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Naming Straight-Chain Alkanals

Key Concepts

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

  • Alkanals belong to the group of organic compounds known as aldehydes.

  • Aldehydes contain a C=O, carbonyl1, functional group.

  • A straight-chain alkanal consists of a chain of 1 or more carbon atoms joined to each other by single covalent bonds, with a C=O functional group attached to the terminal (end) carbon atom in the chain of carbon atoms.

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

        (i) a prefix or stem

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

  • The suffix when naming a straight-chain alkanal is always "al"

  • 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 alkanal is CnH2nO

        where n = number of carbon atoms in the carbon chain

Bonding Concepts

Alkanals are aldehydes, and are compounds containing ONLY carbon, hydrogen and oxygen atoms.

Aldehydes, which include the alkanals, contain the C=O functional group, known as the carbonyl functional group, located on a terminal (end) carbon atom of the chain of carbon atoms.

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

      .  
    .C.
      .  

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

    .H

  • Each oxygen atom in the alkanal molecule has 6 valence electrons.

    :O:
      ..  

  • Each carbonyl (C=O) functional group is made up of 1 atom of carbon and 1 atom of oxygen sharing 2 pairs of electrons (double covalent bond):
    Lewis Structure
    (electron dot diagram)
      Valence Structure
         .
    :O::C.
      ..   
     
         .
    :O=C.
      ..   

    Note that the oxygen atom now has a share in 8 valence electrons in total.
    Note that the carbon atom has 2 unpaired electrons. These electrons are available to form a covalent bond to hydrogen atoms, or other carbon atoms.

Steps for Naming Straight Chain Alkanals

  • The systematic IUPAC name of an alkanal 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 C=O functional group on a terminal carbon atom) : al

  • The name of a straight chain alkanal always ends in the suffix al

  • The first part of the name of a straight chain alkanal, 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 terminal C=O (carbonyl) functional group.

Step 2: Number each carbon atom along the longest carbon chain so that the carbon atom of the C=O (carbonyl) functional group has the number 1.

Step 3: Determine the prefix for the name of the alkanal 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 alkanal. All straight chain alkanals containing one C=O (carbonyl) functional group will end in "al".

Step 5: Write the name for the alkanal in the form of prefixsuffix

Example: 1 Carbon Atom

Name the straight chain alkanal shown below:

 H
|
  
H-C=O

Step 1: Identify the longest carbon chain containing the terminal C=O (carbonyl) functional group.

 H
|
  
H-C=O

Step 2: Number each carbon atom along the longest carbon chain so that the carbon atom of the C=O (carbonyl) functional group has the number 1.

 H
|
  
H-C1=O

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

Parent hydrocarbon is methane.

Prefix is methan

Step 4: Determine the suffix for the name of the alkanal.
All straight chain alkanals containing one C=O (carbonyl) functional group will end in "al".

Suffix is al

Step 5: Write the name for the alkanal in the form of prefixsuffix

methanal
(Note that methanal is a systematic IUPAC name but the preferred IUPAC name for this molecule is formaldehyde).

Example: 2 Carbon Atom Chain

Name the straight chain alkanol shown below:

 H
|
 H
|
  
H-C-C=O
 |
H
    

Step 1: Identify the longest carbon chain containing the terminal C=O (carbonyl) functional group.

 H
|
H
|
  
H-C-C=O
 |
H
   

Step 2: Number each carbon atom along the longest carbon chain so that the carbon atom of the C=O (carbonyl) functional group has the number 1.

 H
|
H
|
 
H-C-2C1=O
 |
H
   

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

Parent hydrocarbon is ethane.

Prefix is ethan

Step 4: Determine the suffix for the name of the alkanal.
All straight chain alkanals containing one C=O (carbonyl) functional group will end in "al".

Suffix is al

Step 5: Write the name for the alkanal in the form of prefixsuffix

ethanal
(Note that ethanal is a systematic IUPAC name but the preferred IUPAC name for this molecule is acetaldehyde).

Example: 3 Carbon Atom Chain

Name the straight chain alkanol shown below:

 H
|
 H
|
 H
|
  
H-C-C-C=O
 |
H
 |
H
    

Step 1: Identify the longest carbon chain containing the terminal C=O (carbonyl) functional group.

 H
|
 H
|
 H
|
  
H-C-C-C=O
 |
H
 |
H
    

Step 2: Number each carbon atom along the longest carbon chain so that the carbon atom of the C=O (carbonyl) functional group has the number 1.

