Isomers

Larry J Scheffler

Lincoln High School

IB Chemistry 3-4

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Structural Isomers

Structural isomers have the same molecular formulas but they differ in their structural formulas.

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Stereoisomers

Stereo isomers have the same structural formulas but they differ in their spatial arrangements.

There are two types of stereoisomerism

1. Geometrical isomerism2. Optical isomerism

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Geometrical isomers

Geometrical isomers occur in organic molecules where rotation around a bond is restricted

This occurs most often around C=C The most common cases are around

asymmetric non-cyclic alkenes

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Geometrical isomers Carbon to carbon double bonds are made up of a and a

bond. The bond is formed from the combination of two p

orbitals, one from each of the two carbon atoms The two p orbitals must be in the same plane to combine

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Geometric Isomers in alkenes

A cis isomer is one in which the substituents are on the same side of the C=C

A trans isomer is one in which the substituents are on the opposite sides of the C=C

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Geometric Isomers in Cycloalkanes Ring structures like C=C restrict

rotation and therefore can result in cis and trans isomers

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Properties of Geometrical Isomers

The chemical properties of geometrical isomers tend to be similar but their physical properties are different

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Properties of Geometrical Isomers

The trans isomer has a much higher melting point. Unlike the cis isomer there is little intra-molecular hydrogen bonding

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Optical Isomers Optical isomerism is present in all

compounds that contain at least one asymmetric (chiral) carbon atom

An asymmetric carbon atom has four different atoms or groups attached

In this case there are two different ways to arrange the four groups around the chiral carbon atom (shown in red)

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Optical Isomers

While these structures may look identical, in three dimensions they are mirror images of each other. Such molecules are called enantiomers.

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Distinguishing EnantiomersOptical isomers can be distinguished by the way they interact with plane polarized light

Polarized lightPolarized light is light that has been passed through a polarizing prism or filter. As a result the light vibrates in a single plane.

Distinguishing EnantiomersOptical isomers can be distinguished by the way they interact with plane polarized light. One enantiomer will rotate polarized light to the right, the other to the left.

Properties of Optical Isomers

Apart from their optical activity enantiomers generally have similar physical and chemical properties.

The chemical properties may be significantly different when the enantiomers interact with other optically active compounds.

Thalidomide has two optical isomers.One is a tranquilizer, the other is aPowerful teratogen.