Classes of Alcohols

Alcohols can contain one or more hydroxyl group. It could be:

1. Monohydric – contain only one hydroxyl group. For example, ethanol.

 

CH₃-CH₂-CH₂-OH

Ethanol

 

1. Dihydric- contain two hydroxyl groups. For example ethane-1,2-diol.

 

CH2 – CH2

    |     |

    OH     OH

 

1. Trihydric- contains three hydroxyl groups. For example, propane-1, 2, 3-triol.

Generally with more than one hydroxyl group, it is said to be polyhydric.

Monohydric alcohols have the general formula C_nH_{2n + 1}OH. They can be classified into three classes – primary (1^o), secondary (2^o) and tertiary alcohol (3^o) according to the number of alkyl groups attached to the hydroxyl-bonded carbon atom.

 

(i) Primary alcohol (l°)

It has one alkyl group attached to the hydroxyl bonded carbon atom.

 

H

|

 

 

R — C—OH

|

H

 

For example, CH₃OH      CH₃CH₂OH Methanol     Ethanol

 

CH3 H

     |     |

CH₃—C— C—OH

     |     |

CH3 H

 

2,2-dimethylpropan-1-ol

 

Secondary alcohol (2^o):

There are two alkyl groups attached to the hydroxyl-bonded carbon atom.

R’

|

R—C—OH

|

H

 

Tertiary (3^o):

There are three alkyl groups, but no hydrogen atom directly attached to the hydroxyl-bonded carbon atom.

 

 

 

R’

|

R—C—OH

|

R”

 

 

 

 

For example:     

CH₃

|

CH₃ —C—OH

|

CH₃

2—methyl propan—2—ol

 

CH₂CH₃

|

CH₃CH₂—C—OH

|

CH₃

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p style=”margin-left: 86pt”>3—methylpentan—3—ol

 

Exercise

(i) Draw the structure of the following alcohols and classify each as1° 2° or 3° alcohol.

1. Butan—1—o1
2. 2—methylpentan—2—ol
3. Pentan—2—ol