 H
|
 H
|
 H
|
  
H-C3-C2-C1=O
 |
H
 |
H
    

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

Parent hydrocarbon is propane.

Prefix is propan

Step 4: Determine the suffix for the name of the alkanal.
All straight chain alkanals containing one C=O (carbonyl) functional group will end in "al".

Suffix is al

Step 5: Write the name for the alkanal in the form of prefixsuffix

propanal
(Note that propanal is both the systematic IUPAC name and the preferred IUPAC name for this molecule. The molecule is also known as propionaldehyde).

Steps for Drawing the Structure3 of Straight Chain Alkanals

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

alkan al
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: Place an oxygen (O) atom at the end of the carbon chain and connect it with a double covalent bond (=) to the last carbon atom in the chain.

Step 5: Draw dashes around each carbon atom in the chain so that each carbon atom makes a total of 4 covalent bonds.

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

Example

Draw a structure for the straight chain alkanal butanal.

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

prefix suffix
butan al

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

Prefix is butan therefore the longest carbon chain has 4 carbon atoms.

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: Place an oxygen (O) atom at the end of the carbon chain and connect it with a double covalent bond (=) to the last carbon atom in the chain.

  C - C - C - C =O

Step 5: Draw dashes around each carbon atom in the chain so that each carbon atom makes a total of 4 covalent bonds.

  |   |   |   |  
- C - C - C - C =O
  |   |   |      

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

  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C =O
  |
H
  |
H
  |
H
     

Steps for Writing the Molecular Formula of Straight Chain Alkanals

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

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

When writing the molecular formula of an alkanal, 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:

CxHyO

Step 1: Draw the structure of the alkanal 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 alkanal 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 alkanal 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 alkanal 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 carbonyl (C=O) group is present, the number of oxygen atoms is 1, and the subscript 1 is NOT included in the molecular formula.

Step 9: Check that your completed molecular formula makes sense (CnH2nO)

Example

Write the molecular formula for pentanal.

Step 1: Draw the structure of the alkanal molecule.

  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C - C =O
  |
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 alkanal molecule.

  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C5 - C4 - C3 - C2 - C1 =O
  |
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 alkanal molecule.

  H5
|
  H4
|
  H3
|
  H2
|
  H1
|
 
6H- C - C - C - C - C =O
  |
H7
  |
H8
  |
H9
  |
H10
     

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 alkanal molecule.

  H
|
  H
|
  H
|
  H
|
  H
|
 
H- C - C - C - C - C =O1
  |
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).
    Note: if only one carbonyl (C=O) group is present, the number of oxygen atoms is 1, and the subscript 1 is NOT included in the molecular formula.

C5H10O

Step 9: Check that your completed molecular formula makes sense (CnH2nO)

n = number of carbon atoms = 5

number of hydrogen atoms = 2 x n = 2 x 5 = 10

number of oxygen atoms = 1

Summay Table: Molecular Formula, Structure and Name of Some Alkanals

no. C atoms
(n)
Molecular Formula
CnH2nO
Structure Preferred IUPAC Name
(other name)
1 CH2O
    H
|
 
H - C =O
formaldehyde
(methanal)

2 C2H4O
    H
|
  H
|
 
H - C - C =O
    |
H
     
acetaldehyde
(ethanal)

3 C3H6O
    H
|
  H
|
  H
|
 
H - C - C - C =O
    |
H
  |
H
     
propanal
(propionaldehyde)

4 C4H8O
    H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C =O
    |
H
  |
H
  |
H
     
butanal
(butyraldehyde)

5 C5H10O
    H
|
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C =O
    |
H
  |
H
  |
H
  |
H
     
pentanal
(valeraldehyde)

6 C6H12O
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C =O
    |
H
  |
H
  |
H
  |
H
  |
H
     
hexanal

7 C7H14O
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C =O
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
     
heptanal

8 C8H16O
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C - C =O
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
     
octanal

9 C9H18O
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C - C - C =O
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
     
nonanal

10 C10H20O
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C - C - C - C =O
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
     
decanal


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1The C=O functional group in alkanals is called the carbonyl group.
Note that if the C=O group is attached to a ring, the molecule is named as a carbaldehyde instead of an alkanal.
Note that when another functional group, such as a carboxyl group, takes precedence over the C=O functional group, the C=O group is then named as the oxy 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.
Since the the C=O functional group is always located on a terminal carbon atom in a straight chain alkanal, no infix is required.
The systematic IUPAC name is derived from a set of general "rules" designed to ensure that each organic molecule can be given an unambiguous name.
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

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