Oxidation of Alcohols
Oxidation of Primary Alcohols: From Aldehyde to Carboxylic Acid
Step 1: Primary Alcohol to Aldehyde
- Primary alcohols, such as ethanol $CH_3CH_2OH$, can be oxidized to aldehydes.
- In this process, the hydroxyl group $-OH$ is converted into a carbonyl group $C=O$, which defines the aldehyde functional group.
- The reaction is represented as:
$$RCH_2OH + [O] \rightarrow RCHO + H_2O$$ - Here, $[O]$ represents the oxidizing agent, commonly acidified potassium dichromate $K_2Cr_2O_7$ or potassium permanganate $KMnO_4$.
Key Condition:
- To stop the reaction at the aldehyde stage, the aldehyde must be distilled out of the reaction mixture as it forms.
- This prevents further oxidation.
Ethanol is oxidized to acetaldehyde (ethanal) when distilled during the reaction: $$CH_3CH_2OH + [O] \rightarrow CH_3CHO + H_2O$$
Step 2: Aldehyde to Carboxylic Acid
- If the reaction continues, the aldehyde undergoes further oxidation to form a carboxylic acid.
- In this step, the carbonyl group $C=O$ in the aldehyde is converted to a carboxyl group $-COOH$.
- The reaction is expressed as:
$$RCHO + [O] \rightarrow RCOOH$$
Key Condition:
- To achieve full oxidation to the carboxylic acid, the reaction mixture is heated under reflux.
- Reflux ensures that any aldehyde vapor condenses and returns to the reaction mixture for further oxidation.
Ethanol is oxidized first to acetaldehyde and then to ethanoic acid (acetic acid) when heated under reflux with an excess of oxidizing agent: $$CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O$$
The orange color of potassium dichromate ($Cr_2O_7^{2-}$) fades to green as it is reduced to $Cr^{3+}$, providing a visual clue that oxidation has occurred.
Self review- What conditions are required to stop the oxidation of a primary alcohol at the aldehyde stage?
- Write the balanced equation for the oxidation of ethanol to ethanoic acid.
Oxidation of Secondary Alcohols: Formation of Ketones
- Secondary alcohols, such as propan-2-ol $CH_3CHOHCH_3$, can be oxidized to ketones.
- Unlike primary alcohols, secondary alcohols cannot be oxidized further to carboxylic acids because the carbon atom bonded to the hydroxyl group lacks a hydrogen atom for further oxidation.
- The reaction is written as:
$$R_2CHOH + [O] \rightarrow R_2C=O + H_2O$$
