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Naming Branched-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 branched-chain alkanol consists of a "stem" and "branches".
    "Branches" are also referred to as "side chains".

  • "Stem" is made up of a chain of 3 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.

  • "Branches" are made up of 1 or more carbon atoms joined to a carbon atom of the "stem" by a covalent bond.

  • The systematic IUPAC name2 of a branched-chain alkanol is made up of three main parts:

    branch name(s) stem name suffix

  • For an alkanol, the suffix is always "ol"

    branch name(s) stem name ol

  • The "stem" name is the name of the parent hydrocarbon with the final "e" removed.

  • For alkanols the parent hydrocarbon is an alkane which is named according to the number of carbon atoms in the chain of carbon atoms:

    Number of carbon atoms:12345678910
    Stem namemethanethanpropanbutanpentanhexanheptanoctannonandecan

  • Branch names are based on the number of carbon atoms in the chain3:

    No. Carbon Atoms
    in branch
    Structure4
    of branch
    Condensed (Semi) Structural
    Formula of branch
    Name
    of branch
    1
      H
    |
     
    - C -H
      |
    H
     
    -CH3 methyl
    2
      H
    |
      H
    |
     
    - C - C -H
      |
    H
      |
    H
     
    -CH2-CH3 ethyl
    3
      H
    |
      H
    |
      H
    |
     
    - C - C - C -H
      |
    H
      |
    H
      |
    H
     
    -CH2-CH2-CH3 propyl

    If more than one of the same branch is present, a multiplying prefix is added to the beginning of the name of that banch:

    No. of identical branches Multiplying Prefix
    2 di
    3 tri
    4 tetra

Steps for Naming Branched Chain Alkanols

  • The systematic IUPAC name of a branched chain alkanol can be represented as:
    branch
    name
    stem
    name
    suffix
    name
    infix- alkyl alkan -n- ol

    The systematic IUPAc name of a branched chain alkanol is therefore made up of 3 main parts:

        (i) a suffix, "ol" , and an infix which tells us the location of the hydroxyl (hydroxy) functional group along the stem.

        (ii) a "stem", the name of the longest alkane chain containing the OH functional group with the "e" of alkane removed

        (iii) "branch" or "branches", any chain or chains of carbon atoms that do not make up the stem, an infix is required to tell us the location of each branch chain along the stem.

  • The name of a straight chain alkanol always ends in the suffix ol

  • The name of an alkyl branch (side chain) and the name of the alkane stem is determined by the number of carbon atoms in the chain:

    Number of carbon atoms in chain:12345678910
    Stem Namemethanethanpropanbutanpentanhexanheptanoctannonandecan
    Branch Namemethylethylpropylbutylpentylhexylheptyloctylnonyldecyl

Step 1: Identify the longest carbon chain containing the OH (hydroxy or hydroxyl) functional group (this is the "stem")

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

Stem name is alkan

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 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 5: Determine the infix for the "ol" suffix based on the location of the OH (hydroxy or hydroxyl) functional group.

Write this infix in the form of -number-

Step 6: Write the name of the "stem"

Stem name: alkan-number-ol

Step 7: Identify all alkyl groups (branches or side-chains).

Step 8(a): Write the name(s) of these alkyl groups (branches or side chains) in alphabetical order:

butyl before ethyl before methyl before pentyl before propyl

Step 8(b): If there is more than one of the same alkyl group, add a suitable multiplying prefix (di, tri, tetra) to the beginning of the name of that alkyl group.
Note: adding a multiplying prefix to the name of a branch (side) chain DOES NOT change the alphabetical order of the alkyl groups above.

dibutyl before diethyl before dimethyl before dipentyl before dipropyl

dibutyl before ethyl before methyl before pentyl before propyl

butyl before ethyl before methyl before pentyl before dipropyl

Step 9(a): Determine the infix for each alkyl group (branch or side chain) based on its location along the "stem"

