IUPAC Name, Structure and Formula of Straight Chain Alkanoic Acids Chemistry Tutorial
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 carboxyl(1) (COOH) functional group.
- 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 name(2) 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: | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|
| Prefix: | meth | eth | prop | but | pent | hex | hept | oct | non | dec |
|---|
- The general molecular formula for a straight-chain alkanoic acid is CnH2nO2
where n = number of carbon atoms in the carbon chain
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Structure and Bonding in Alkanoic Acids (carboxylic acids)
Alkanoic acids are carboxylic acids, and are compounds containing ONLY carbon, hydrogen and oxygen atoms.
Carboxylic acids, which include the alkanoic acids, contain the COOH functional group, known as the carboxyl functional group.
- Carbon belongs to group 14 of the periodic table.
Each carbon atom in the alkanoic acid molecule has 4 valence electrons.
- Each hydrogen atom in the alkanoic acid molecule has 1 valence electron.
- Oxygen belongs to group 16 of the periodic table.
Each oxygen atom in the alkanoic acid molecule has 6 valence electrons.
- Each carboxyl (COOH) functional group is made up of 1 atom of carbon, 2 atoms of oxygen and 1 atom of hydrogen sharing a pair of electrons (covalent bond):
Note that each oxygen (O) atom now has a share in 8 valence electrons, and the hydrogen (H) atom has a share in 2 valence electrons.
The unpaired electron on the carbon atom is available to bond with another carbon atom, or with a hydrogen atom.
- If the unpaired electron on the carbon atom is shared with a hydrogen atom, the resulting molecule is known as formic acid (preferred IUPAC name, or methanoic acid which is the systematic IUPAC name):
Lewis Structure
(electron dot diagram) |
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Valence Structure |
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:O:
.. |
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.. |
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.. |
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| H |
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C |
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O |
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. |
H |
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.. |
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- If the unpaired electron on the carbon atom of the carboxyl group is shared with a carbon atom, the resulting molecule is acetic acid (preferred IUPAC name, or, systematic IUPAC name ethanoic acid):
Lewis Structure
(electron dot diagram) |
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Valence Structure |
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H |
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:O:
.. |
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.. |
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.. |
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.. |
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| H |
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C |
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C |
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O |
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H |
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.. |
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.. |
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H |
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H
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:O:
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| H |
- |
C |
- |
C |
- |
..
O
.. |
- |
H |
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H |
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- It is possible to continue adding more carbon atoms to the chain, as long as the carboxyl (COOH) group is always at the end of the chain.
Lewis Structure
(electron dot diagram) |
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Valence Structure |
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H |
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H |
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:O:
.. |
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.. |
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.. |
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.. |
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.. |
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| H |
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C |
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C |
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C |
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O |
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H |
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.. |
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.. |
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.. |
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H |
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H |
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H
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H
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:O:
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| H |
- |
C |
- |
C |
- |
C |
- |
..
O
.. |
- |
H |
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H |
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H |
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Steps for IUPAC Naming of Straight Chain Alkanoic Acids (caboxylic 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: | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
|---|
| Prefix: | meth | eth | prop | but | pent | hex | hept | oct | non | dec |
|---|
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
Worked Example of IUPAC Name of Straight-Chain Alkanoic Acid: 1 Carbon Atom in the Chain
Name the straight chain alkanoic acid shown below using IUPAC nomenclature rules:
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)
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.)
Worked Example of IUPAC Naming of Straight-Chain Alkanoic Acid: 2 Carbon Atom Chain
Name the straight chain alkanoic acid shown below using IUPAC nomenclature rules:
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)
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.)
Worked Example of IUPAC Naming of Straight-Chain Alkanoic Acids: 3 Carbon Atom Chain
Name the straight chain alkanoic acid shown below using IUPAC nomenclature rules:
| | H
| | | H
| | | O
|| | | |
| H- | C | - | C | - | C | - | OH |
| | |
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 Structure(3) 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.
