Oxidative Cleavage of Alkenes

Overview

This lecture covers the oxidative cleavage of alkenes, focusing on ozonolysis under reducing and oxidizing conditions and permanganate cleavage.

Oxidative cleavage breaks a carbon-carbon double bond (alkene) to form two carbonyl compounds.
These reactions are not addition reactions but rather involve splitting the alkene into two fragments.

Ozonolysis uses ozone (O₃) to react with alkenes, forming an intermediate called an ozonide.
Step 1: Ozone reacts with the alkene to form a melosinide, which rearranges into an ozonide.
Step 2: The ozonide is treated with a reducing or oxidizing agent, which determines the final products.

Common reducing agents: dimethyl sulfide (DMS) or zinc and water.
Under reducing conditions, the alkene is cleaved, yielding ketones and aldehydes.
Aldehydes remain unchanged (do not oxidize further).

Common oxidizing agent: hydrogen peroxide (H₂O₂).
Under oxidizing conditions, ketones remain ketones, but aldehydes are further oxidized into carboxylic acids.

Hot, concentrated potassium permanganate (KMnO₄) under acidic conditions also cleaves alkenes oxidatively.
This produces the same products as ozonolysis under oxidizing conditions: ketones and carboxylic acids.
Mechanistic details for permanganate cleavage are not usually required for exams.

To determine the products, split the carbon-carbon double bond and attach double-bonded oxygens to both fragments.
Ketones are always formed where applicable.
The nature of the conditions (reducing or oxidizing) affects whether aldehydes stay the same or become carboxylic acids.

Oxidative cleavage — Breaking a double bond by oxidation to form two carbonyl compounds.
Ozonolysis — Reaction of alkenes with ozone to form ozonides, then cleaving into carbonyl compounds.
Ketone — Carbonyl compound with two alkyl groups attached to the carbonyl carbon.
Aldehyde — Carbonyl compound with one alkyl group and one hydrogen attached to the carbonyl carbon.
Carboxylic acid — Functional group with a carbonyl and hydroxyl on the same carbon (COOH).
Reducing agent — Substance that prevents further oxidation (e.g., DMS, Zn/H₂O).
Oxidizing agent — Substance that promotes further oxidation (e.g., H₂O₂, KMnO₄).