Write the infix (including the hyphen) to the left of the name of its alkyl group in the aphabetical list:

infix-alkyl

Step 9(b): If there is more than one of the same alkyl group (as indicated by a multiplying prefix such as di, tri, etc), the location of each instance of this alkyl group is written down in ascending order and each number is separated from the other numbers by a comma.

lower number,higher number-dialkyl

lowest number,middle number,highest number-trialkyl

Step 10: Write the complete systematic IUPAC name for the branched chain alkanol in the form of

infix(s)-branch name(s) stem name -infix-suffix
infix(s)-branch name(s) stem name -infix-ol

Note that the name is written as one word.
There are no spaces between:
- letters and letters
- numbers and numbers
- letters and numbers
- hyphens and numbers
- hyphens and letters
- numbers and commas

Example: Naming a Branched Chain Alkanol with Only One Branch (side chain)

Name the branched chain alkanol molecule shown below:

      H
|
       
  H
|
H
 
-C-
|
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 (this is the "stem")

      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C - C - C - OH
  |
H
  |
H
  |
H
   
3 carbon alkane chain
is the same as
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C - C - C - OH
  |
H
  |
H
  |
H
   
3 carbon alkane chain

Step 2: Determine the name of the alkane "stem" based on the number of carbon atoms in this longest chain.
Parent alkane is propane (3 carbon alkane chain)

Stem name 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.

Numbering from right to left
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C3 - C2 - C1 - OH
  |
H
  |
H
  |
H
   
 
Numbering from left to right
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C1 - C2 - C3 - OH
  |
H
  |
H
  |
H
   

Numbering from right to left is preferred: OH group attached to carbon number 1 in the chain.
Numbering from left to right is NOT preferred: OH attached to carbon number 3 in the chain.
(3 is larger than 1, so the carbon chain will be numbered from right to left, NOT from left to right)

Step 4: 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 5: Determine the infix for the "ol" suffix based on the location of the OH (hydroxy or hydroxyl) functional group.

Numbering from right to left
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C3 - C2 - C1 - OH
  |
H
  |
H
  |
H
   

OH group attached to C1

Infix is -1-

Step 6: Write the name of the "stem" : alkan-infix-ol

Stem name: propan-1-ol

Step 7: Identify all alkyl groups "branching off" the propan-1-ol "stem" (these alkyl groups are the branches or side-chains).

Numbering from right to left
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C3 - C2 - C1 - OH
  |
H
  |
H
  |
H
   

Step 8(a): Write the name(s) of these alkyl groups (branches or side chains) in alphabetical order:

One branch.
Branch is made up of an alkyl group with only 1 carbon atom.

Branch name is methyl

Step 8(b): If there is more than one of the same alkyl group, add a suitable multiplying prefix (di, tri, tetra) to the beginning of the name of that alkyl group.

Only one branch so this step is NOT required.

Step 9(a): Determine the infix for each alkyl group (branch or side chain) based on its location along the "stem"

Numbering from right to left
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C3 - C2 - C1 - OH
  |
H
  |
H
  |
H
   

The methyl group is attached to C2

Infix is 2-

Name of this branch is 2-methyl

Step 9(b): If there is more than one of the same alkyl group (as indicated by a multiplying prefix such as di, tri, etc), the location of each instance of this alkyl group is written down in ascending order and each number is separated from the other numbers by a comma.

Only one branch (side chain) is present so this step is NOT required.

