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Naming Straight Chain Alkanes

Key Concepts

  • Alkanes are hydrocarbons
    (compounds containing ONLY carbon and hydrogen)

  • A single covalent bond joins one carbon atom in the chain to another carbon atom.

  • The name of an alkane is made up of two parts:

        (i) a prefix or stem (first part of the name, alk)

        (ii) a suffix (last part of the name, ane)

  • The suffix when naming an alkane is always "ane"

  • The prefix or stem is dependent on the number of carbon atoms in the chain of carbon atoms:
    Number of carbon atoms:12345678910
    Prefix:methethpropbutpenthexheptoctnondec

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

        where n = number of carbon atoms in the carbon chain

Bonding Concepts

Alkanes are hydrocarbons, that is, alkanes are compounds containing ONLY carbon and hydrogen atoms.

  • Each carbon atom in the alkane molecule has 4 valence electrons.

      .  
    .C.
      .  

  • Each hydrogen atom in the alkane molecule has 1 valence electron.

    .H

  • Each carbon atom in the alkane molecule makes 4 covalent bonds by sharing a valence electron with another atom to make an octet of electrons (4 electron pairs).

    Lewis Structure
    (electron dot diagram)
      Valence Structure
      .   .  
    .
    .
    C.
    .
      .   .  
     
      |  
    -C-
      |  

  • Each carbon atom can covalently bond to hydrogen atoms and to other carbon atoms:

      C-C   C-H
    Lewis Structure
    . .
    C
    .
    .. .
    C
    .
    .
     
    . .
    C
    .
    .. H
    Valence Structure
    . .
    C
    .
    - .
    C
    .
    .
     
    . .
    C
    .
    - H

  • In a straight chain alkane there are only single covalent bonds made up of one pair of electrons joining one carbon atom to another carbon atom.

    Lewis Structure
    (electron dot diagram)
      Valence Structure
      .   .   .  
    .C..C..C.
      .   .   .  
     
      .   .   .  
    .C-C-C.
      .   .   .  

  • In order to complete the octet of electrons around each carbon atom, any unpaired electron will pair up with the valence electron from an atom of hydrogen.

    Lewis Structure
    (electron dot diagram)
      Valence Structure
        H   H   H   
        . .  . .  . .  
    H..C..C..C..H
        . .  . .  . .  
        H   H   H   
     
        H   H   H   
        |   |   |   
    H -C-C-C- H
        |   |   |   
        H   H   H   

Steps for Naming Straight Chain Alkanes

  • The name of an alkane is made up of two parts:

        (i) a prefix or stem (first part of the name) : alk

        (ii) a suffix (last part of the name) : ane

  • The name of a straight chain alkane always ends in the suffix ane

  • The first part of the name of a straight chain alkane, its prefix or stem, is determined by the number of carbon atoms in the chain:

    Number of carbon atoms:12345678910
    Prefix:methethpropbutpenthexheptoctnondec

Step 1: Write the general name for your organic (carbon) compound (alkane)

Step 2: Number the carbon atoms in the carbon chain from left to right (or from right to left).

Step 3: Use the table of prefixes above to convert the number of carbon atoms to a prefix

Step 4: Substitute the alk prefix in the general name for the prefix determined by the number of carbon atoms.

Example

Name the straight chain alkane shown below:

  H
|
 H
|
  
H-C-C-H
  |
H
 |
H
  

Step 1: Write the general name for your organic (carbon) compound

alkane (the name of straight chain alkanes always ends in "ane")

Step 2: Number the carbon atoms in the carbon chain from left to right (or from right to left).

  H
|
 H
|
  
H-C1-C2-H
  |
H
 |
H
  

Step 3: Use the table of prefixes above to convert the number of carbon atoms to a prefix

Number of carbon atoms:2
Prefix:eth

Step 4: Substitute the alk prefix in the general name for the prefix determined by the number of carbon atoms.

General name :     alkane
Name of this alkane :     ethane

Steps for Drawing the Structure of Straight Chain Alkanes

Step 1: Divide the name of the straight chain alkane into a prefix and a suffix

Step 2: The suffix "ane" tells us that there are ONLY single covalent bonds between the carbon atoms in the chain.

