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Naming Straight-Chain Alkanols

Key Concepts

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

  • Alkanols belong to the group of organic compounds known as alcohols.

  • Alcohols are hydroxy compounds, they contain an OH, hydroxy (or hydroxyl)1, functional group.

  • A straight-chain alkanol consists of a chain of 1 or more carbon atoms joined to each other by single covalent bonds, with an OH functional group attached to one of the carbon atoms in the chain.

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

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

    (ii) an infix
    (a number which tells us the location of the OH functional group, -n-)

    (iii) a suffix
    (last part of the name, ol)

  • The suffix when naming a straight-chain alkanol is always "ol"

  • 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 alkanol is CnH2n+2O

        where n = number of carbon atoms in the carbon chain

Steps for Naming Straight Chain Alkanols

Step 1: Identify the longest carbon chain containing the OH (hydroxy or hydroxyl) functional group.

Step 2: Determine the prefix for the name of the alkanol 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 bonded to the OH (hydroxy or hydroxyl) functional group has the lowest possible number.

Step 4: Determine the infix for the name of the alkanol based on the location of the OH (hydroxy or hydroxyl) functional group.

Note that the infix may not be required if the longest carbon chain contains only one or two carbon atoms.

Step 5: Determine the suffix for the name of the alkanol. All straight chain alkanols containing one OH (hydroxy or hydroxyl) functional group will end in "ol".

Step 6: Write the name for the alkanol in the form of prefix-infix-suffix

Example: Naming a Straight-Chain Alkanol

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 OH (hydroxy or hydroxyl) functional group.

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

Step 2: Determine the prefix for the name of the alkanol 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 3: Number each carbon atom along the longest carbon chain so that the carbon atom bonded to the OH (hydroxy or hydroxyl) functional group has the lowest possible number.

chain numbered from right to left   chain numbered from left to right
 H
|
 H
|
 H
|
  
H-C3-C2-C1-OH
 |
H
 |
H
 |
H
  
 
 H
|
 H
|
 H
|
  
H-C1-C2-C3-OH
 |
H
 |
H
 |
H
  
OH group attached to first carbon atom.
1 is less than 3
This numbering is preferred.
  OH group attached to third carbon atom.
3 is greater than 1
This numbering is NOT preferred.

Step 4: Determine the infix for the name of the alkanol based on the location of the OH (hydroxy or hydroxyl) functional group.

Note that the infix may not be required if the longest carbon chain contains only one or two carbon atoms.

 H
|
 H
|
 H
|
  
H-C-C-C1-OH
 |
H
 |
H
 |
H
  

An infix IS required since the OH functional group could be attached to either the first or second carbon atom in the chain.

In this molecule, the OH group is attached to the first carbon atom in the chain, C1, so the infix is -1-

Step 5: Determine the suffix for the name of the alkanol.
All straight chain alkanols containing one OH (hydroxy or hydroxyl) functional group will end in "ol".

Suffix is ol

Step 6: Write the name for the alkanol in the form of prefix-infix-suffix

Systematic IUPAC name is propan-1-ol

Other names: 1-propanol and n-propyl alcohol (a functional name)

Steps for Drawing the Structure3 of Straight Chain Alkanols

Step 1: Break the systematic IUPAC name of the alkan-n-ol into its three parts:

alkan -n- ol
prefix infix suffix

Note that the infix may be absent of the parent alkane chain contains only 1 or 2 carbon atoms.

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: Draw dashes around each carbon atom in the chain such that each carbon atom makes 4 bonds.

Step 5: Number the carbon atoms in the chain from left to right.

Step 6: Determine the location of the OH (hydroxy or hydroxyl) functional group using the infix.
We know there is an OH functional group because the molecule's name has the suffix ol.

Step 7: Draw the OH group at the end of a dash on the carbon with the same number as the infix.

Step 8: Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

Example: Drawing the Structure of a Straight-Chain Alkanol

Draw a structure for the straight chain alkanol propan-2-ol (2-propanol or i-propyl alcohol).

Step 1: Break the systematic IUPAC name of the alkan-n-ol into its three parts:

propan -2- ol
alkan -n- ol
prefix infix suffix

Note that the infix may be absent of the parent alkane chain contains only 1 or 2 carbon atoms.

Step 2: Determine the number of carbon atoms in the longest alkane carbon chain using the prefix.
Prefix is propan
propan indicates the parent alkane chain is propane, a carbon chain containing 3 carbon atoms with single covalent bonds between each carbon atom 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  

Step 4: Draw dashes around each carbon atom in the chain such that each carbon atom makes 4 bonds.

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

Step 5: Number the carbon atoms in the chain from left to right.

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

Step 6: Determine the location of the OH (hydroxy or hydroxyl) functional group using the infix.
We know there is an OH functional group because the molecule's name has the suffix ol.

infix is -2- therefore the OH group is attached to the second atom in the propane chain.

  |   |   |  
- C - C2 - C -
  |   |   |  

Step 7: Draw the OH group at the end of a dash on the carbon with the same number as the infix.

infix is -2- therefore the OH group is attached to the second atom in the propane chain.

