R3.2.9 Oxidation of organic compounds

Oxidation of Alcohols

Oxidation of Primary Alcohols: From Aldehyde to Carboxylic Acid

Step 1: Primary Alcohol to Aldehyde

Primary alcohols, such as ethanol $C H_{3} C H_{2} O H$ , can be oxidized to aldehydes.
In this process, the hydroxyl group $- O H$ is converted into a carbonyl group $C = O$ , which defines the aldehyde functional group.

The reaction is represented as:
$R C H_{2} O H + [O] \to R C H O + H_{2} O$

Here, $[O]$ represents the oxidizing agent, commonly acidified potassium dichromate $K_{2} C r_{2} O_{7}$ or potassium permanganate $K M n O_{4}$ .

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

Oxidation of Ethanol to Acetaldehyde

$C H_{3} C H_{2} O H + [O] \to C H_{3} C H O + H_{2} O$
Ethanol is oxidized to acetaldehyde (ethanal) when distilled during the reaction.

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 $- C O O H$ .

The reaction is expressed as:
$R C H O + [O] \to R C O O H$

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

Oxidation of Ethanol to Ethanoic Acid

$C H_{3} C H_{2} O H + 2 [O] \to C H_{3} C O O H + H_{2} O$
Ethanol is oxidized first to acetaldehyde and then to ethanoic acid (acetic acid) when heated under reflux with an excess of oxidizing agent.

Note

The orange color of potassium dichromate ( $C r_{2} O_{7}^{2 -}$ ) fades to green as it is reduced to $C r^{3 +}$ , 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 $C H_{3} C H O H C H_{3}$ , 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:
$R_{2} C H O H + [O] \to R_{2} C = O + H_{2} O$

Tip

Key Condition:

This reaction typically requires heating under reflux with an oxidizing agent, such as acidified potassium dichromate or potassium permanganate.

Example

Oxidation of Propan-2-ol to Propanone

$C H_{3} C H O H C H_{3} + [O] \to C H_{3} C O C H_{3} + H_{2} O$
Propan-2-ol is oxidized to propanone (acetone), a ketone, under reflux.

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 $(C H_{3})_{3} C O H$ , 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.

No possible reaction for a tertiary alcohol, 2-methylpropan-2-ol.

Analogy

Think of oxidation as peeling layers off an onion. With tertiary alcohols, there are no layers left to peel from the carbon atom connected to the $- O H$ group.

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.

Reflection

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?

Theory of Knowledge

Consider how the same chemical process (oxidation) can lead to vastly different products depending on the conditions.

How does this reflect the role of human choice and creativity in scientific experimentation?

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R3.2.9 Oxidation of organic compounds

Oxidation of Alcohols

Oxidation of Primary Alcohols: From Aldehyde to Carboxylic Acid

Step 1: Primary Alcohol to Aldehyde

Oxidation of Ethanol to Acetaldehyde

Step 2: Aldehyde to Carboxylic Acid

Oxidation of Ethanol to Ethanoic Acid

Oxidation of Secondary Alcohols: Formation of Ketones

Oxidation of Propan-2-ol to Propanone

Why Tertiary Alcohols Cannot Be Oxidized

Experimental Techniques: Reflux vs. Distillation

Reflux

Distillation

Reflection

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Introduction to Oxidation of Organic Compounds