Step 10: Write the complete systematic IUPAC name for the branched chain alkanol in the form of

infix(s)-branch name(s) stem name -infix-suffix
infix(s)-branch name(s) stem name -infix-ol
2-methyl propan -1-ol

Systematic IUPAC name is 2-methylpropan-1-ol.
Note that there are no spaces between numbers and hyphens, no spaces between letters and hyphens, and there is NOT a space between "methyl" and "propan-1-ol"

Example: Naming a Branched Chain Alkanol with Two Identical Alkyl Branches

Name the branched chain alkanol molecule shown below:

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

Step 1: Identify the longest carbon chain containing the OH (hydroxy or hydroxyl) functional group (this is the "stem")

      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C - C - C - OH
  |
H
 
H-
|
-C-
 
-H
|
H
   
      |
H
       
is the same as
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C - C - C - OH
  |
H
 
H-
|
-C-
 
-H
|
H
   
      |
H
       
is the same as
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C - C - C - OH
  |
H
 
H-
|
-C-
 
-H
|
H
   
      |
H
       

The longest carbon chain containing the OH functional group has 3 carbon atoms.

Step 2: Determine the name of the alkane "stem" based on the number of carbon atoms in this longest chain.
Parent alkane is propane (3 carbon atom alkane chain)

Stem name 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.

numbering from right to left   numbering from left to right
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C3 - C2 - C1 - OH
  |
H
 
H-
|
-C-
 
-H
|
H
   
      |
H
       
 
      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C1 - C2 - C3 - OH
  |
H
 
H-
|
-C-
 
-H
|
H
   
      |
H
       

Preferred numbering is from right ro left because it results in the OH group being attached to the carbon atom of the chain with the lowest number (number 1 instead of number 3)

Step 4: 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 5: Determine the infix for the "ol" suffix based on the location of the OH (hydroxy or hydroxyl) functional group.

OH group attached to carbon 1 of the alkane chain.

Infix is -1-

Step 6: Write the name of the "stem" : alkan-infix-ol

Stem name: propan-1-ol

Step 7: Identify all alkyl groups (branches or side-chains).

      H
|
       
  H
|
H
 
-C-
|
H
 
H
|
   
H- C3 - C2 - C1 - OH
  |
H
 
H-
|
-C-
 
-H
|
H
   
      |
H
       

There are 2 branches, one is shown in red and the other in orange

Step 8(a): Write the name(s) of these alkyl groups (branches or side chains) in alphabetical order:

Branch shown in red has only 1 carbon atom, it is a methyl group

Branch shown in orange also has only 1 carbon atom, it is also a methyl group

Step 8(b): If there is more than one of the same alkyl group, add a suitable multiplying prefix (di, tri, tetra) to the beginning of the name of that alkyl group.
Note: adding a multiplying prefix to the name of a branch (side) chain DOES NOT change the alphabetical order of the alkyl groups above.

There are 2 identical branches (side chains), 2 methyl groups

Multiplying prefix is di

Branch name: dimethyl

Step 9(a): Determine the infix for each alkyl group (branch or side chain) based on its location along the "stem"

Write the infix (including the hyphen) to the left of the name of its alkyl group in the aphabetical list:

Branch shown in red attached to C2, infix is 2-

Branch shown in red attached to C2, infix is also 2-

Step 9(b): If there is more than one of the same alkyl group (as indicated by a multiplying prefix such as di, tri, etc), the location of each instance of this alkyl group is written down in ascending order and each number is separated from the other numbers by a comma.

Completed prefix for name is: 2,2-dimethyl

Step 10: Write the complete systematic IUPAC name for the branched chain alkanol in the form of

infix(s)-branch name(s) stem name -infix-suffix
infix(s)-branch name(s) stem name -infix-ol
2,2-dimethyl propan -1-ol

Systematic IUPAC name is 2,2-dimethylpropan-1-ol

Note that the name is written as one word.
There are no spaces between:
- letters and letters
- numbers and numbers
- letters and numbers
- hyphens and numbers
- hyphens and letters
- numbers and commas

Example: Naming Branched Chain Alkanol with Different Alkyl Branches

Name the branched chain alkanol molecule shown below:

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

Step 1: Identify the longest carbon chain containing the OH (hydroxy or hydroxyl) functional group (this is the "stem")

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

Step 2: Determine the name of the alkane "stem" based on the number of carbon atoms in this longest chain.
Longest carbon chain contains 6 carbon atoms.
Parent alkane is hexane

Stem name is hexan

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.

