Naming Of Organic Compound

How to name organic compounds in chemistry

In chemistry, naming organic compounds somewhat becomes challenging for students especially when taking their baby steps in chemistry.

 IUPAC nomenclature is divided into nomenclature in inorganic chemistry and organic chemistry.

The naming of organic compounds becomes a little confusing when the basics are misconstrued or not understood at all. I want to try my best to help you understand the basics of naming these compounds with some naming organic compounds worksheets.

It becomes even more confusing either when naming organic compounds with multiple methyls or naming organic compounds with functional groups or structures with strokes or dashes.

The first thing I would like to do is to lay down the IUPAC rules for naming organic compounds and then walk you through these basics with examples.

5 IUPAC rules for naming organic compounds

These are the rules or precepts that guide us through in the naming of organic compounds and once we adhere to these guidelines then there will be no issue with the naming of compounds.

Rule 1: Take the longest continuous carbon chain

In naming organic compounds, there is a need to emphasize that the longest carbon chain should be taken into consideration first. The longest carbon chain indicates the number of carbon chain and determines the prefix for example if you have CH3CH2CH2CH3 it means that the longest carbon chain is five and it means the prefix is going to be a pentane but how do we know if it is actually pentane, pentene or pentyne.

The longest carbon chain could be branched and not necessarily straight. As a matter of fact, there is no rule that states that the longest carbon chain must be straight. In counting the carbon atoms in the longest carbon chain, it is always required to start the numbering from the place where the functional group is and then proceed afterwards.

Take a look at some examples of these illustration

In this one, the longest carbon chain is branched.

Example 1

Picture1

The longest carbon chain here for example is not the straight carbon chain but the branched carbon chain as a result it is a decane and not an octane

But how did I do deduce this, take a closer look here so you can understand how

Picture2

So, taking the longest carbon chain from the branched-chain reveals that the organic compound is a decane.

And guys remember that I am yet to name the compound.  The name of the compound is actually 4 methyl decane, don’t worry about the nomenclature we will get to see how the compound is named.

Another example is seen below

Rule 3: Identify the functional group in the chain

Another important rule is to identify the functional group in the chain and the number accordingly. The functional; group takes consideration first and thus numbering should start from the end the functional group is closest to.

Butanol
Butanol

You can see that the functional group alkanol (R-OH) took the first consideration in the numbering of the longest carbon chain and this could be taken as butanol or butan-1-ol.

The but- prefix is due to the four carbon atoms which is as a result of naming the prefix and functional group of compounds .

Rule 3: Identify the substituent groups attached to the chain

Ideally, the common substituents are the alkyl groups like methyl, ethyl, propyl etc

Also, the halo groups are commonly attached too and they are called choro group when chlorine is attached, the fluoro group when fluorine is attached and finally iodo group when iodine is attached.

Take a look at the example shown below.

Picture3

You will see that there are four substituent groups in the structure above, actually, we have three methyl groups and one halo(fluoro) group.

How do I name this compound and which substituents should come first? I will show you in a jiffy how we can name this compound.

However, the name of the compound is 4 fluoro-2,2,6 trimethyl heptane, and hope you noticed the alphabetical trend in the nomenclature.

Let’s look at the second example on how to identify substituents.

Picture4

Here too, we have four substituents two methyl groups, one ethyl group, and one chloro group.

Rule 4: Number the continuous carbon chain consecutively, starting at the end nearest a substituent group

The numbering is quite important because we have situations where we get two or more nomenclatures from the same compound due to wrong numbering.

In fact, it is important I mention that only one numbering will be correct. The next important rule is the ability to number the substituent groups attached to the largest carbon chain.

In doing this I would like to emphasize three guidelines that will help us in doing this.

Guideline 1

Start the numbering of the carbon chain and the substituent positioning from the substituent closest to the end of the chain.

Take a look at this example.

Wrong numbering

Picture5

correct numbering

Picture6

So, the correct numbering is the numbering that recognizes where the substituents stem from.

The rule of thumb is that you start the numbering from the end the substituent is closest to.

Guideline 2

There are situations that occur when you have substituents placed from the same positions when counting from both ends and then you don’t know where to start from.

Now listen when you have alkyl groups placed from same positions at both ends. Start from the bulky group.

Take a look at this

Picture7

Where do I start numbering because the two alkyl groups viz methyl groups (CH3) and ethyl group (C2H5) are placed at the same positions from both ends?

The bulkier group should be considered in the numbering because that will yield the correct numbering.

So this is the correct numbering

Picture8

Please do not be tempted to start from the methyl group numbering or always start from the right in numbering.

Picture9

There is no rule that states that one should always start numbering from the right… No! There is no rule like that.

Guideline 3

When you have a halo group and an alkyl group on the same positions from both ends then you should start with the more inductive which is the halo group.

Take a look at this example.

Picture10

It should be this way

Picture11

Rule 5: Name the organic compound but take the naming of the substituents alphabetically.

There is a salient point I wish to make here and that is to name the substituents alphabetically.

Regardless of the positioning, for example, a chloro group should come before a fluoro group and an ethyl group should come before a methyl group in alphabetical trend.

Let’s then put these rules into practice and name these few compounds.

You can find 20 examples on naming organic compounds but here I will just give few examples.

Example 1

Picture12

The numbering and name of this compound should be this

Picture13

You can see I started from the ethyl substituents and now the nomenclature will be

3 ethyl -7 methyl nonane

Example 2

Picture15

The correct numbering should as shown below

Picture16

Then the naming of the compound is 3 chloro-7 methyl nonane

Generally naming of compounds requires practice and I am going to do a naming of organic compounds worksheet that will help you to practice and see the answers and your mistakes which will help you a great deal in your journey in chemistry.

I would love it if you will like me to do an e-book o naming organic compounds so I can know I should embark on it.

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