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Formula to Name
To work on determining the name of a compound starting from its formula, let's go
through some examples. These are also shown in Example 20 in your workbook.
First Example
| This first example is a compound which contains four carbon atoms and nine
hydrogen atoms and one chlorine atom arranged in a particular way, as shown. In order to
name this, we first have to select the longest chain. The longest carbon chain in that
molecule is a three-carbon chain. This means that the last part of the name of this
compound will be propane. |
CH3
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CH3-CH-CH2-Cl
-propane |
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| We will number these three carbon atoms from right to left in this case
because that order will give us the smallest numbers in the name when we look at where
groups are attached to the main chain. |
CH3
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CH3-CH-CH2-Cl
3 2 1
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| Now, what about the rest of the name? Well, there are two functional
groups attached to this three-carbon chain; there is a chloro group and a methyl group.
The chloro group in this compound is attached to the first carbon and the methyl group is
attached to the second carbon. Putting all this information together, you come up with the
name: 1-chloro-2-methylpropane. Note that chloro comes
before methyl because of alphabetical order, not because of the 1-
and 2-. |
CH3
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CH3-CH-CH2-Cl
3 2 1
1-chloro-2-methylpropane |
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Let's take the name and dissect it to see what the name represents. 1-chloro
means that there is a chlorine atom attached to the first carbon in the chain. 2-methyl
means that there is a methyl group attached to the second carbon in the chain, and propane
means that the chain is three carbon atoms long and has all single bonds.
Second Example
| This example brings up the "tie-breaker" rule for numbering the
carbon atoms in the longest continuous chain. Note that we have two chloro groups and a
methyl group attached to a four-carbon chain. Because there is a chloro group at each end
of the chain numbering from left-to-right or from right-to-left is a "tie". To
break the "tie" we look at which direction would give us the lowest second
number. If there were two chloro groups attached to one end and only one at the other end,
that would be the deciding factor. |
CH3
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Cl-CH2-CH-CH2-CH2-Cl |
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| In this case the location of the methyl group dictates that the
four-carbon chain is numbered from left-to-right. Putting all of the parts together the
name of this compound is 1,4-dichloro-2-methylbutane. |
CH3
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Cl-CH2-CH-CH2-CH2-Cl
1 2 3 4 |
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Third Example
| Let's go on to the next example. You can see that this looks much more
complex, and, of course, it is. But if we just take the method of naming this compound one
step at a time, it will cause no difficulties. |
CH3
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CH3 CH2 Cl
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CH3-C - C - CH2-C-Cl
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H CH3 Cl |
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| First of all, let's select the longest chain of carbon atoms. There are five
carbons in this chain, so pentane will be the last part of the
name. In this case the carbon atoms in the chain are numbered from the end that has three
chlorine atoms attached. What about the functional groups that are attached to this
pentane chain? There are three chloro groups attached to the first carbon. There is a
methyl group attached below the third carbon, an ethyl group also attached to the third
carbon, and another methyl group attached to the fourth carbon. |
CH3
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CH3 CH2 Cl
| | |
CH3-C - C - CH2-C-Cl
| | |
H CH3 Cl
5 4 3 2 1 |
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| The name will start with 1,1,1-trichloro to indicate
three chlorine atoms all attached to the number one carbon atom. Note that there is a
number for each group even though it is the same number.
Alphabetically, ethyl comes after chloro and before methyl. There is an ethyl group on
the third carbon, so we continue with 3-ethyl.
The two methyl groups are indicated by writing dimethyl. To indicate
that those methyl groups are on the third and fourth carbon atoms, we write the 3,4-dimethyl.
Then we use pentane to indicate that there are five carbons in the
chain.
The total name is shown there, and it is 1,1,1-trichloro-3-ethyl-3,4-dimethylpentane.
Again, please note the use of commas between numbers and hyphens between numbers and
letters. |
CH3
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CH3 CH2 Cl
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CH3-C - C - CH2-C-Cl
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H CH3 Cl
5 4 3 2
1
1,1,1-trichloro-3-ethyl-3,4-dimethylpentane
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E-mail instructor:
Eden Francis
Clackamas Community College
©2001, 2003 Clackamas Community College, Hal Bender
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