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Naming Branched-Chain Alkanes

Key Concepts

Naming Branched-Chain Alkanes:

  • Name of a branched-chain alkane is made up of two parts:

    alkyl alkane
    branch name stem name
    prefix suffix

  • Name the longest alkane carbon chain (stem) first.

  • Identify the branches (side chains).

  • Name each branch (side chain).
    Number of carbon atoms in branch (side chain) Structure Name
    (alkyl)
    1
    -stem-
    |
    H-C-H
    |
    H
    methyl

    2
    -stem-
    |
    H-C-H
    |
    H-C-H
    |
    H
    ethyl

    3
    -stem-
    |
    H-C-H
    |
    H-C-H
    |
    H-C-H
    |
    H
    propyl

  • Number the carbon atoms making up the stem so that the branches have the lowest possible numbers.

  • Separate the number of the stem carbon atom from the name of the branch by a hyphen (-).

    If two or more identical branches are present:
    (i) Use the prefixes di, tri, tetra, etc before the branch name.

    number of identical branchesprefix
    2di
    3tri
    4tetra

    (ii) Indicate the location of the stem carbon atoms to which each branch is attached by writing the numbers separated by a comma (,) in ascending order.
    (iii) Separate the final number from the prefix of the branch name by a hyphen (-).

    If two or more different branches (side chains) are present:
    (i) Identify and name each branch (side chain).
    (ii) Indicate the location of the stem carbon atoms to which each branch is attached by writing the number then a hyphen and then the name of the branch.
    (ii) Write the names of the branches in alphabetical order (ignoring any di, tri, tetra, etc prefix) separating numbers from letters by a hyphen.

  • Branch name(s) added as a prefix to the name of the stem.

Drawing the structure1 of branched-chain alkanes:

  • Divide the name into the name of the branches and the name of the stem.

  • Draw the straight chain alkane stem.

  • Number the carbon atoms making up the stem's chain.

  • Break the rest of the name up into individual branch names.

  • Determine the number of identical branches (if any).

  • Determine the location of the carbon atom on the stem to which each branch (side chain) is attached.

  • Note the locations of the branches on the numbered stem chain you have already drawn.

  • Draw the required number of carbon atoms for each branch at the specified location along the stem's carbon chain.

  • Complete the structure by adding a hydrogen atom at the end of any vacant covalent bond.

Molecular formula of branched-chain alkanes:2

  • Draw the structure of the branched-chain alkane.

  • Write a skeletal molecular formula using the symbols for carbon and hydrogen:
    C   H

  • Count the number of carbon atoms in total in the structure.
    total number of carbon atoms = n

  • Write the number of carbon atoms as a subscript number to the right of the symbol for carbon
    CnH

  • Count the number of hydrogen atoms in total in the structure.
    total number of hydrogen atoms = y

  • Write the number of hydrogen atoms as a subscript number to the right of the symbol for carbon
    CnHy

  • The general molecular formula for a branched-chain alkane is CnH2n+2

        where n = number of carbon atoms in the carbon chain

Animated Tutorial

Examples: Naming a Branched-Chain Alkane

Name the molecule shown below:

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

Step 1. Name the longest alkane carbon chain (stem) first.

          H
|
  H
|
         
  H
|
  H
|
  H-C-H
|
  H-C-H
|
  H
|
  H
|
 
H- C - C - C - C - C - C -H
  |
H
  |
H
  |
H-C-H
  |
H
  |
H
  |
H
 
          |
H-C-H
             
          |
H
             
  • 6 carbon atoms in the longest carbon chain.
            6 carbon atoms prefix = hex

  • Only single bonds between carbon atoms in the chain
            suffix = ane

  • Name of longest alkane carbon chain, stem, is hexane

Step 2. Identify the branches (side chains).

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

Step 3. Name each branch (side chain).

methyl           H
|
  H
|
          methyl
    H
|
  H
|
  H-C-H
|
  H-C-H
|
  H
|
  H
|
   
  H- C - C - C - C - C - C -H  
    |
H
  |
H
  |
H-C-H
  |
H
  |
H
  |
H
   
            |
H-C-H
               
            |
H
              ethyl

Step 4. Number the carbon atoms making up the stem so that the branches have the lowest possible numbers.

methyl           H
|
  H
|
          methyl
    H
|
  H
|
  H-C-H
|
  H-C-H
|
  H
|
  H
|
   
  H- C1 - C2 - C3 - C4 - C5 - C6 -H  
    |
H
  |
H
  |
H-C-H
  |
H
  |
H
  |
H
   
            |
H-C-H
               
            |
H
              ethyl

Step 5. Separate the number of the stem carbon atom from the name of the branch by a hyphen (-).

3-methyl           H
|
  H
|
          4-methyl
    H
|
  H
|
  H-C-H
|
  H-C-H
|
  H
|
  H
|
   
  H- C1 - C2 - C3 - C4 - C5 - C6 -H  
    |
H
  |
H
  |
H-C-H
  |
H
  |
H
  |
H
   
            |
H-C-H
               
            |
H
              3-ethyl

Step 6 (a) Branch name(s) added as a prefix to the name of the stem.

