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Naming Straight-Chain Alkanoic Acids

Key Concepts

  • Alkanoic acids are organic molecules containing only carbon (C), hydrogen (H) and oxygen (O) atoms.

  • Alkanoic acids belong to the group of organic compounds known as carboxylic acids.

  • All alkanoic acids contain the carboxyl1 (COOH) functional group.
     O
    ||
     
    -C-OH

  • A straight-chain alkanoic acid consists of a chain of 1 or more carbon atoms joined to each other by single covalent bonds, with an OH and a =O functional group attached to a terminal (end) carbon atom in the chain.
  • Numbering of the carbon chain begins with the carbon atom of the carboxyl functional group.

  • The systematic IUPAC name2 of an alkanoic acid is made up of two parts:

        (i) a prefix or stem
            (the name of the parent hydrocarbon (the alkane chain) without the "e" ending, alkan)

        (ii) a suffix
            (last part of the name, oic acid)

  • The suffix when naming a straight-chain alkanoic acid is always "oic acid"

  • The prefix or stem is dependent on the number of carbon atoms in the longest chain of carbon atoms (the parent hydrocarbon, or parent alkane):
    Number of carbon atoms:12345678910
    Prefix:methethpropbutpenthexheptoctnondec

  • The general molecular formula for a straight-chain alkanoic acid is CnH2nO2

        where n = number of carbon atoms in the carbon chain

Steps for Naming Straight Chain Alkanoic Acids

  • The systematic IUPAC name of an alkanoic acid is made up of two parts:

        (i) a prefix or stem (first part of the name, the name of the parent alkane without the "e" ending) : alkan

        (ii) a suffix (last part of the name indicating the presence of the COOH functional group) : oic acid

  • The name of a straight chain alkanoic acid always ends in the suffix oic acid

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

    Number of carbon atoms:12345678910
    Prefix:methethpropbutpenthexheptoctnondec

Step 1: Identify the longest carbon chain containing the COOH (carboxyl) functional group.

Step 2: Number each carbon atom along the longest carbon chain so that the carbon atom of the COOH (carboxyl) functional group has the number 1.

Step 3: Determine the prefix for the name of the alkanoic acid based on the number of carbon atoms in the chain.
(The name of the parent alkane without the "e" ending)

Step 4: Determine the suffix for the name of the alkanoic acid.
All straight chain alkanoic acids containing one COOH (carboxyl) functional group will end in "oic acid".

Step 5: Write the name for the alkanoic acid in the form of prefixsuffix

Example: 1 Carbon Atom

Name the straight chain alkanoic acid shown below:

 O
||
  
H-C-OH

Step 1: Identify the longest carbon chain containing the COOH (carboxyl) functional group.

 O
||
  
H-C-OH

Step 2: Number each carbon atom along the longest carbon chain so that the carbon atom of the COOH (carboxyl) functional group has the number 1.

 O
||
  
H-C1-OH

Step 3: Determine the prefix for the name of the alkanoic acid based on the number of carbon atoms in the chain.
(The name of the parent alkane without the "e" ending)

Longest carbon chain has 1 carbon atom.

Parent hydrocarbon is methane.

Prefix is methan

Step 4: Determine the suffix for the name of the alkanoic acid.
All straight chain alkanoic acids containing one COOH (carboxyl) functional group will end in "oic acid".

Suffix is oic acid

Step 5: Write the name for the alkanoic acid in the form of prefixsuffix

Systematic IUPAC name is methanoic acid
(Preferred IUPAC name is formic acid.)

Example: 2 Carbon Atom Chain

Name the straight chain alkanoic acid shown below:

 H
|
 O
||
  
H-C-C-OH
 |
H
    

Step 1: Identify the longest carbon chain containing the COOH (carboxyl) functional group.

 H
|
 O
||
  
H-C-C-OH
 |
H
    

Step 2: Number each carbon atom along the longest carbon chain so that the carbon atom of the COOH (carboxyl) functional group has the number 1.

 H
|
 O
||
  
H-C2-C1-OH
 |
H
    

Step 3: Determine the prefix for the name of the alkanoic acid based on the number of carbon atoms in the chain.
(The name of the parent alkane without the "e" ending)

Longest carbon chain has 2 carbon atoms.

