Interpreting Names
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Interpreting Names

Determining Structural Formulas

Another aspect of nomenclature is to use the name of a compound to derive the structure of that compound. When doing this, I generally recommend that you read the name backwards. Start at the end of the name to see what kind of compound it is. Then find out how long the main chain is. Then see what groups are attached to it and where they are attached.

Example

Let's use 2-methylbutane as an example.

Start at the end of the name. It ends with -ane. This means that it has carbon, hydrogen, and single bonds. Keep going. Butane means that we have a four-carbon chain, so write out four carbons in a row and connect them by single bonds.

C - C - C - C
The name also says 2-methyl. That means a methyl group is attached to the second carbon. Attach that one carbon group to the second carbon in the butane chain and you will see that you have the skeleton of the structure for 2-methylbutane.

C - C - C - C
|   
C   

All you have to do now is add hydrogens to each carbon to bring the total number of bonds on each carbon up to four.

H   H   H   H
|   |   |   |
H - C - C - C - C - H
|   |   |   |
H H-C-H H   H
|    
H   

Practice

Set 1

Now try drawing structural formulas for these compounds (they are also shown in exercise 15 in your workbook).

3-ethyl-2,4-dimethylheptane
4-isopropyloctane

Answers

3-ethyl-2,4-dimethylheptane   CH3   CH3         
|     |        
CH3-CH-CH-CH-CH2-CH2-CH3
|         
CH2-CH3   
4-isopropyloctane
CH3-CH2-CH2-CH-CH2-CH2-CH2-CH3
|      
CH3-CH-CH3     

Set 2

Next, answer these questions (they are also shown in exercises 16 and 17 in your workbook).

A compound was mistakenly named 2-ethylbutane. What is its correct name? What error was made in coming up with this name?
Gasoline is mainly a mixture of hydrocarbons. One of these hydrocarbons is 2,2,4-trimethylpentane. What is the structural formula of this compound? (also called iso-octane)

Answers

The skeletal structure for 2-ethylbutane is shown to the right. The error made was in not selecting the longest continuous carbon chain. The correct name is 3-methylpentane.

C - C - C - C
|    
C    
|    
C   

2,2,4-trimethylpentane CH3   CH3
|     |  
CH3-C-CH2-CH-CH3
|        
CH3      

If you had trouble with any of these, ask the instructor for help and for additional names and/or formulas with which to practice.

 

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