More than 1 branch is present, go to step 6 (b)

Step 6 (b) If two or more identical branches are present:
(i) Use the prefixes di, tri, tetra, etc before the branch name.

Two methyl branches
dimethyl

(ii) Indicate the location of the stem carbon atoms to which each branch is attached by writing the numbers in ascending order separated by a comma (,).
3,4

(iii) Separate the final number from the prefix of the branch name by a hyphen (-).
3,4-dimethyl

Step 6 (c) If two or more different branches (side chains) are present:
(i) Identify and name each branch (side chain).

1 x ethyl
2 x methyl named above as 3,4-dimethyl

(ii) Indicate the location of the stem carbon atoms to which each branch is attached by writing the number then a hyphen and then the name of the branch.
3-ethyl
3,4-dimethyl

(ii) Write the names of the branches in alphabetical order (ignoring any di, tri, tetra, etc prefix) separating numbers from letters by a hyphen.
3-ethyl-3,4-dimethyl

Write the complete name by adding the prefix (name of all the branches) to the suffix (name of the stem alkane)

3-ethyl-3,4-dimethylhexane

Examples: Drawing the Structure of a Branched-Chain Alkane

Draw the structure of 2,2-dimethylpropane.

Step 1: Divide the name of the branched-chain alkane into two parts:

general name: alkyl alkane
  branch name stem name
  prefix suffix
example 2,2-dimethyl propane

Step 2: Draw the straight chain alkane stem

  • stem = propane

  • prefix = number of carbon atoms in chain = 3 (prop)

  • suffix = type of bonds between carbon atoms in chain = single bonds (ane)

  |   |   |  
- C - C - C -
  |   |   |  

Note that each carbon atom can make 4 covalent bonds and that we are drawing in all 4 bonds, even the ones that are not joining two carbon atoms together in the chain.

Step 3: Number the carbon atoms in the stem

  |   |   |  
- C1 - C2 - C3 -
  |   |   |  

Step 4: Break the the branch name up into individual branch (side chain) names:

2,2-dimethyl

Only one type of branch (side chain): methyl

Methyl branches contain only 1 carbon atom.

Step 5: Determine the number of identical branches (side chains):

2,2-dimethyl

There are two methyl groups: di

Step 6: Determine the location of the carbon atom on the stem to which each branch (side chain) is attached:

2,2-dimethyl

Both methyl branches are attached to carbon number 2 along the stem: 2,2-

Step 7: Identify the location of each branch along the stem:

  |   |   |  
- C1 - C2 - C3 -
  |   |   |  

Both methyl branches (side chains) will be attached to C2

Step 5: Draw in the required number of carbon atoms for each branch at the specified location along the stem's carbon chain:

      |
-C-
     
  |   |   |  
- C1 - C2 - C3 -
  |   |   |  
      -C-
|
     

Notice that we are still drawing in all 4 covalent bonds for each carbon atom, even when the bond is not between two carbon atoms.

Step 6: Complete the structure by placing a hydrogen atom (H) at the end of any vacant covalent bond:

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

Examples: Writing the Molecular Formula of Branched-Chain Alkanes

Write the molecular formula for 2-methylpropane.

Step 1: Draw the structure of the branched alkane molecule.

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

Step 2: Write a skeleton molecular formula using the symbols for carbon (C) and hydrogen (H)

C H

Step 3: Count the total number of carbon atoms in the branched-chain alkane molecule.

      H
|
H-C1-H
     
  H
|
  |   H
|
 
H- C2 - C3 - C4 -H
  |
H
  |
H
  |
H
 

There are a total of 4 carbon atoms in the molecule.

Step 4: Write the number of of carbon atoms into the skeleton molecular formula as a subscript number to the right of the symbol for carbon (C).

C4H

Step 5: Count the total number of hydrogen atoms in the branched-chain alkane molecule.

      H4
|
H3-C-H5
     
  H2
|
  |   H6
|
 
H1- C - C - C -H7
  |
H10
  |
H9
  |
H8
 

There are a total of 10 hydrogen atoms in the molecule.

Step 6: Write the number of of hydrogen atoms into the skeleton molecular formula as a subscript number to the right of the symbol for hydrogen (H).

C4H10

Step 7: Check that your completed molecular formula makes sense (CnH2n+2)

when n = 4
2n + 2 = (2 x 4) + 2 = 8 + 2 = 10
So, C4H10 is the molecular formula for the alkane


What would you like to do now?


1"Structure" here will refer to a valence structure, which can be used to represent the 2-dimensional structural formula.

Once you have drawn the valence structure or 2-dimensional structural formula you can use this to draw

a condensed (semi) structural formula

or a skeletal structure

2We are ignoring the possibility of cyclic compounds in this discussion.

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