Parent hydrocarbon is ethane.

Prefix is ethan

Step 4: Determine the suffix for the name of the alkanoic acid.
All straight chain alkanoic acids containing one COOH (carboxyl) functional group will end in "oic acid".

Suffix is oic acid

Step 5: Write the name for the alkanoic acid in the form of prefixsuffix

Systematic IUPAC name is ethanoic acid
(Preferred IUPAC name is acetic acid.)

Example: 3 Carbon Atom Chain

Name the straight chain alkanol shown below:

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

Step 1: Identify the longest carbon chain containing the COOH (carboxyl) functional group.

 H
|
 H
|
 O
||
  
H-C-C-C-OH
 |
H
 |
H
    

Step 2: Number each carbon atom along the longest carbon chain so that the carbon atom of the COOH (carboxyl) functional group has the number 1.

 H
|
 H
|
 O
||
  
H-C3-C2-C1-OH
 |
H
 |
H
    

Step 3: Determine the prefix for the name of the alkanoic acid based on the number of carbon atoms in the chain.
(The name of the parent alkane without the "e" ending)

Longest carbon chain has 3 carbon atoms.

Parent hydrocarbon is propane.

Prefix is propan

Step 4: Determine the suffix for the name of the alkanoic acid.
All straight chain alkanoic acids containing one COOH (carboxyl) functional group will end in "oic acid".

Suffix is oic acid

Step 5: Write the name for the alkanoic acid in the form of prefixsuffix

Systematic IUPAC name is propanoic acid
(Preferred IUPAC name is also propanoic acid.)

Steps for Drawing the Structure3 of Straight Chain Alkanoic Acids

Step 1: Break the systematic IUPAC name of the alkanoic acid into its two parts:

alkan oic acid
prefix suffix

Step 2: Determine the number of carbon atoms in the longest alkane carbon chain using the prefix.

Step 3: Draw a chain of carbon atoms of the required length using dashes to represent a single covalent bond between each pair of carbon atoms.

Step 4: For the "oic acid" suffix, on the first carbon atom, draw 2 vertical lines connected to an O atom (representing the double bonded oxygen atom), and a horizontal dash to the OH group (covalently bonded hydroxyl group).

Step 5: Place dashes representing single covalent bonds around the other carbon atoms in the chain so that each of these carbon atoms is surrounded by 4 dashes.

Step 6: Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

Example

Draw a structure for butanoic acid.

Step 1: Break the systematic IUPAC name of the alkanoic acid into its two parts:

butan oic acid
prefix suffix

Step 2: Determine the number of carbon atoms in the longest alkane carbon chain using the prefix.

Prefix is butan, therefore this alkanoic acid has 4 carbon atoms in the chain.

Step 3: Draw a chain of carbon atoms of the required length using dashes to represent a single covalent bond between each pair of carbon atoms.

  C - C - C - C  

Step 4: For the "oic acid" suffix, on the first carbon atom, draw 2 vertical lines connected to an O atom (representing the double bonded oxygen atom), and a horizontal dash to the OH group (covalently bonded hydroxyl group).

  O
||
             
HO- C - C - C - C  

Step 5: Place dashes representing single covalent bonds around the other carbon atoms in the chain so that each of these carbon atoms is surrounded by 4 dashes.

  O
||
   
|
   
|
   
|
 
HO- C - C - C - C -
      |   |   |  

Step 6: Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

  O
||
  H
|
  H
|
  H
|
 
HO- C - C - C - C -H
      |
H
  |
H
  |
H
 

Steps for Writing the Molecular Formula of Straight Chain Alkanoic Acids

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

For a straight-chain alkanoic acid, only three elements are present, carbon (C), hydrogen (H) and oxygen (O).

When writing the molecular formula of an alkanoic acid, the number of carbon atoms is written before the number of hydrogen atoms which is written before the number of oxygen atoms, that is, C is written before H which is written before O4.
A straight chain alkanoic acid will always have 2 oxygen atoms.

CxHyO2

Step 1: Draw the structure of the alkanoic acid molecule.

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

C H O

Step 3: Count the number of carbon atoms in the alkanoic acid 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).

Step 5: Count the number of hydrogen atoms in the alkanoic acid 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).

Step 7: Count the number of oxygen atoms in the alkanoic acid molecule.

