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IUPAC Nomenclature of Alkens – Naming Organic Compounds

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One of the first and most important topics that you will learn as an organic chemistry student is all about naming organic compounds. You will be expected to follow a naming system to classify compounds and provide their IUPAC names. In this article I will provide you with a simple approach for the iupac nomenclature of alkenes

An alkene is defined as a molecule containing at least one carbon to carbon double bond, or pi bond.

When naming alkenes you have to start with the standard nomenclature approach which includes a first and last name.

The first name refers to the number of carbons in the parent chain. The first name used for the numbers 1 through 10 are as follows:

meth, eth, prop, but, pent, hex, hept, oct, non, dec

The last name for an alkene is ‘ene’ when just one pi bond is present.

Any additional pi bonds must include a prefix as a numerical identifier as follows:

  1. understood, no prefix required
  2. di
  3. tri
  4. tetra

While the list goes on, this is typically where you stop at the beginner organic chemistry level

When you analyze a simple (non-substituted) carbon chain, simply count the number of carbons and chose a first name, then identify the type of bonds and chose a last name

For example, the molecule CH3CH2CH2CH=CHCH3 will be named as follows:

6 carbons for a first name of hex
a pi or double bond for a last name of ene

Putting this together we get a hexene

Since the pi bond occurs on the second carbon when counting from the right, we must include a 2 prior to the molecule name so as not to confuse this with a hexene that contains a double bond on the first or third carbon

The final name for this molecule is 2-hexene

Now for a more complex example,

Say you are given the molecule CH3CH2CH2CH2CH=CHCH2CH=CH3

We first count the carbon atoms for a total of 9
This results in the first name of ‘non’
We have at least one (in this case 2) pi bonds on the molecule for a last name of ene. Since we have 2 pi bonds we call it diene where the ‘di’ indicates 2 double bonds

The first double bond occurs between carbon 1 and 2 when counting from the right. This gives it the lower number of ‘1’
The second double bond occurs between carbons 4 and 5,
so we assign a number ‘4’

Putting the name together we have 1,4-nonadiene