Naming the Parent Chain

Stereoisomerism Substituents Parent Un saturation Functional group 

When naming the parent of the compound, we are looking for the chain of carbon atoms that is going to be the root of our name. Everything else in the compound is connected to that chain at a specific location, designated by numbers. So we need to know how to choose the parent carbon chain and number it correctly. 

The first step is learning how to say a chain of three carbons or a chain of seven carbons. Here is a table showing the appropriate names  

If we have carbon atoms in a ring, we add the term cyclo, so a ring of six carbon atoms is called cyclohex- as the parent and a ring of five carbon atoms is cyclopent-. 

You must commit these terms to memory. I am not a big advocate of memorization, but for now, you must memorize these terms. After a while, it will become habitual, like a phone number that you dial all of the time, and you won t have to think about it anymore. 

Stereoisomerism Substituents Parent Unsaturation Functional group 

The tricky part comes when you need to figure out which carbon chain to use. Consider the following example, which has three different possibilities for the parent chain  

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EXERCISE 5.18 Name the parent chain in the following compound ... 

Name the parent chain as an alkane, with the halogen as a substituent bonded to the longest chain. 

When an organic compound is named, the parent chain is identified, and the carbon atoms are numbered so that any branches or multiple bonds have the lowest possible numbers. 

Number the carbon atoms in the parent chain starting with the end closest to the double or triple bond and name the parent chain as you would for an alkane, but change the ending to -ene or -yne. 

In the lUPAC name, the parent chain is named and the —OR group is named as an alkoxy substituent. 

In this example, the parent chain has nine carbon atoms. When naming the parent chain of a compound, the names in Table 4.1 are used. These names will be used very often in this course. Parent chains of more than 10 carbon atoms will be less common, so it is essential to commit to memory at least the first 10 parents on the fist in Table 4.1. 

Name the parent chain by finding the longest C chain. 

Name the parent hydrocarbon. Find the longest carbon chain containing the double bond, and name the compound accordingly, using the suffix -me ... 

Find the longest chain, and name it as the parent. If a double or triple bond is present, the parent chain must contain it. 

Aldehydes are named by replacing the terminal -e of the corresponding alkane name with -at. The parent chain must contain the -CHO group, and the -CHO carbon is numbered as carbon 1. For example ... 

The carbon atoms of the parent chain are numbered according to 2-Carb-2.2.1. If a unique numbering is required for the branch(es) (e.g. for X-ray orNMR work), the carbon atoms may be given the locant of the appropriate branch point, with the internal branch locant as superscript, e.g. 42 for position 2 of the branch at position 4 of the main chain. This style of branch numbering is not to be used for naming purposes e.g. the side-chain-methylated derivative of compound 5 is named 4,6-dideoxy-3-C-[(I )-1 -methoxyethyl]-D-n7w-hexose, and not as a s -O-methyl derivative. 

So a compound wifh five chlorine atoms would be pentachloro. Each and every substituent needs to be numbered so that we know where it goes on the parent chain, but we will learn about this after we have finished going through the five parts of the name. At that time, we will also discuss in what order to place substituents in the name. 

We re almost ready to start naming molecules. We finished learning about the individual parts of a name, and now we need to know how to identify how the pieces are connected. For example, let s say you determine that the functional group is OH (therefore, the suffix is -ol), there is one double bond (-en-), the parent chain is six carbon atoms long (hex), there are four methyl groups attached to the parent chain (tetramethyl), and the double bond is cis. Now you know all of the pieces, but we must find a way to identify where all of the pieces are on the parent chain. Where are all of those methyl groups (and so on). This is where the numbering system comes in. First we will learn how to number the parent chain, and then we will learn the rules of how to apply those numbers in each part of the name. 

Now that we know how to number the parent chain, we need to see how to apply those numbers to the various parts of the name. 

Next we need to name the parent. We locate the longest chain that includes the functional group and double bond. In this example, it is an obvious choice. The parent has 7 carbon atoms, so the parent is -hept-. 

Table 11.2. Prefix for the number of carbons in the parent chain when naming organic compounds.

Step 1 Locate the longest chain that contains an — OH group attached to one of the carbon atoms. Name the parent alkane. 

Step 1 Name the parent alkane. Remember that the main chain must contain the C = 0 group. 

First, one has to identify and name the groups attached to the chain, and number the chain so that the substituent gets the lowest possible number. For example, one of the isomers of pentane is 2-methylbutane, where the parent chain is a four-carbon butane chain, and is numbered starting from the end nearer the substituent group (methyl group). Therefore, the methyl group is indicated as being attached to carbon atom number 2. 

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