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Classification of Alcohols (alkanols)

Key Concepts

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

  • All alcohols contain the OH functional group which is called the hydroxyl, or hydroxy, functional group1.

  • An alcohol can be classified as a:

    • Primary alcohol (also referred to as 1o alcohol)

    • Secondary alcohol (also referred to as 2o alcohol)

    • Tertiary alcohol (also referred to as 3o alcohol)

    • Classification of an alcohol depends on the location of the OH (hydroxyl or hydroxy) functional group:

      primary alcohol
      (1o alcohol)
      secondary alcohol
      (2o alcohol)
      tertiary alcohol
      (3o alcohol)
      General formula:

      where R, R' and R" represent
      alkyl groups (alkane chains)

        H
      |
       
      R- C -OH
        |
      H
       
        OH
      |
       
      R- C -R'
        |
      H
       
        OH
      |
       
      R- C -R'
        |
      R"
       
      Number of alkyl groups attched to
      same carbon atom as OH group:
      1
      (only R)
      2
      (R and R')
      3
      (R, R' and R")

    Classification of Alcohol by Steps

    If you have been given the structural formula of an alcohol, all you need to do in order to classify the alcohol is decide how many alkyl groups are attached to the same carbon atom as the OH (hydroxyl or hydroxy) functional group.

    If you have been given the name of an alcohol, you will need to draw its structure first before you classify it.

    You can use the steps below to decide if an alcohol should be classified as primary, secondary, or, tertiary.

    Step 1: Locate the OH (hydroxyl or hydroxy) functional group.

    Step 2 Identify the carbon atom to which the OH (hydroxyl or hydroxy) functional group is covalently bonded.

    Step 3: Count the number of carbon atoms covalently bonded to the same carbon atom as the OH functional group.

    Step 4: Classify the Alcohol

    • 1 carbon atom bonded to C-OH = 1 alkyl group = 1o alcohol = primary alcohol

    • 2 carbon atoms bonded to C-OH = 2 alkyl groups = 2o alcohol = secondary alcohol

    • 3 carbon atoms bonded to C-OH = 3 alkyl groups = 3o alcohol = tertiary alcohol

    Example: Primary Alcohol

    Classify the propan-1-ol (1-propanol) alcohol molecule shown below:

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

    Step 1: Locate the OH (hydroxyl or hydroxy) functional group.

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

    Step 2 Identify the carbon atom to which the OH (hydroxyl or hydroxy) functional group is covalently bonded.

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

    Step 3: Count the number of carbon atoms covalently bonded to the same carbon atom as the OH functional group.

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

    Only 1 carbon atom is covalently bonded to the same carbon atom as the OH functional group.

    Step 4: Classify the Alcohol

    1 alkyl group = 1o alcohol = primary alcohol

    This is a primary alcohol.

    Example: Secondary Alcohol

    Classify the propan-2-ol (2-propanol) alcohol molecule shown below:

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

    Step 1: Locate the OH (hydroxyl or hydroxy) functional group.

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

    Step 2 Identify the carbon atom to which the OH (hydroxyl or hydroxy) functional group is covalently bonded.

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

    Step 3: Count the number of carbon atoms covalently bonded to the same carbon atom as the OH functional group.

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

    2 carbon atoms are covalently bonded to the same carbon atom as the OH functional group.

    Step 4: Classify the Alcohol

    2 alkyl groups = 2o alcohol = secondary alcohol

    This is a secondary alcohol.

    Example: Tertiary Alcohol

    Classify the 2-methylpropan-2-ol (2-methyl-2-propanol) alcohol molecule shown below:

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

    Step 1: Locate the OH (hydroxyl or hydroxy) functional group.

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

    Step 2 Identify the carbon atom to which the OH (hydroxyl or hydroxy) functional group is covalently bonded.

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

    Step 3: Count the number of carbon atoms covalently bonded to the same carbon atom as the OH functional group.

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

    3 carbon atoms are covalently bonded to the same carbon atom as the OH functional group.

    Step 4: Classify the Alcohol

    3 alkyl groups = 3o alcohol =tertiary alcohol

    This is a tertiary alcohol.

    Summary Table: Classification of Some Alkanols

    Molecular Formula Primary Alcohol Secondary Alcohol Tertiary Alcohol
    CH4O
      H
    |
     
    H- C -OH
      |
    H
     

    methanol

       

    C2H6O
      H
    |
      H
    |
     
    H- C - C -OH
      |
    H
      |
    H
     

    ethanol

       

    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)

     

    C4H10O
      H
    |
      H
    |
      H
    |
      H
    |
     
    H- C - C - C - C -OH
      |
    H
      |
    H
      |
    H
      |
    H
     

    butan-1-ol
    (1-butanol)

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

    butan-2-ol
    (2-butanol)

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

    2-methylpropan-2-ol
    (2-methyl-2-propanol)

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

    2-methylpropan-1-ol
    (2-methyl-1-propanol)


    C5H12O
      H
    |
      H
    |
      H
    |
      H
    |
      H
    |
     
    H- C - C - C - C - C -OH
      |
    H
      |
    H
      |
    H
      |
    H
      |
    H
     

    pentan-1-ol
    (1-pentanol)

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

    pentan-2-ol
    (2-pentanol)

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

    2-methylbutan-2-ol
    (2-methyl-2-butanol)

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

    2-methylbutan-1-ol
    (2-methyl-1-butanol)

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

    pentan-3-ol
    (3-pentanol)

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

    3-methylbutan-1-ol
    (3-methyl-1-butanol)

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

    3-methylbutan-2-ol
    (3-methyl-2-butanol)


    YouTube Animation


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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.

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