5.6: Classification of alcohols
- Identify the general structure for an alcohol.
- Identify the structural feature that classifies alcohols as primary, secondary, or tertiary.
An alcohol is an organic compound with a hydroxyl (OH) functional group on an alkane type of carbon atom or a tetrahedral carbon atom. Because OH is the functional group of all alcohols, we often represent alcohols by the general formula ROH , where R is an alkyl group (R stands for Rest of the Molecule). Alcohols are common in nature. Most people are familiar with ethyl alcohol (ethanol), the active ingredient in alcoholic beverages, but this compound is only one of a family of organic compounds known as alcohols. The family also includes such familiar substances as cholesterol and the carbohydrates. Methanol (CH 3 OH) and ethanol (CH 3 CH 2 OH) are a couple of examples of alcohols.
Classification of Alcohols
Some of the properties of alcohols depend on the number of carbon atoms attached to the specific carbon atom that is attached to the OH group. Alcohols can be grouped into three classes on this basis.
- A primary (1°) alcohol is one in which the carbon atom (in red) with the OH group is attached to one other carbon atom (in blue). Its general formula is RCH 2 OH.
- A secondary (2°) alcohol is one in which the carbon atom (in red) with the OH group is attached to two other carbon atoms (in blue). Its general formula is R 2 CHOH .
- A tertiary (3°) alcohol is one in which the carbon atom (in red) with the OH group is attached to three other carbon atoms (in blue). Its general formula is R 3 COH .
Table \(\PageIndex{1}\) names and classifies some of the simpler alcohols. Some of the common names reflect a compound’s classification as secondary ( sec -) or tertiary ( tert -). These designations are not used in the IUPAC nomenclature system for alcohols. IUPAC stands for International Union of Pure and Applied Chemistry. Note that there are four butyl alcohols in the table, corresponding to the four butyl groups. One butyl group is (CH 3 CH 2 CH 2 CH 2 -), and three others are:
| Condensed Structural Formula | Class of Alcohol | Common Name | IUPAC Name |
|---|---|---|---|
| CH 3 OH | — | methyl alcohol | methanol |
| CH 3 CH 2 OH | primary | ethyl alcohol | ethanol |
| CH 3 CH 2 CH 2 OH | primary | propyl alcohol | 1-propanol |
| (CH 3 ) 2 CHOH | secondary | isopropyl alcohol | 2-propanol |
| CH 3 CH 2 CH 2 CH 2 OH | primary | butyl alcohol | 1-butanol |
| CH 3 CH 2 CHOHCH 3 | secondary | sec -butyl alcohol | 2-butanol |
| (CH 3 ) 2 CHCH 2 OH | primary | isobutyl alcohol | 2-methyl-1-propanol |
| (CH 3 ) 3 COH | tertiary | tert -butyl alcohol | 2-methyl-2-propanol |
| secondary | cyclohexyl alcohol | cyclohexanol |
Nomenclature of Alcohols
Alcohols with one to four carbon atoms are frequently called by common names, in which the name of the alkyl group is followed by the word alcohol :
Biochemistry Link
The citric acid cycle is a series of reactions important to metabolism. One step in the process involves the conversion of citrate ion (3 o -alcohol) to isocitrate ion (2 o -alcohol). Citrate and isocitrate are isomers of each other.
Citrate is a 3 o -alcohol, the alcohol (-OH) functional group is attached to a carbon atom (red) to three other carbon atoms (blue).
Isocitrate is a 2 o -alcohol, the alcohol (-OH) functional group is attached to a carbon atom (red) to two other carbon atoms (blue).
Summary
Alcohols are classified according to the number of carbon atoms attached to the carbon atom bears the OH group.