Step 8: Write the number of of oxygen atoms into the skeleton molecular formula as a subscript number to the right of the symbol for oxygen (O).
    Note: if only one carboxyl (COOH) group is present, the number of oxygen atoms is 2, and the subscript 2 is included in the molecular formula.

Step 9: Check that your completed molecular formula makes sense (CnH2nO2)

Example

Write the molecular formula for pentanoic acid.

Step 1: Draw the structure of the alkanoic acid molecule.

  O   H   H   H   H  
  ||   |   |   |   |  
HO- C - C - C - C - C -H
      |   |   |   |  
      H   H   H   H  

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

C H O

Step 3: Count the number of carbon atoms in the alkanoic acid molecule.

  O   H   H   H   H  
  ||   |   |   |   |  
HO- C1 - C2 - C3 - C4 - C5 -H
      |   |   |   |  
      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).

C5H O

Step 5: Count the number of hydrogen atoms in the alkanoic acid molecule.

  O   H2   H3   H4   H5  
  ||   |   |   |   |  
1HO- C - C - C - C - C -H6
      |   |   |   |  
      H10   H9   H8   H7  

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).

C5H10O

Step 7: Count the number of oxygen atoms in the alkanoic acid molecule:

  O2   H   H   H   H  
  ||   |   |   |   |  
HO1- C - C - C - C - C -H
      |   |   |   |  
      H   H   H   H  

Step 8: Write the number of of oxygen atoms into the skeleton molecular formula as a subscript number to the right of the symbol for oxygen (O).

2 oxygen atoms are present in this molecule.

Molecular formula for pentanoic acid is C5H10O2

Step 9: Check that your completed molecular formula makes sense (CnH2nO2)
n = number of carbon atoms = 5
number of hydrogen atoms = 2 x n = 2 x 5 = 10
number of oxygen atoms = 2

Summay Table: Molecular Formula, Structure and Name of Some Alkanoic Acids

no. C atoms
(n)
Molecular Formula
CnH2nO2
Structure Preferred IUPAC Name
(alternative name)
1 CH2O2
    O
||
   
H - C - OH
formic acid
(methanoic acid)

2 C2H4O2
    H
|
  O
||
   
H - C - C - OH
    |
H
       
acetic acid
(ethanoic acid)

3 C3H6O2
    H
|
  H
|
  O
||
   
H - C - C - C - OH
    |
H
  |
H
       
propanoic acid
(propionic acid)

4 C4H8O2
    H
|
  H
|
  H
|
  O
||
   
H - C - C - C - C - OH
    |
H
  |
H
  |
H
       
butanoic acid
(butyric acid)

5 C5H10O2
    H
|
  H
|
  H
|
  H
|
  O
||
   
H - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
       
pentanoic acid

6 C6H12O2
    H
|
  H
|
  H
|
  H
|
  H
|
  O
||
   
H - C - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
  |
H
       
hexanoic acid

7 C7H14O2
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  O
||
   
H - C - C - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
       
heptanoic acid

8 C8H16O2
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  O
||
   
H - C - C - C - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
       
octanoic acid

9 C9H18O2
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  O
||
   
H - C - C - C - C - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
       
nonanoic acid

10 C10H20O2
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  O
||
   
H - C - C - C - C - C - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
       
decanoic acid


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1The COOH functional group in alkanoic acids is called the carboxyl group.
If another functional group takes precedence over the carboxyl group in a molecule, such as in the name of radical, carboxy is used as the prefix.

2IUPAC is the abbreviation for the International Union of Pure and Applied Chemistry

The preferred IUPAC systematic name places the infix for the locant immediately before that part of the name to which it relates, except when the preferred IUPAC name is the traditional contracted name in which case the infix is placed at the front of the name.
For the simple straight chain alkanoic acids we are discussing, no infix is required since the carboxyl group is, by definition, at the end of the carbon chain, the locant will always be 1 and the infix will always be 1.
The rules for naming organic compounds are still being developed. The most recent document for referral is "Preferred names in the nomenclature of organic compounds" (Draft 7 October 2004).

3"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

4The molecular formula of an alkanoic acid is CnH2nO2, while the often used CxHyCOOH is, strictly speaking, not a molecular formula but a condensed structural formula.

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