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  • Topic: Stereochemistry, Isomer, Conformational isomerism
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  • Published : January 2, 2013
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Experiment 1

Stereochemistry is the study of the three-dimensional configuration that makes up a molecule and the ways in which this arrangement affects the physical and chemical properties of molecules. Isomerism is the main concern of stereochemistry. Isomers are sets of chemical compounds with identical molecular formulas but different structural properties. There are two types of isomers, structural isomers and stereoisomers.

Structural isomers are subdivided into two: position and functional isomers. Position isomers occur among substituted hydrocarbons. The substituents are attached to different positions on the carbon chain.

1-propanol 2-propanol

Functional isomers, on the other hand, have very different chemical properties because differences in the structures give rise to different functional groups.

ethanol dimethyl ether

Stereoisomers are molecules which have the same basic arrangement of atoms in their molecules but differ in the way the atoms are arranged in space. There are three basic types of stereoisomers such as conformational isomers, geometric isomers and optical isomers. Molecules that exhibit free rotation about a single bond give rise to conformational isomerism. This is usually illustrated by alkanes due to their C-C single bond. It is usually represented using the Newman projection.



chair boat
The second type of stereoisomerism is geometric isomerism which is common in alkenes. Geometric isomers differ in physical properties such as melting points and boiling points.

ex. cis (same side)trans (opposite side)

For alkenes where cis-trans configuration does not apply, the geometry of the alkene may be designated by E (entgegen, opposite) or Z (zussamen, together). This is applied by determining which of the two groups of atoms, attached to the carbon of the double bond, acquire the higher priority according to the Cahn-Ingold-Prelog rules. If the two groups with higher priority are located at the same side of the double bond, the isomer is said to be Z. If they are located on the opposite sides of the double bond, then the isomer is of the E-type.

ex. Z-3-chloro-2-penteneE-3-chloro-2-pentene

The last type of stereoisomerism is optical isomerism. When plane-polarized light is passed through an optical isomer, it is rotated into a different plane of polarization. Optical isomers exhibit this optical activity in varying degree. These isomers have similar properties and differ only in the direction in which they rotate light. Molecules that are non-superimposable mirror images are called enantiomers. They are also known as optical isomers. They rotate the plane-polarized light in opposite directions but with the same magnitude. All enantiomers have chiral centers and are considered chiral molecules. Chiral centers are carbon atoms that bear four different atoms or groups of atoms. If one or more atoms or group of atoms are the same, they are considered achiral. Steroisomers that are not mirror images of each other are called diasteriomers. M












1. Draw all the possible structural isomers of a compound with the molecular formula C4H8O. a. How many are positional isomers? Functional group isomers?

2. Make a model of butane. Examine the internal bonds and carefully rotate the single bond. b. Draw the Newman projection of the different conformations of butane. c. Which is the most stable conformer? Why?

d. Which is the least stable? Why?

3. Draw all possible structural isomers of the compound C2H4FBr. e. Label whether the isomer is a chiral or achiral molecule by putting an asterisk (*) on the chiral center. f. Make a...
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