Organic Chemistry, Form, Part 9 - Beginning Nomenclature For Alkenes

Greetings,

This post will continue the nomenclature of the last lesson, which featured alkene hydrocarbons with the double bond on the ending carbon atom.  Here, the focus is on alkene hydrocarbons with 4 or more carbon atoms and a double bond not on the terminal carbon atom.  Two new specifications become necessary: 1) Indication of the double bond location and 2) Indication of beginning and terminal alkyl group relative positions across the double bond.

Indication of the Double Bond Location

For simplicity of understanding, we will ignore specification 2) for now.  Let's begin with a condensed formula for a seven carbon alkene. ==> 

CH3CH2CH2CH=CHCH2CH3 The rule to indicate the double bond location is to assign numbers to the carbon atoms, beginning with the end closest to the double bond.  We must start counting from the right side and we can stop when we reach the double bond.

CH3CH2CH2CH=CHCH2CH3 

                              3    2     1

The double bond for this example is at carbon number 3 and the chain has 7 carbons total, therefore we have the compound 3-Heptene.

Indication of the "Relative Alkyl Group Positions" - Cis/Trans Isomer Labeling

When we view the complete Lewis Structure of 3-Heptene it becomes apparent that there are two possible structural formulas (isomers), therefore we also must include a label indicating which isomer we have.  This is summarized in the following diagram:

The two isomers shown in the chart have an obvious structural difference and they are, in fact, different compounds with different properties.

That's all for now.  My next post will explain nomenclature for alkenes consisting of additional alkyl groups attached to the double bond.

As always, thank you for reading!

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