Clasification
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Classification

A valuable set of phrases that is used when talking about alcohols is pointed out here (and in Example 5 in your workbook). These are the words primary, secondary and tertiary. The word primary is applied to an alcohol which has one alkyl group bonded to the carbon atom that has the -OH group on it. This is indicated this by using an R- to represent an alkyl group. A secondary alcohol is an alcohol with two alkyl groups attached to the carbon atom that has the alcohol group on it. A tertiary alcohol is one which has three alkyl groups attached to the carbon atom with the hydroxyl group on it. Note that 1°, 2° and 3° are quite often used as abbreviations for primary, secondary and tertiary.
H
|
R-C-OH
|
H

primary
1°

 R
|
R-C-OH
|
H

secondary
2°

 R
|
R-C-OH
|
R

tertiary
3°

 

Practice

I would like you to go through the structural formulas for the alcohols shown below and identify each one as primary, secondary or tertiary. Check your answers below and then continue.

H  H  CH3 H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
OH H  H  H		 H  H  CH3 H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
H  OH H  H		 H  H  CH3 H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
H  H  OH H		 H  H  CH3 H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
H  H  H  OH

 

Answers

H  H  CH3 H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
OH H  H  H		 H  H  CH3 H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
H  OH H  H		 H  H  CH3 H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
H  H  OH H		 H  H  CH3 H
|  |  |  |
H-C--C--C--C-H
|  |  |  |
H  H  H  OH
primary secondary tertiary primary

 

Polyhydroxy Alcohols

It is possible to have more than one hydroxyl group in a molecule. I would like to spend a little bit of time talking about the two most common of these multi-alcohols. (These are shown below and in Example 6 in your workbook.) Actually, they should be called polyhydroxy alcohols to indicate that there are two or more hydroxyl groups in each molecule. The IUPAC system does a good job of naming these.

In this case the name 1,2-ethandiol indicates quite well what is contained in the molecule. Ethan- indicates two carbons, -diol indicates two hydroxyl groups, and the 1,2- lets you know where the hydroxyl groups are attached to the carbon chain.
 

CH2-CH2
|    | 
OH   OH 

 

 1,2-ethanediol

ethylene glycol
The name 1,2,3-propantriol also does a very good job of describing the structure of the compound.
 

CH2-CH-CH2
|   |  |  
OH  OH OH 

 

 1,2,3-propanetriol

glycerol or glycerine
However, these two particular compounds are very common and usually go by their common names of ethylene glycol and glycerol (or glycerine). I particularly wanted to bring glycerol to your attention because it becomes very important when we start talking about animal fats and vegetable oils (triglycerides) in a later lesson.

 

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E-mail instructor: Eden Francis

Clackamas Community College
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