A tertiary alcohol is generally resistant to oxidation under typical conditions. Unlike primary and secondary alcohols, which readily undergo oxidation to form aldehydes, ketones, or carboxylic acids, tertiary alcohols lack the necessary structural features for this common chemical transformation.
Understanding Alcohol Oxidation
Oxidation in organic chemistry often involves the loss of hydrogen atoms, the gain of oxygen atoms, or an increase in the number of bonds to oxygen for a carbon atom. For alcohols, the ability to be oxidized hinges on the presence of hydrogen atoms bonded to the carbon atom that carries the hydroxyl (-OH) group, known as the carbinol carbon.
- Primary alcohols (R-CH₂OH) have two hydrogen atoms attached to the carbinol carbon. They can be oxidized first to aldehydes and then further to carboxylic acids.
- Example: Ethanol (a primary alcohol) can be oxidized to ethanal (an aldehyde), and then to ethanoic acid (a carboxylic acid).
- Secondary alcohols (R₂-CHOH) have one hydrogen atom attached to the carbinol carbon. They can be oxidized to ketones.
- Example: Propan-2-ol (a secondary alcohol) can be oxidized to propanone (a ketone).
The Unoxidizable: Tertiary Alcohols
In contrast to primary and secondary alcohols, a tertiary alcohol (R₃-COH) possesses a carbinol carbon that is bonded to three other carbon atoms and no hydrogen atoms. This crucial structural difference is why tertiary alcohols cannot be oxidized in the same manner.
Without a hydrogen atom on the carbinol carbon, the typical mechanism for alcohol oxidation, which involves the removal of hydrogen and the formation of a carbon-oxygen double bond (a carbonyl group, C=O), cannot occur. Under harsh oxidizing conditions, tertiary alcohols might undergo dehydration (loss of water) to form alkenes, or even cleavage of carbon-carbon bonds, but these reactions are not considered standard oxidation.
- Example: 2-methylpropan-2-ol (also known as tert-butyl alcohol) is a common tertiary alcohol. Its central carbon, bonded to the -OH group, is also bonded to three methyl (-CH₃) groups, leaving no hydrogens to be removed during oxidation.
Summary of Alcohol Oxidation
The table below summarizes the key differences in the oxidizability of various alcohol types:
| Alcohol Type | General Structure | Hydrogens on Carbinol Carbon | Typical Oxidation Product(s) | Can Be Oxidized? |
|---|---|---|---|---|
| Primary | R-CH₂OH | Two | Aldehyde, Carboxylic Acid | Yes |
| Secondary | R₂-CHOH | One | Ketone | Yes |
| Tertiary | R₃-COH | None | None (under typical conditions) | No |
Practical Implications
The distinct reactivity of different alcohol types, particularly the resistance of tertiary alcohols to oxidation, is a fundamental concept in organic chemistry. This difference allows chemists to selectively react primary or secondary alcohols in the presence of tertiary alcohols, which is valuable in multi-step organic synthesis. For instance, if a molecule contains both a primary and a tertiary alcohol, a chemist could choose an oxidizing agent to specifically target the primary alcohol without affecting the tertiary one.
