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Organic Isomers

An isomer is one particular form of a compound which can take on numerous forms based on a single chemical formula.

Structural Isomers

For instance butane has the formula C4H10. Using the same number of carbons and hydrogens we can create two different arrangements of these atoms one called n-butane and the other methyl propane (see diagram left). Each of these arrangements generates a compound with its own unique properties including boiling point, density, viscosity etc.

This type of isomer is called a "structural isomer".

Geometric Isomers

On the other hand the illustration on the left also shows two arrangements for 1,2 dichloropropene. If this arrangement of atoms was based on an alkane structure the two versions would be the same because a simple twist around the carbon bond would transform one isomer into the other.

However because the double bond does not rotate in the way that a single bond would, these two isomers are different from one another in the same way the two structural isomers above are different from one another. These types of isomers are referred to as "geometric isomers". When the two chlorines are on opposite sides of the molecule the name of the compound begins with "trans" and when they are on the same side the name begins with "cis"; and "yes" this is the origin of the word "trans fats" which describes the particularly harmful kind of fat found in many fast foods.

A useful aid in remembering the significance of the double bond in forming structural isomers, is to visualize two oranges as the two carbons involved in a bond. When they are connected by a single pencil jabbed into each orange the two oranges can spin freely relative to one another. If they are connected by two pencils jabbed into their flesh, then they cannot spin with respect to one another and where you attach the chlorine atoms now makes a distinguishable difference between the two forms.

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