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Naming Straight-Chain Alkanones

Key Concepts

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

  • Alkanones belong to the group of organic compounds known as ketones.

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

  • A straight-chain alkanone consists of a chain of 3 or more carbon atoms joined to each other by single covalent bonds, with a =O functional group attached to a non-terminal carbon atom in the chain of carbon atoms.

     O
    ||
     
    R-C-R'

    R and R' represented chains of carbon atoms

  • The systematic IUPAC name2 of an alkan-n-one is made up of three parts:

        (i) a prefix or stem

        (ii) an infix (location of C=O along the chain)

        (iii) a suffix (last part of the name, one which sounds like "own" not like "won")

  • The suffix when naming a straight-chain alkanone is always "one"

  • 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

    Note that it is not possible to construct an alkanone with only 1 or 2 carbon atoms in the chain, because, in both these cases, the carbonyl functional group would occur on a terminal (end) carbon atom so the molecules would be alkanals NOT alkanones.

  • The general molecular formula for a straight-chain alkanone is CnH2nO

        where n = number of carbon atoms in the carbon chain

Bonding Concepts

Alkanones are ketones, and are compounds containing ONLY carbon, hydrogen and oxygen atoms.

Ketones, which include the alkanones, contain the C=O functional group, known as the carbonyl functional group, located on a non-terminal carbon atom of the chain of carbon atoms.

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

      .  
    .C.
      .  

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

    .H

  • Each oxygen atom in the alkanone 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 will be used to form covalent bonds to other carbon atoms.

  • If the carbon atom of the carbonyl (C=O) functional group covalently bonds to 1 carbon atom on each side, the resulting molecule is known as acetone:
    Lewis Structure
    (electron dot diagram)
      Valence Structure
     H
    ..
    :O:
    ..
    ..
    H
    ..
     
    H..C:C:C..H
     ..
    H
     ..
    H
     
     
     H
    |
    :O:
    ||
    H
    |
     
    H-C-C-C-H
     |
    H
     |
    H
     

    Note the each carbon atom now has a share in 8 valence electrons, and that each hydrogen atom has a share in 2 valence electrons.

  • It is therefore possible to build up even longer carbon chains by replacing one or other or both of the terminal (end) hydrogen atoms with a carbon atom.

Steps for Naming Straight Chain Alkanones

  • The systematic IUPAC name of an alkan-n-one is made up of three parts:

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

        (ii) an infix (the location of the carbonyl functional group)

        (iii) a suffix (last part of the name indicating the presence of the C=O functional group on a non-terminal carbon atom) : one

  • The name of a straight chain alkanone always ends in the suffix one

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

Step 2: Determine the prefix for the name of the alkanone based on the number of carbon atoms in the chain.
(The name of the parent alkane without the "e" ending)

Step 3: Number each carbon atom along the longest carbon chain so that the carbon atom of the C=O (carbonyl) functional group has the lowest possible number.

Step 4: The locant (the number of the carbon atom which is double bonded to the oxygen atom) becomes the infix, separated from the prefix and the suffix of the name by hyphens.

Step 5: Determine the suffix for the name of the alkanone. All straight chain alkanones containing one C=O (carbonyl) functional group will end in "one".

Step 6: Write the name for the alkanone in the form of prefixinfixsuffix

Note that an infix is NOT required in the final name if the alkanone is made up of only 3 carbon atoms, since the functional group can ONLY occur on the second carbon atom.

Example: 3 Carbon Atoms

Name the straight chain alkanone shown below:

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

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

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

Step 2: Determine the prefix for the name of the alkanone based on the number of carbon atoms in the chain.
(The name of the parent alkane without the "e" ending)

3 carbon atoms in the chain, the parent hydrocarbon is propane.
Prefix is propan

Step 3: Number each carbon atom along the longest carbon chain so that the carbon atom of the C=O (carbonyl) functional group has the lowest possible number.

Numbering:left to right
 H
|
 O
||
 H
|
 
H-C1-C2-C3-H
 |
H
   |
H
 
locant is 2
 
Numbering:right to left
 H
|
 O
||
 H
|
 
H-C3-C2-C1-H
 |
H
   |
H
 
locant is 2

Step 4: The locant (the number of the carbon atom which is double bonded to the oxygen atom) becomes the infix, separated from the prefix and the suffix of the name by hyphens.

infix is -2-

Step 5: Determine the suffix for the name of the alkanone.
All straight chain alkanones containing one C=O (carbonyl) functional group will end in "one".

suffix is one

Step 6: Write the name for the alkanone in the form of prefixinfixsuffix

Systematic name is propan-2-one

BUT, because there is ONLY one possible location for the carbonyl functional group in this molecule, the infix is NOT required and the systematic IUPAC name is propanone

Note that the retained name, acetone, can also be used3.

Steps for Drawing the Structure4 of Straight Chain Alkanones

Step 1: Break the systematic IUPAC name of the alkanone into its three parts:

alkan -n- one
prefix infix 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: Number the carbon atoms in this chain from left to right in ascending order.

Step 5: Place an oxygen (O) atom above the carbon atom at the location in the chain as indicated by the infix and join it to the carbon atom using a covalent double bond (=)

Step 6: Draw dashes around each carbon atom in the chain so that each carbon atom makes a total of 4 covalent bonds.

Step 7: 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 alkanone pentan-2-one.