Numbering from left to right   Numbering from right to left
              H
|
       
            H-
 
C
|
-H
 
     
  H
|
  H
|
  OH
|
H-
 
C
|
-H
 
H
|
  H
|
H- C1 - C2 - C3 - C4 - C5 - C6 -H
  |
H
 
H-
|
-C-
 
-H
|
H
  |
H
  |
H
  |
H
      |
H
               
 
              H
|
       
            H-
 
C
|
-H
 
     
  H
|
  H
|
  OH
|
H-
 
C
|
-H
 
H
|
  H
|
H- C6 - C5 - C4 - C3 - C2 - C1 -H
  |
H
 
H-
|
-C-
 
-H
|
H
  |
H
  |
H
  |
H
      |
H
               

Preferred numbering is from left to right because this result in the OH group being attached to carbon number 3 instead of carbon number 4.

Step 4: 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 5: Determine the infix for the "ol" suffix based on the location of the OH (hydroxy or hydroxyl) functional group.

              H
|
       
            H-
 
C
|
-H
 
     
  H
|
  H
|
  OH
|
H-
 
C
|
-H
 
H
|
  H
|
H- C1 - C2 - C3 - C4 - C5 - C6 -H
  |
H
 
H-
|
-C-
 
-H
|
H
  |
H
  |
H
  |
H
      |
H
               

OH group is attached to carbon 3

Infix is -3-

Step 6: Write the name of the "stem" : alkan-number-ol

Stem name: hexan-3-ol

Step 7: Identify all alkyl groups (branches or side-chains).

              H
|
       
            H-
 
C
|
-H
 
     
  H
|
  H
|
  OH
|
H-
 
C
|
-H
 
H
|
  H
|
H- C1 - C2 - C3 - C4 - C5 - C6 -H
  |
H
 
H-
|
-C-
 
-H
|
H
  |
H
  |
H
  |
H
      |
H
               

Step 8(a): Write the name(s) of these alkyl groups (branches or side chains) in alphabetical order:

Alkyl group branch shown in red is made up of a chain of 2 carbon atoms: ethyl

Alkyl group branch shown in orange is made up of a chain of 1 carbon atom: methyl

"e" comes before "m" in the alphabet so the order will be:

ethyl   methyl

Step 8(b): If there is more than one of the same alkyl group, add a suitable multiplying prefix (di, tri, tetra) to the beginning of the name of that alkyl group.

These two branches are made up of different alkyl groups so this step is NOT required.

Step 9(a): Determine the infix for each alkyl group (branch or side chain) based on its location along the "stem"

              H
|
       
            H-
 
C
|
-H
 
     
  H
|
  H
|
  OH
|
H-
 
C
|
-H
 
H
|
  H
|
H- C1 - C2 - C3 - C4 - C5 - C6 -H
  |
H
 
H-
|
-C-
 
-H
|
H
  |
H
  |
H
  |
H
      |
H
               

ethyl group is attached to C5, infix is 5-
methyl group is attached to C3, infix is 3-

branches: 5-ethyl-3-methyl

Step 9(b): If there is more than one of the same alkyl group (as indicated by a multiplying prefix such as di, tri, etc), the location of each instance of this alkyl group is written down in ascending order and each number is separated from the other numbers by a comma.

The alkyl groups making up the brnaches (side chains) are different so this step is NOT required.

Step 10: Write the complete systematic IUPAC name for the branched chain alkanol in the form of

infix(s)-branch name(s) stem name -infix-suffix
infix(s)-branch name(s) stem name -infix-ol
5-ethyl-3-methyl hexan -3-ol

The systematic IUPAC name of this branched chain alkanol is 5-ethyl-3-methylhexan-3-ol

Note that the name is written as one word.
There are no spaces between:
- letters and letters
- numbers and numbers
- letters and numbers
- hyphens and numbers
- hyphens and letters
- numbers and commas

Steps for Drawing the Structure of Branched Chain Alkanols

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

infix-branch(s) stem -infix-suffix
n-alkyl alkan -n-ol

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

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: Identify the name and location of each branch (side chain) by breaking the name of each branch (side chain) up into its infix-branch name

Step 9: Use the infix for each branch to position the branch along the "stem" and draw a chain of carbon atoms of the required length at each position.

