Classification of Alcohols (alkanols) Chemistry Tutorial

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 group⁽¹⁾.
• An alcohol can be classified as a:

  ⚛ Primary alcohol (also referred to as 1^o alcohol)

  ⚛ Secondary alcohol (also referred to as 2^o alcohol)

  ⚛ Tertiary alcohol (also referred to as 3^o 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")

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Classification of Alcohol (alkanol) by Steps

If you have been given the structural formula of an alcohol (alkanol), 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 (alkanol)

• 1 carbon atom bonded to C-OH = 1 alkyl group = 1^o alcohol = primary alcohol
• 2 carbon atoms bonded to C-OH = 2 alkyl groups = 2^o alcohol = secondary alcohol
• 3 carbon atoms bonded to C-OH = 3 alkyl groups = 3^o alcohol = tertiary alcohol

Worked Example of Classifying an Alcohol (alkanol): Primary Alcohol (alkanol)

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

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.

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 = 1^o alcohol = primary alcohol

This is a primary alcohol.

Worked Example of Classification of an Alcohol (alkanol): Secondary Alcohol (alkanol)

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

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.

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

Step 4: Classify the Alcohol

2 alkyl groups = 2^o alcohol = secondary alcohol

This is a secondary alcohol.

Worked Example of Classifying an Alcohol (alkanol): Tertiary Alcohol (alkanol)

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

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.

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

Step 4: Classify the Alcohol

3 alkyl groups = 3^o alcohol =tertiary alcohol

This is a tertiary alcohol.

Summary Table: Classification of Some Alkanols (alcohols)