Worked Example of Drawing the Strcuture of a Straight-Chain Alkanoic Acid
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.
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.
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O
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| HO- |
C |
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C |
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C |
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C |
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Step 6: Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.
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O
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H
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H
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H
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| HO- |
C |
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C |
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C |
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C |
-H |
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H |
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H |
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H |
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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 O(4).
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)
Worked Example of Writing the Molecular Formula of a Straight-Chain Alkanoic Acid
Write the molecular formula for pentanoic acid.
Step 1: Draw the structure of the alkanoic acid molecule.
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O |
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H |
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H |
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H |
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H |
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| HO- |
C |
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C |
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C |
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C |
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C |
-H |
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H |
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H |
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H |
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H |
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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.
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O |
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H |
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H |
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H |
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H |
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| HO- |
C1 |
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C2 |
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C3 |
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C4 |
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C5 |
-H |
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H |
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H |
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H |
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H |
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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.
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O |
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H2 |
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H3 |
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H4 |
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H5 |
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| 1HO- |
C |
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C |
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C |
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C |
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C |
-H6 |
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H10 |
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H9 |
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H8 |
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H7 |
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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:
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O2 |
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H |
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H |
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H |
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H |
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| HO1- |
C |
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C |
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C |
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C |
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C |
-H |
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H |
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H |
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H |
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H |
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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 × n = 2 × 5 = 10
number of oxygen atoms = 2
Summay Table: Molecular Formula, Structure and IUPAC Name of Some Alkanoic Acids
no. C atoms
(n) |
Molecular Formula
CnH2nO2 |
Structure |
Preferred IUPAC Name
(alternative name) |
| 1 |
CH2O2 |
|
formic acid
(methanoic acid) |
|
| 2 |
C2H4O2 |
|
acetic acid
(ethanoic acid) |
|
| 3 |
C3H6O2 |
| |
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H
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H
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O
|| |
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| H |
- |
C |
- |
C |
- |
C |
- |
OH |
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H |
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H |
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propanoic acid
(propionic acid) |
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| 4 |
C4H8O2 |
| |
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H
| |
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H
| |
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H
| |
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O
|| |
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| H |
- |
C |
- |
C |
- |
C |
- |
C |
- |
OH |
| |
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H |
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H |
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H |
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butanoic acid
(butyric acid) |
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| 5 |
C5H10O2 |
| |
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H
| |
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H
| |
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H
| |
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H
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O
|| |
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| H |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
OH |
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H |
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H |
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H |
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H |
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pentanoic acid |
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| 6 |
C6H12O2 |
| |
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H
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H
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H
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H
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H
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O
|| |
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| H |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
OH |
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H |
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H |
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H |
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H |
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H |
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hexanoic acid |
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| 7 |
C7H14O2 |
| |
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H
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H
| |
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H
| |
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H
| |
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H
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H
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O
|| |
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| H |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
OH |
| |
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H |
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H |
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H |
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H |
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H |
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H |
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heptanoic acid |
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| 8 |
C8H16O2 |
| |
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H
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H
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H
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H
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H
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H
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H
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O
|| |
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| H |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
OH |
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H |
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H |
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H |
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H |
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H |
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H |
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H |
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octanoic acid |
|
| 9 |
C9H18O2 |
| |
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H
| |
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H
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H
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H
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H
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H
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H
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H
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O
|| |
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| H |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
OH |
| |
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H |
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H |
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H |
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H |
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H |
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H |
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H |
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H |
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nonanoic acid |
|
| 10 |
C10H20O2 |
| |
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H
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H
| |
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H
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H
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H
| |
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H
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H
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H
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H
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O
|| |
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| H |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
C |
- |
OH |
| |
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H |
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H |
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H |
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H |
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H |
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H |
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H |
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H |
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H |
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decanoic acid |
Footnotes:
(1) The 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.
(2) IUPAC 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
(4) The 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.
