessarily agree with the (d) or (l) designations which are based upon the actual rotation of polarized light.

L-Alanine is a representative amino acid that is used in human and animal metabolism. The D isomer is not used. In the metabolism of carbohydrates only the D isomer is utilized and not the L isomer.

Example: Chiral Drugs - Ibuprofen (Advil)

As a further example, the drug Ibuprofen, is sold as a mixture of isomers, although the (S) isomer is most effective and the (R) isomer is also utilized after conversion to the (S) isomer in the body.

See ibuprofen graphic in new window

Quiz: Which carbon in ibuprofen is chiral? Check answer using pull down menu.