OXIDATION AND REDUCTION REACTIONS

SELENIUM DIOXIDE SeO₂

        Selenium dioxide is an important oxidizing agent, specific for the oxidation of reactive CH₂ and CH₃ groups.

Preparation: It is prepared by the direct oxidation of metallic selenium burns with  a flame in air producing SeO₂.

                        

Properties:

i)  It is a white crystalline solid which sublimes on heating.

ii) It is soluble in solvents like dioxane, ethanol, acetic acid and water.

iii)  It is a white crystalline solid dissolves in water forming selenius acid (H₂SeO₃) and highly toxic.

iv) It is a selective oxidant.

Synthetic applications:

i)  It oxidizes allylic and benzylic CH₃ and CH₂ and groups to corresponding alcohols.

ii)  It oxidizes active CH₃ and CH₂  groups to carbonyl groups.

iii) It oxidizes alkynes  to dicarbonyls

iv) It also acts as a dehydrogenating agent.

Oxidation of allylic and benzylic methyl and methylene groups to alcohols:

    The primary use of SeO₂ is to oxidize allylic and benzylic C-H fragments to the corresponding alcohols. Double and triple bonds and aromatic rings activate CH₂ and CH₃ groups for this oxidation. The reaction is carried out in HOAc  at 100-140^oC. Alcohols or dioxane can also be used.

   The CH₂ or CH₃ groups α-to the most highly substituted end of the double is hydroxylated according to the order of preference of oxidation CH2 > CH3 > CH groups.

    

 Mechanism: The reaction proceeds via an initial Ene reaction of allylic compounds with SeO₂ to give allylic selenic acid, which undergoes a 2,3-sigmatropic shift to give selenoxylic acid allyl ester. This ester on hydrolysis affords the allyl alcohol while on heating yields aldehyde.

If the CH₂ group is flanked by aromatic rings SeO₂ oxidation leads to aldehyde or ketone

     

Oxidation of Carbonyl Compounds: Another application of SeO₂ is the oxidation of the methyl and methylene groups adjacent to the carbonyl group to aldehydic and ketonic groups respectively.

 Mechanism: The oxidation is carried out in aqueous acetic acid medium and hence the actual reagent is selenious acid H₂SeO₃. It attacks on the enol form of the substrate to yield selenate ester which eliminates water and Se to give the product.

    

Dehydrogenating Agent:

In certain cases selenium oxide has been used for dehydrogenation at elevated temperature.

 

Oxidation of Alkynes:

Selenium dioxide is used to oxidize Alkynes to vicinal dicarbonyl compounds. Diphenylacetylene is readily oxidized at110^oC in presence of aqueous acetic acid containing sulphuric acid to benzil ie. Internal alkynes are converted to α-diketones.

JONES REAGENT

      Jones reagent is an aqueous solution of CrO₃ and H₂SO₄ in stoichiometric proportions in acetone solvent. It is a good method to oxidize 2^o alcohol(not acid sensitive) to ketone in which sodium dichromate in dil. H₂SO₄ is added to a solution of the alcohol in acetone.

                     

The mechanism starts with the formation of HCrO^4-  ions,  ie. CrO₃, from dichromate ion in solution. In acid, these form chromate ester with  alcohol. (The ester Decom-

poses by elimination of the Cr IV H₂CrO₃)

Alcohols containing double or triple bonds may be selectively oxidized using

 Jones’ reagent.

   

PYRIDINIUM CHLOROCHROMATE  (PCC)

Preparation: PCC is prepared by dissolving CrO₃ in HCl and then treating with pyridine.

      

             PCC is less reactive than other oxidizing chromium reagents and hence it is highly selective. It oxidizes primary alcohol to corresponding aldehyde. Since it is mild oxidising agent other acid sensitive groups such as double bonds are unaffected. It is also known as Corey’s reagent and it is used in solvents such as CH₂Cl₂.

          PCC is also useful for oxidizing an allylic carbon to keto group. However, in such cases the alcoholic group if present must be protected before oxidation.

    Jones’ reagent is not suitable for the oxidation of primary alcohol since the aldehyde that forms is further oxidized to acid via its hydrate. The key point is to avoid water so PCC in CH₂Cl₂ and PDC work quite well.