Parent chains

Identify the substituent groups attached to the parent chain... 

Both remaining CgHi4 isomers have two methyl groups as substituents on a four carbon chain Thus the parent chain is butane When the same substituent appears more than once use the multiplying prefixes di tri tetra and so on A separate locant is used for each substituent and the locants are separated from each other by commas and from the words by hyphens... 

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

Number the carbons of the parent chain beginning at the end nearer the first substituent, whether alkyl or halo. Assign each substituent a number according to its position on the chain. 

Step 3 If the parent chain can be properly numbered from either end by step 2, begin at the end nearer the substituent that has alphabetical precedence. 

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 ... 

If other functional groups are present and the doubly bonded oxygen is considered a substituent on a parent chain, the prefix oxo- is used. 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. 

Microstmcmral changes on irradiation have been observed by IR and UV spectroscopy. Changes in absorption bands due to vinyhdene double bonds [356,357], substituted double bonds, and ethyl and methyl groups give a measure of modifications in the presence of radiation. The ratio of the double bonds (located mainly at the end of a polymer chain) and scission is reported by some investigators [356-358] and found to be independent of temperature and dose. This is beheved to be due to the reaction of the methyl radical side group with hydrogen atoms on the backbone of the parent chain. 

We will systematically go throngh all hve parts to every name, starting at the end (functional group) and working onr way backward to the first part of the name (stereoisomerism). It is important to do it backward like this, because the position of the functional group affects which parent chain yon choose. 

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 ... 

First we need to find the functional group and make sure that the functional group is connected directly to the parent chain. Remember from the last section that if there are two functional groups, one of them gets priority. The functional group that gets priority is the one that needs to be connected to our parent chain. Of the three possibilities shown above, this rule eliminates the last possibility, because the functional group (OH) is not connected directly to the parent chain. 

If there are stiU more choices of possible parent chains (as there are in this case), then we look for the chain that also includes the double bond (if there is one in the compound). In our case, there is a double bond, and this rule determines for us which chain to use ... 

Of the remaining two possibilities, we must choose a parent that includes both carbon atoms of the double bond. Only one parent chain contains both the functional group and the two carbon atoms of the double bond. Containing the functional group means that the OH is connected to a carbon that is part of the chain. We do not count the oxygen atom itself as part of the chain. It is only attached to the chain. So the chain above is made up of four carbon atoms. 

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

Answer First we look for a functional group. This compound is a carboxylic acid, so we know the parent chain must include the carboxylic acid group. Next we look for a double bond. The parent chain should include that as weU. This gives us our answer. The triple bond will not be included in the parent chain, because the functional group and the double bond are a higher priority than a triple bond. 

Once we have identified the functional group and the parent chain, then everything else connected to the parent chain is called a substituent. In the following example, all of the highlighted groups are substituents, because they are not part of the parent chain ... 

We start by learning how to name the alkyl substituents. These groups are named with the same terminology that we used for naming parent chains, but we add yl to the end to indicate that it is a snbstituent ... 

Look at the propyl gronp above and you will notice that it is a small chain of 3 carbon atoms that is attached to the parent chain by the hrst carbon of the small chain. Bnt what if it is attached by the middle carbon Then it is not called propyl anymore ... 

It is still a chain of three carbon atoms, but it is attached to the parent chain differently than a propyl group is attached, so we call it isopropyl. This is an example of a branched substituent (branched, because it is not connected in one straight line to the parent chain, like a propyl group is). 

The tert-butyl group is made up of four carbon atoms, just like a butyl group, but the tert-hutyl group is not a straight line connected to the parent. Rather, the group has three methyl groups attached to one carbon atom, which is itself attached to the parent chain. So, we call this group tert-butyl. 

There are two other ways to attach four carbon atoms to a parent chain (other than butyl and tert-hutyl). As a small assignment, see if you can find their names in your textbook. 

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. 

Once we have chosen the parent chain, there are only two ways to number it right to left or left to right. But how do we choose To number the parent chain properly, we begin with the same hierarchy that we used when choosing the parent in the first place ... 

If there is a functional group, then number the parent chain so that the functional group gets the lower number ... 

If there is more than one substituent on the parent chain, then we should number the chain so that the substituents get the lowest numbers possible ... 

EXERCISE 5.46 For the compound below, choose the parent chain and then number it correctly ... 

Answer To choose the parent chain, remember that we need to choose the longest chain that contains the functional group ... 

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. 

We include this information in the name of a molecule, very much like we did for R and S conhgurations. For example, if the double bond is between carbons numbered 5 and 6 on a parent chain, then we would add the term (5 ) or (5Z) at the beginning of the name. 

When we number the parent chain, we see that the double bond is at the third carbon in the parent chain, and the stereocenter is at the fifth carbon in the parent chain ... 

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