Oxidation

1) Introduction

Oxidation is any chemical reaction that involves the moving of electrons. Specifically, it means the side that gives away electrons. When iron reacts with oxygen it forms a chemical called rust. The iron is oxidized and the oxygen is reduced.

OR

Oxidizationing is any chemical reaction that involves the moving of electrons. Specifically, it means the substance that gives away electrons is oxidized.

OR

Addition of Oxygen to the corresponding moiety and removal of Hydrogen atom, OR removal of electron from corresponding moiety.

The classical procedure for oxidizing primary alcohols to aldehydes and secondary alcohols to ketones involves treatment of the appropriate alcohol with a chromium(V1) and KMnO4 reagent.'

Oxidation of primary alcohols to aldehydes requires anhydrous conditions.

In the presence of water, the resultant aldehyde can form the hydrate, which may be further oxidized to the carboxylic acid.

Recently, procedures for oxidation of alcohols to aldehydes and ketones have been developed that obviate the toxicity associated with the use of chromium reagents.

Because of the greater stability of ketones to most oxidizing conditions, the conversion of secondary alcohols to ketones can be accomplished with a wide variety of

Reagents and Conditions:

2) SOME REAGENTS FOR 1°,2°,3° ALCOHOL OXlDATION:

2.1) Jones Reagent:

The Jones reagent is an excellent reagent for the oxidation of secondary alcohols which do not contain acid-sensitive groups such as acetal Oxidation of primary alcohols with Jones reagent may result in the conversion of the aldehydes initially formed to the corresponding carboxylic acids.

Chromic acid oxidation may also be performed in the presence of water immiscible solvents.

Addition of a stoichiometric amount of aqueous sodium dichromate and sulfuric acid to a solution of the secondary alcohol in diethyl ether at 25°C affords the corresponding ketone.

2.2) Collins Reagent:

CrO3‧ 2C5H5N is a mild reagent for the oxidation of alcohols that contain acid-sensitive groups.

The reagent is prepared by adding CrO3 to a mixture of pyridine-CH2CI2 and the RCH2OH or R2CHOH is then added to the oxidant in solution.

2.3) Pyridiaium Chlolrochromate PCC:

Primary and secondary alcohols are readily oxidized in CH2Cl2 .

Since PCC is slightly acidic, oxidations of compounds containing acid-sensitive groups should be carried out in the presence of powdered NaOAc.

2.4) Pyridinium Dichromate (PDC):

The reagent is less acidic than PCC.

Hence, oxidations in CH2C12 can be carried out under nearly neutral conditions.

This permits the conversion of primary alcohols and secondary alcohols containing acid-sensitive groups into the corresponding aldehydes or ketones.

In DMF solution, however, PDC oxidizes nonconjugated primary alcohols to the corresponding carboxylic acids.

2.5) Swern Oxidation:

Activation of dimethylsulfoxide (DMSO) by oxalyl chloride produces intermediate (A) that decomposes rapidly at -78°C to furnish chlorodimethylsulfonium chloride along with CO and CO2.

Reaction with RCH2OH (or R2CHOH) leads to intermediate (B) which upon addition of Et3N and warming affords the corresponding carbonyl compound, Me2S and Et3NH+Cl-.

The Swern oxidation proceeds rapidly at low temperatures and thus can be employed for the preparation of a-keto aldehydes and acylsilanes, which are hyperactive carbonyl compounds and prone to hydration, polymerization, and air oxidation.

2.6) Dess-Martin Periodinane (DMP):

The Dess-Martin oxidation of alcohols has proven to be an efficient method for the conversion of primary and secondary alcohol to aldehydes and ketones, respectively.

The rate of oxidation is markedly accelerated in the presence of water.

The oxidation proceeds un der mild reaction conditions and is especially suitable for multifunctional substrates containing acid-sensitive groups.

2.7) IBX:

IBX, the precursor of DMP, is a valuable oxidant of functionalized alcohols and nitrogen- and sulfur-containing substrate when dissolved in DMSO.

In contrast to the oxidative cleavage observed with 1,2-diols by the DMP reagent, IBX converts glycols to a-ketos or a-diketonesl"'and 1,4-diols to lactols.

2.8) Tetrapropylammonium perruthenate (TPAP) (Pr4N+Ru04-):

(TPAP) (Pr4N+Ru04-) is an air-stable oxidant for primary and secondary alcohols is commercially available and environmentally friendly since it is used in catalytic amounts in the presence of a co-oxidant such as N-methylmorpholine-N-oxide (NMO).

TPAP tolerates a wide variety of functional groups, including double bonds, enones, halides, epoxides, esters, and lactones. Protecting groups, such as MEM, trityl, silyl and benzyl ethers, THP, and acetals, are not affected.