Reactivity 3.2.9 — Oxidation of Organic Compounds

Certain functional groups in organic compounds undergo oxidation reactions, particularly alcohols. These reactions result in a transformation of the functional group, depending on the type of alcohol and reaction conditions.

Types of Alcohols and Their Oxidation Behavior

• Primary alcohols (1°) are oxidized first to aldehydes, and then to carboxylic acids under prolonged heating.
• Secondary alcohols (2°) are oxidized to ketones.
• Tertiary alcohols (3°) do not oxidize under typical conditions because they lack a hydrogen atom on the carbon bearing the hydroxyl group.

General Equations for Oxidation

Primary Alcohol

• • Step 1: Partial oxidation (mild conditions)

\( \text{CH}_3\text{CH}_2\text{OH} \rightarrow \text{CH}_3\text{CHO} \) (ethanol → ethanal)

• • Step 2: Full oxidation (stronger heating or excess oxidant)

\( \text{CH}_3\text{CHO} \rightarrow \text{CH}_3\text{COOH} \) (ethanal → ethanoic acid)

• • Overall (in presence of oxidizing agent):

\( \text{CH}_3\text{CH}_2\text{OH} \xrightarrow{[\text{O}]} \text{CH}_3\text{COOH} \)

Secondary Alcohol

• • Example: Propan-2-ol to propanone

\( \text{CH}_3\text{CH(OH)}\text{CH}_3 \xrightarrow{[\text{O}]} \text{CH}_3\text{COCH}_3 \)

Tertiary Alcohol

• No oxidation under standard conditions.
• This is due to the absence of a hydrogen atom on the carbon attached to the –OH group.

Experimental Set-ups

1. Distillation (To Obtain Aldehydes)

When oxidizing a primary alcohol to an aldehyde, the product is distilled off as it forms to prevent further oxidation.

• Setup includes: round-bottom flask, condenser angled downward, thermometer, heat source.
• Oxidizing agent: acidified potassium dichromate (\( \text{K}_2\text{Cr}_2\text{O}_7 \) with H₂SO₄)
• Observation: Orange dichromate solution turns green as Cr⁶⁺ is reduced to Cr³⁺.

2. Reflux (To Complete Oxidation to Carboxylic Acid)

Refluxing allows heating for a prolonged time without losing volatile compounds, promoting full oxidation to the carboxylic acid.

• Setup includes: round-bottom flask, vertical condenser, heat source.
• Used for: Oxidation of aldehydes to acids or complete oxidation of primary alcohols.