IUPAC Nomenclature of Alkens – Naming Organic Compounds

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

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