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SeO₂ and MnO₂

Specialized Oxidizing Agents for Allylic and Benzylic Positions.

By chemca Team • Updated Jan 2026

While strong oxidizers like $KMnO_4$ oxidize entire chains, Selenium Dioxide ($SeO_2$) and Manganese Dioxide ($MnO_2$) are selective reagents. They target specific positions like allylic or benzylic carbons, leaving other parts of the molecule intact.

1. Selenium Dioxide ($SeO_2$)

Riley Oxidation

Selenium dioxide is primarily used for the oxidation of allylic or benzylic C-H bonds and $\alpha$-methylene groups adjacent to carbonyls.

A. Oxidation of Allylic Position: It converts an allylic group ($-CH_2-$) into an Allylic Alcohol or Aldehyde/Ketone.

$$ R-CH_2-CH=CH_2 \xrightarrow{SeO_2} R-CH(OH)-CH=CH_2 $$

B. Oxidation of Carbonyl Compounds ($\alpha$-Oxidation): It oxidizes the $\alpha$-methylene ($CH_2$) or methyl ($CH_3$) group adjacent to a carbonyl group into a carbonyl group, forming 1,2-Dicarbonyls.

$$ CH_3-CHO \text{ (Acetaldehyde)} \xrightarrow{SeO_2} \underset{\text{Glyoxal}}{CHO-CHO} $$ $$ CH_3-CO-CH_3 \text{ (Acetone)} \xrightarrow{SeO_2} \underset{\text{Methyl Glyoxal}}{CH_3-CO-CHO} $$

Key Feature: $SeO_2$ does NOT break the C=C double bond. It only attacks the reactive allylic position.

2. Manganese Dioxide ($MnO_2$)

Selective Oxidation of Alcohols

Active Manganese Dioxide is a specific reagent for the oxidation of Allylic and Benzylic primary and secondary alcohols to aldehydes and ketones, respectively.

A. Allylic Alcohols:

$$ CH_3-CH=CH-CH_2OH \xrightarrow{MnO_2} \underset{\text{Crotonaldehyde}}{CH_3-CH=CH-CHO} $$

B. Benzylic Alcohols:

$$ C_6H_5-CH_2OH \xrightarrow{MnO_2} \underset{\text{Benzaldehyde}}{C_6H_5-CHO} $$

Selectivity:

It does NOT oxidize normal saturated alcohols (like Ethanol or Propanol) under mild conditions. It leaves C=C double bonds and triple bonds unaffected.

3. Quick Comparison

Feature	Selenium Dioxide ($SeO_2$)	Manganese Dioxide ($MnO_2$)
Substrate	Alkenes, Alkynes, Carbonyls	Allylic & Benzylic Alcohols
Target Site	Allylic C-H bond, $\alpha$-C of Carbonyl	-OH group at Allylic/Benzylic position
Product	Allylic Alcohol / 1,2-Dicarbonyl	Conjugated Aldehyde/Ketone
Effect on C=C	Intact	Intact

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