Step 10: Surround each new carbon atom with 4 dashes to represent covalent bonds.

Step 11: Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

Example

Draw a structure for the branched chain alkanol 2,3-dimethylbutan-2-ol

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

infix-branch(s) stem -infix-suffix
n-alkyl alkan -n-ol
2,3-dimethyl butan -2-ol

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

Stem name is butan so there are 4 carbon atoms in the alkane 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: Draw dashes around each carbon atom in the chain such that each carbon atom makes 4 bonds.

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

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

  |   |   |   |  
- C1 - C2 - C3 - C4 -
  |   |   |   |  

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.

-2-ol tells us the OH group is covalently bonded to C2

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

  |   OH
|
  |   |  
- C1 - C2 - C3 - C4 -
  |   |   |   |  

Step 8: Identify the name and location of each branch (side chain) by breaking the name of each branch (side chain) up into its infix-branch name

2,3-dimethyl tells us there are 2 (di) methyl groups.

One methyl group, -CH3, is covalently bonded to C2

The other methyl group, -CH3, is covalently bonded to C3

Step 9: Use the infix for each branch to position the branch along the "stem" and draw a chain of carbon atoms of the required length at each position.

  |   OH
|
  |   |  
- C1 - C2 - C3 - C4 -
  |   |
C
  |
C
  |  
                 

Step 10: Surround each new carbon atom with 4 dashes to represent covalent bonds.

  |   OH
|
  |   |  
- C1 - C2 - C3 - C4 -
  |   |
-C-
  |
-C-
  |  
      |   |      

Step 11: Complete the structure by placing a hydrogen atom (H) at the end of all the vacant dashes.

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

Steps for Writing the Molecular Formula of Branched Chain Alkanols

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

For a branched-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 O5:

CxHyO

Step 1: Draw the structure of the branched chain alkanol (n-alkylalkan-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.

Example

Write the molecular formula for 3-ethyl-4,4-dimethylpentan-2-ol

Step 1: Draw the structure of the 3-ethyl-4,4-dimethylpentan-2-ol

          H
|
         
          H-C-H
|
  H
|
     
  H
|
  OH
|
  H-C-H
|
  H-C-H
|
  H
|
 
H- C - C - C - C - C -H
  |
H
  |
H
  |
H
  |
H-C-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-C1-H
|
  H
|
     
  H
|
  OH
|
  H-C2-H
|
  H-C3-H
|
  H
|
 
H- C9 - C8 - C7 - C6 - C4 -H
  |
H
  |
H
  |
H
  |
H-C5-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).

C9H O

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

          1H
|
         
          2H-C-H3
|
  6H
|
     
  19H
|
  OH20
|
  4H-C-H5
|
  7H-C-H8
|
  9H
|
 
18H- C - C - C - C - C -H10
  |
H17
  |
H16
  |
H15
  |
14H-C-H13
  |
H11
 
              |
H12
     

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).

C9H20O

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

          H
|
         
          H-C-H
|
  H
|
     
  H
|
  1OH
|
  H-C-H
|
  H-C-H
|
  H
|
 
H- C - C - C - C - C -H
  |
H
  |
H
  |
H
  |
H-C-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 3-ethyl-4,4-dimethylpentan-2-ol is C9H20O


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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 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 The name of a branch is the name of the parent hydrocarbon with the final "e" of the name replaced and the suffix "yl" added.
Nomenclature of branches (substituents) is discussed on the Introduction to Organic Nomenclature page.

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

5The 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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