Module 1   

Lesson 1.2  Naming the Alkanes


When you are provided with a diagram of an organic molecule, you have to be able to name it correctly. Three types of diagrams are used to represent organic molecules - structural, condensed structural, and line.


For example, study the illustrations below. All three diagrams depict hexane (C6H14)



A structural diagram shows all carbon and hydrogen atoms and all bonds.

 

Fig. 1   hexane

 


A condensed structural diagram shows all carbon and hydrogen atoms and all carbon-carbon bonds. However, it does not show carbon-hydrogen bonds.

Learning Tip:  Longer hydrocarbon chains can be condensed further by combining the repeating units, such as CH3- (CH2)4-CH3

                 

 Fig. 2  hexane 

 

 



A line diagram shows only carbon-carbon bonds. The intersections and ends of the line drawing indicate carbon atoms.


 

Fig. 3  hexane

 

  1.  Naming Unbranched Alkanes and/or Cycloalkanes



    A diagram of an unbranched alkane is quite straightforward to name.  Just follow these steps:


    1. Count the number of carbons in the chain or ring

    2. Choose the appropriate prefix from the table shown in the section "Introducing the Alkanes".  Remember that the prefix corresponds to the number of carbon atoms.

    3. Add the suffix "ane"

    4. If the alkane is in a closed ring, add the prefix "cyclo" to the front of the name.


      Fig. 4  octane


      Fig. 5  cyclopentane



Watch





  1.  
  2.  Naming Branched Alkanes and/or Cycloalkanes



    The ability of a carbon atom to form four bonds creates the possibility of branching along the length of a hydrocarbon molecule. An alkyl branch is not part of the parent chain and contains only singly-bonded carbon and hydrogen atoms.


    Naming branched alkanes is trickier than naming unbranched alkanes! Fortunately,  the IUPAC (International Union of Pure and Applied Chemists) has set standards relating to organic nomenclature.


    To name branched alkanes, follow the steps below:

    1. Identify the longest continuous chain of carbon atoms. This is the parent  chain (also referred to as the main chain). 

    2. Number the carbon atoms in the parent chain such that the branches have the lowest set of numbers. The lowest set of numbers is defined by IUPAC as being the set that starts with the lowest number. Start numbering the longest continuous chain from the end with the closest branch!

    3. Identify any alkyl branches. Count the number of carbon atoms in the branch, use the appropriate prefix, and add the ending yl (methyl, ethyl, propyl, etc.). List the alkyl branches in alphabetical order.
       
    4.  Indicate the locations of the alkyl branches using numbers.  If there is more than one branch of the same type, use additional prefixes di, tri, etc. Please note that these additional prefixes do not affect the alphabetical order of the branches.

    5. When naming a branched cycloalkane, keep the following in mind: 

          • Because the cyclic structure is the parent chain, the prefix cyclo is placed in front of the alkane name. 

          • Number the branches in order to obtain the lowest possible carbon numbers. Begin with the location of one of the branches.
          • If there is only one branch present, omit the "1" from the name.

Watch

 


Examples of Branched Alkanes and Cycloalkanes



2-methylpentane


2,2-dimethylbutane


3,4-diethyl-3-methyl-4-propylheptane


1-ethyl-3-methylcyclopentane

Read pages 367 to 372 of the textbook, giving close attention to the sample problems and communication examples.

Check Your Understanding


Go to your textbook and complete "Practice" questions 9 and 11 on page 371 and Section 9.2 Question 6 on page 373. Check your answers by clicking the banner beneath.

Page 371 Practice Question 9

  1. 2-methylheptane
  2. 3,7-dimethylnonane
  3. 2,4,6-trimethylheptane
  4. 3-ethyl-6-methyloctane

You may have found (b) and (d) tricky because of the way the molecules were displayed.   Not all carbons in the longest chain were in a horizontal line.



Page 371 Practice Question 11

  1. This is incorrect. A number is required for each branch. The correct name is 2,2-dimethylhexane.
  2. This is incorrect. A "di" prefix should be used when two of the same branch are present. The correct name is 3,3-diethylpentane.
  3. This is correct.
  4. This is incorrect. There are four carbons in the longest chain, not three. The correct name is 2,2-dimethylbutane.
  5. This is incorrect. There are six carbons in the longest chain, not five. The correct name is 3-ethylhexane.
  6. This is incorrect. The longest chain has four carbons, not three. The correct name is methylbutane.

Page 373 Section 9.2 Question 6

  1. ethylpentane
  2. trimethylbutane
  3. 4-ethyl-2,4-dimethylhexane
  4. 3-methyl-5-propyloctane

Although the numbers are unnecessary, it is acceptable to name (a) as 3-ethylpentane and (b) as 2,2,3-trimethylbutane.