Step 3: Use the table above to convert the prefix to the number of carbon atoms in the carbon chain

Step 4: Draw the required number of carbon atoms in a line with a dash (-) between each pair of carbon atoms to represent the single covalent bond

Step 5: Place more dashes (- or | ) to represent other covalent bonds around each carbon atom so that each carbon atom is surrounded by 4 dashes (covalent bonds)

Step 6: Place a hydrogen atom (H) at the end of any covalent bond (dash) that is not used to bond to a carbon atom (C)

Example

Draw a structure1 for the straight chain alkane butane.

Step 1: Divide the name of the straight chain alkane into a prefix and a suffix

name: butane

prefix is but

suffix is ane

Step 2: The suffix ane tells us that there are ONLY single covalent bonds between the carbon atoms in the chain.

Step 3: Use the table above to convert the prefix to the number of carbon atoms in the carbon chain

Number of carbon atoms:4
Prefix:but

Step 4: Draw the required number of carbon atoms in a line with a dash (-) between each pair of carbon atoms to represent the single covalent bond

C1-C2-C3-C4

Step 5: Place more dashes (- or | ) to represent other covalent bonds around each carbon atom so that each carbon atom is surrounded by 4 dashes (covalent bonds)

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

Step 6: Place a hydrogen atom (H) at the end of any covalent bond (- or |) that is not used to bond to a carbon atom (C)

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

Steps for Writing the Molecular Formula of Straight Chain Alkanes

A molecular formula tells us the number of atoms of each element present in a molecule of the compound.

For an alkane, only two elements are present, carbon (C) and hydrogen (H).

When writing the molecular formula of an alkane, the number of carbon atoms is written before the number of hydrogen atoms, that is, C is written before H

Step 1: Draw the structure of the alkane molecule.

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

Step 3: Count the number of carbon atoms in the straight chain.

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).

Step 5: Count the number of hydrogen atoms in the straight chain.

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).

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

Example

Write the molecular formula for propane.

Step 1: Draw the structure of the alkane molecule.

    propane

    prefix or stem: prop means there are 3 carbon atoms (C) in the chain

    suffix: ane means there are only single covalent bonds between carbon atoms in the chain (C-C)

    structure of propane:

    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 number of carbon atoms in the straight chain.

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

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).

C3H

Step 5: Count the number of hydrogen atoms in the straight chain.

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

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).

C3H8

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

propane: 3 carbon atoms so n=3
general alkane formula: CnH2n+2
for n=3: C3H2x3+2 which is C3H8

Animated YouTube Tutorial

Examples

Name no. C atoms
(n)
no. H atoms
(2n + 2)
Formula
CnH2n+2
Structure
methane 1 (2 x 1) + 2 = 4 CH4
 H
|
 
H-C-H
 |
H
 

ethane 2 (2 x 2) + 2 = 6 C2H6
 H
|
 H
|
 
H-C-C-H
 |
H
 |
H
 

propane 3 (2 x 3) + 2 = 8 C3H8
 H
|
 H
|
 H
|
 
H-C-C-C-H
 |
H
 |
H
 |
H
 

butane 4 (2 x 4) + 2 = 10 C4H10
 H
|
 H
|
 H
|
 H
|
 
H-C-C-C-C-H
 |
H
 |
H
 |
H
 |
H
 

pentane 5 (2 x 5) + 2 = 12 C5H12
 H
|
 H
|
 H
|
 H
|
 H
|
 
H-C-C-C-C-C-H
 |
H
 |
H
 |
H
 |
H
 |
H
 

hexane 6 (2 x 6) + 2 = 14 C6H14
 H
|
 H
|
 H
|
 H
|
 H
|
 H
|
 
H-C-C-C-C-C-C-H
 |
H
 |
H
 |
H
 |
H
 |
H
 |
H
 

heptane 7 (2 x 7) + 2 = 16 C7H16
 H
|
 H
|
 H
|
 H
|
 H
|
 H
|
 H
|
 
H-C-C-C-C-C-C-C-H
 |
H
 |
H
 |
H
 |
H
 |
H
 |
H
 |
H
 

octane 8 (2 x 8) + 2 = 18 C8H18
 H
|
 H
|
 H
|
 H
|
 H
|
 H
|
 H
|
 H
|
 
H-C-C-C-C-C-C-C-C-H
 |
H
 |
H
 |
H
 |
H
 |
H
 |
H
 |
H
 |
H
 

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

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