      OH      
  |   |       |  
- C - C2  - C -
  |   |       |  
OR
      O-H      
  |   |       |  
- C - C2  - C -
  |   |       |  

Step 8: Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

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

Steps for Writing the Molecular Formula of Straight Chain Alkanols

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

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

When writing the molecular formula of an alkanol, 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:

CxHyO

Step 1: Draw the structure of the alkan-n-ol 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 alkanol 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 alkanol 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 alkanol 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 hydroxy (or hydroxyl, OH) 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 (CnH2n+2O)

Example: Writing the Molecular Formula of a Straight-Chain Alkanols

Write the molecular formula for butan-1-ol (1-butanol or n-butyl alcohol).

Step 1: Draw the structure of the alkan-n-ol molecule.

  H   H   H   H  
  |   |   |   |  
HO- 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 alkanol molecule.

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

C4H O

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

  H2   H3   H4   H5  
  |   |   |   |  
1HO- 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).

C4H10O

Step 7: Count the number of oxygen atoms in the alkanol molecule:

  H   H   H   H  
  |   |   |   |  
HO1- 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).
    Note: if only one hydroxy (or hydroxyl, OH) group is present, the number of oxygen atoms is 1, and the subscript 1 is NOT included in the molecular formula.

Only 1 oxygen atom is present in this molecule so no subscript 1 is required in the molecular formula.

Molecular formula for butan-1-ol is C4H10O

Step 9: Check that your completed molecular formula makes sense (CnH2n+2O)
n = number of carbon atoms = 4
number of hydrogen atoms = 2 x n + 2 = 2 x 4 + 2 = 10
number of oxygen atoms = 1

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

no. C atoms
(n)
Molecular Formula
CnH2n+2O
Structure Name
1 CH4O
    H
|
   
H - C - OH
    |
H
   
methanol

2 C2H6O
    H
|
  H
|
   
H - C - C - OH
    |
H
  |
H
   
ethanol
(ethan-1-ol)

3 C3H8O
    H
|
  H
|
  H
|
   
H - C - C - C - OH
    |
H
  |
H
  |
H
   
propan-1-ol
(1-propanol)
    H
|
  OH
|
  H
|
   
H - C - C - C - H
    |
H
  |
H
  |
H
   
propan-2-ol
(2-propanol)

4 C4H10O
    H
|
  H
|
  H
|
  H
|
   
H - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
   
butan-1-ol
(1-butanol)
    H
|
  H
|
  OH
|
  H
|
   
H - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
   
butan-2-ol
(2-butanol)

5 C5H12O
    H
|
  H
|
  H
|
  H
|
  H
|
   
H - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
  |
H
   
pentan-1-ol
(1-pentanol)
    H
|
  H
|
  H
|
  OH
|
  H
|
   
H - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
   
pentan-2-ol
(2-pentanol)
    H
|
  H
|
  OH
|
  H
|
  H
|
   
H - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
   
pentan-3-ol
(3-pentanol)

6 C6H14O
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
   
H - C - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
hexan-1-ol
(1-hexanol)
    H
|
  H
|
  H
|
  H
|
  OH
|
  H
|
   
H - C - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
hexan-2-ol
(2-hexanol)
    H
|
  H
|
  H
|
  OH
|
  H
|
  H
|
   
H - C - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
hexan-3-ol
(3-hexanol)

7 C7H16O
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
   
H - C - C - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
heptan-1-ol
(1-heptanol)
    H
|
  H
|
  H
|
  H
|
  H
|
  OH
|
  H
|
   
H - C - C - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
heptan-2-ol
(2-heptanol)
    H
|
  H
|
  H
|
  H
|
  OH
|
  H
|
  H
|
   
H - C - C - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
heptan-3-ol
(3-heptanol)
    H
|
  H
|
  H
|
  OH
|
  H
|
  H
|
  H
|
   
H - C - C - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
heptan-4-ol
(4-heptanol)

8 C8H18O
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  H
|
   
H - C - C - C - C - C - C - C - C - OH
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
octan-1-ol
(1-octanol)
    H
|
  H
|
  H
|
  H
|
  H
|
  H
|
  OH
|
  H
|
   
H - C - C - C - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
octan-2-ol
(2-octanol)
    H
|
  H
|
  H
|
  H
|
  H
|
  OH
|
  H
|
  H
|
   
H - C - C - C - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
octan-3-ol
(3-octanol)
    H
|
  H
|
  H
|
  H
|
  OH
|
  H
|
  H
|
  H
|
   
H - C - C - C - C - C - C - C - C - H
    |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
  |
H
   
octan-4-ol
(4-octanol)

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1The OH functional group in alkanols is called the hydroxy group or hydroxyl group.
While the IUPAC document refers to the hydroxy group, hydroxyl is also a possible name because of the retention of the unpaired electron on the oxygen atom.
Hydroxide is NOT a possible name for the OH group because the "ide" suffix refers to the gain of an electron, that is, it refers to the negatively charged OH- ion.
Note that when another functional group takes precedence over the OH functional group, the OH group is then named as the hydroxy group.

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 alkanols we are discussing, either nomenclature is acceptable since the name in each case is unambiguous.
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

4The molecular formula of an alkanol is CnH2n+2O, while the often used CnH2n+1OH is, strictly speaking, not a molecular formula but a condensed structural formula.

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