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 $KMnO_4$.
Tip
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.
Example
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:
$$RCHO + [O] \rightarrow RCOOH$$
Tip
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.
Example
Ethanol is oxidized first to acetaldehyde and then to ethanoic acid (acetic acid) when heated under reflux with an excess of oxidizing agent: $$CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O$$
Note
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:
$$R_2CHOH + [O] \rightarrow R_2C=O + H_2O$$
Tip
Key Condition:
- This reaction typically requires heating under reflux with an oxidizing agent, such as acidified potassium dichromate or potassium permanganate.
Example
Propan-2-ol is oxidized to propanone (acetone), a ketone, under reflux: $$CH_3CHOHCH_3 + [O] \rightarrow CH_3COCH_3 + H_2O$$
Common Mistake
- Many students mistakenly assume that secondary alcohols can be oxidized to carboxylic acids.
- Remember, secondary alcohols stop at the ketone stage because there is no hydrogen atom on the carbonyl carbon for further oxidation.
Self review
- Why can't secondary alcohols be oxidized to carboxylic acids?
- Write the balanced equation for the oxidation of propan-2-ol.
Why Tertiary Alcohols Cannot Be Oxidized
- Tertiary alcohols, such as 2-methylpropan-2-ol $(CH_3)_3COH$, cannot be oxidized under normal conditions.
- This is because the carbon atom bonded to the hydroxyl group lacks a hydrogen atom, making further oxidation impossible.
Experimental Techniques: Reflux vs. Distillation
Reflux
- Reflux is used when complete oxidation is desired, such as converting a primary alcohol to a carboxylic acid.
- The reaction mixture is heated with a reflux condenser, which cools any vapors and returns them to the reaction flask.
Distillation
- Distillation is used to isolate intermediate products such as aldehydes.
- The aldehyde, having a lower boiling point than the alcohol or carboxylic acid, vaporizes and is collected in a separate container.
Tip
Use reflux for complete oxidation to a carboxylic acid and distillation to isolate an aldehyde.
Self review
- What experimental setup is used to isolate an aldehyde during the oxidation of a primary alcohol?
- Why is reflux necessary for the complete oxidation of a primary alcohol to a carboxylic acid?