Step 1: Break the systematic IUPAC name of the alkanone into its three parts:

alkan -n- one
prefix infix suffix
pentan -2- one

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

Prefix = pentan

There must be 5 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 - C

Step 4: Number the carbon atoms in this chain from left to right in ascending order.

C1 - C2 - C3 - C4 - C5

Step 5: Place an oxygen (O) atom above the carbon atom at the location in the chain as indicated by the infix and join it to the carbon atom using a covalent double bond (=)

infix is -2-
Double bonded oxygen atom is attached to carbon 2 in the chain.

    O
||
           
C1 - C2 - C3 - C4 - C5

Step 6: Draw dashes around each carbon atom in the chain so that each carbon atom makes a total of 4 covalent bonds.

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

Step 7: Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

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

Steps for Writing the Molecular Formula of Straight Chain Alkanones

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

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

When writing the molecular formula of an alkanone, 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 alkanone 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 alkanone 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 alkanone 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 alkanone 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 butan-2-one.

Step 1: Draw the structure of the alkanone molecule.

  H
|
  O
||
  H
|
  H
|
 
H- C - C - C - C -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 alkanone molecule.

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

C4H O

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

  2H
|
  O
||
  3H
|
  4H
|
 
1H- C - 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).

C4H8O

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

  H
|
  1O
||
  H
|
  H
|
 
H- C - C - C - C -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.

C4H8O

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

n = number of carbon atoms = 4

number of hydrogen atoms = 2 x n = 2 x 4 = 8

number of oxygen atoms = 1

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

no. C atoms
(n)
Molecular Formula
CnH2nO
Structure Systematic IUPAC Name
(alternative Systematic Name)
[functional class name5]
3 C3H6O
    H
|
  O
||
  H
|
 
H - C - C - C -H
    |
H
      |
H
 
propan-2-one
(propanone)
[dimethyl ketone]

4 C4H8O
    H
|
  O
||
  H
|
  H
|
 
H - C - C - C - C -H
    |
H
      |
H
  |
H
 
butan-2-one
(butanone)
[ethyl methyl ketone]

5 C5H10O
    H
|
  O
||
  H
|
  H
|
  H
|
 
H - C - C - C - C - C -H
    |
H
      |
H
  |
H
  |
H
 
pentan-2-one
(2-pentanone)
[methyl propyl ketone]
    H
|
  H
|
  O
||
  H
|
  H
|
 
H - C - C - C - C - C -H
    |
H
  |
H
      |
H
  |
H
 
pentan-3-one
(3-pentanone)
[diethyl ketone]

6 C6H12O
    H
|
  O
||
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C -H
    |
H
      |
H
  |
H
  |
H
     
hexan-2-one
(2-hexanone)
[butyl methyl ketone]
    H
|
  H
|
  O
||
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C -H
    |
H
  |
H
      |
H
  |
H
  |
H
 
hexan-3-one
(3-hexanone)
[ethyl propyl ketone]

7 C7H14O
    H
|
  O
||
  H
|
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C -H
    |
H
      |
H
  |
H
  |
H
  |
H
  |
H
 
heptan-2-one
(2-heptanone)
[methyl pentyl ketone]
    H
|
  H
|
  O
||
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C -H
    |
H
  |
H
      |
H
  |
H
  |
H
  |
H
 
heptan-3-one
(3-heptanone)
[butyl ethyl ketone]
    H
|
  H
|
  H
|
  O
||
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C -H
    |
H
  |
H
  |
H
      |
H
  |
H
  |
H
 
heptan-4-one
(4-heptanone)
[dipropyl ketone]

8 C8H16O
    H
|
  O
||
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C - C -H
    |
H
      |
H
  |
H
  |
H
  |
H
  |
H
  |
H
 
octan-2-one
(2-octanone)
[hexyl methyl ketone]
    H
|
  H
|
  O
||
  H
|
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C - C -H
    |
H
  |
H
      |
H
  |
H
  |
H
  |
H
  |
H
 
octan-3-one
(3-octanone)
[ethyl pentyl ketone]
    H
|
  H
|
  H
|
  O
||
  H
|
  H
|
  H
|
  H
|
 
H - C - C - C - C - C - C - C - C -H
    |
H
  |
H
  |
H
      |
H
  |
H
  |
H
  |
H
 
octan-4-one
(4-octanone)
[butyl propyl ketone]


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1The C=O functional group in alkanones is called the carbonyl group.
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 uses substitutive nomenclature and 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 alkanones being considered here, it really doesn't make much difference whether the infix is at the beginning of the name or in the middle since the name will not be ambiguous.
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).

3It is not clear in the IUPAC document cited above as to which name for CH3COCH3 is the preferred IUPAC name since each of the names is stated to be the preferred IUPAC name at different places in the document. (page 2 acetone (PIN), page 46 propan-2-one (PIN), page 505 acetone (not preferred), page 612 propan-2-one (PIN))
It is likely that the preferred IUPAC name is acetone, but that when the molecule is modified the preferred stem name becomes the substitutive IUPAC name propan-2-one.

4"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

5Functional class nomenclature is an alternative IUPAC system for naming compounds.
For each alkanone, the C=O functional group is named last as the ketone.
Each carbon chain attached to this "central" functional group is then named in accordance with the rules for naming alkanes, with the "e" of alkane removed and replaced with "yl" as per naming branched-chain alkanes.
Each chain's name is then listed alphabetically and separated by a space.
The general name for an alkanone using functional class nomenclature will be alkyl alkyl ketone.
You will also find the functional class names for alkanones referred to as "common names".

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