Oxidation reactions, Chemistry tutorial

Introduction:

Oxidation as stated in the ionic and free radical reactions is a technique through which an element experiences a total loss of electrons. The concept as applied to organic covalent compounds, in which elements share electrons instead of losing or gaining them is similar, however it is often simplified and narrowed down to make it simpler to identify this technique. Thus it should be remember that, while the definition described below is grossly simplified, it serves up the goal of quickly recognizing oxidation procedure in numerous organic reactions.

Definitions:

Oxidation in the inorganic chemistry is considered as a loss of electrons. For inorganic ions, as whenever Fe2+ is oxidized to Fe3+, this concept works fine. As most of the organic compounds are uncharged, electron loss or gain is not apparent. The organic chemists tend to think of oxidation as an outcome of adding an oxidizing agent like O2, F2, Cl2 and Br2.

In reference to the organic molecules thus, oxidation is widely stated as either the elimination of hydrogen or the substitution of a hydrogen atom linked to a carbon atom by the other more electronegative element, most often oxygen.

This definition is described by the given illustrations: (a) Successive dehydrogenation of ethane and (b) Oxidative sequence of transformation of methane.

Successive (oxidative) dehydrogenation of ethane:

Whenever in an organic compound a carbon atom loses a bond to hydrogen and makes a new bond to a heteroatom (or to the other carbon), the compound is stated to have been dehydrogenated or oxidized. The loss of C-H bonds to make new C-C bonds in succeeding dehydrogenation (elimination of hydrogen) of ethane to ethylene is oxidation

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Fig: Successive dehydrogenation of ethane

Note: The bigger the bond multiplicity the more oxidized the molecule, that is, single bond < double bond < triple bond. In this sequence of two-carbon system each of the two carbon atoms loses a C-H bond and acquires a C-C bond in a stepwise way and therefore is oxidized.

Oxidative transformation of methane:

Redox reactions in the organic chemistry mostly deal by a small set of very identifiable functional group transformations. Acquaintance by the idea of 'oxidation states' as applied to the organic functional groups is thus vital. Well-known functional groups can be set in order of increasing or decreasing oxidation state via comparing the relative number of bonds to hydrogen atoms. Let's take a series of single carbon compounds as an illustration. The degree of oxidation rises as we move methane, by four carbon-hydrogen (C-H) bonds, is highly reduced. Following in the series is methanol (that is, three carbon-hydrogen bonds, one carbon-oxygen bond), followed via methanal, methanoic acid and lastly carbon-dioxide at the highly oxidized end of the group.

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Fig: Oxidative transformation of methane

Note: The more the C-O bonds or the less the less the C-H bonds, the greater oxidized the molecule.

We can now comprehend by considering the descriptions above that oxidation of a Carbon atom in an organic compound comprises one or more of the given changes: (a) The increase in multiple bond order of the Carbon atom (b) Addition of Oxygen to a Carbon atom (c) Replacement of an Hydrogen on a Carbon atom via a more electronegative element particularly Oxygen.

Oxidizing Reagents:

Some of the illustrations of oxidizing agents (that is, reagents) exist. This part represents some of these reagents, a simple process of preparation and some applications of each in the functional group transformation.

Chromic Acid (H2CrO4):

a) Preparation:

This reagent is made by reacting sodium or potassium dichromate by sulphuric acid as illustrated below:

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Fig: Preparation of Chromic Acid (H2CrO4)

b) Application in functional group transformation:  

i)  Oxidation of the secondary alcohols to ketones:

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Fig: Oxidation of secondary alcohols to ketones

ii)  Oxidation of primary alcohols to carboxylic acids:

The alcohol is initially oxidized to an aldehyde. In the reaction conditions, a molecule of water adds to the carbonyl group to make a hydrate which is afterward oxidized to the carboxylic acid.

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Fig: Oxidation of primary alcohols to carboxylic acids

Pyridinium Chlorochromate (PCC) - C5H5NH [CrO3Cl]:

a) Preparation:

The initial preparation comprises the reaction of pyridine by chromium trioxide and concentrated hydrochloric acid:

C5H5N + HCl + CrO3 → [C5H5NH][CrO3Cl]

b) Application in functional group transformation:

By pyridinium chlorochromate, the oxidation of primary alcohols can be stopped at aldehydes. In order to prevent aldehydes from extra oxidation, it is essential to keep anhydrous condition (that is, avoid the addition of water to the carbonyl group). PCC was made up as a non-aqueous alternative to chromic acid. PCC thus offered the benefit of the selective oxidation of alcohols to aldehydes. By employing this reagent, 2-phenylethanol might be oxidized to phenyl acetaldehyde devoid of following oxidation to phenylacetic acid:

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Fig: Pyridinium Chlorochromate

Potassium permanganate (KMnO4) and Osmium tetroxide (OsO4):

a) Preparation:

Dissolve around 3.3 g of reagent grade potassium permanganate (KMnO4) in 1 L of deionized water and heat on a steam bath for 2 hours. Cover and let the solution to stand for 24 hrs. Filter via a fine porosity sintered glass crucible. Store the solution in a glass-stoppered, amber-colored bottle. Keep away from exposure to direct sunlight; cover the neck of the bottle by a small beaker as a protection against dust. Whenever manganese dioxide precipitates on standing, re-filter before use.

b) Application in functional group transformation:

Osmium tetroxide (OsO4) or potassium permanganate (KMnO4) in the aqueous base, reacts by alkenes to result 1,2-diols. Such reagents are employed to transform alkenes to the corresponding 1,2-diols (glycols) via a Stereospecific procedure known as  Syn hydroxylation or Syn addition of two OH groups as they comprise the formation of intermediate cyclic inorganic 'esters' which decompose to the diol in following steps as illustrated in the given explanations. The reaction is assumed to comprise the formation of an intermediate cyclic permanganate ester that is readily hydrolyzed under the reaction conditions to result the 1,2-diol. A cyclic osmate ester is produced by OsO4

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Fig: Potassium permanganate-Osmium tetroxide

Osmium tetraoxide provides excellent results of 1,2-diols, however it is toxic (that is, it causes blindness) and costly. Potassium permanganate is economical and safer to use, however it provides much lower outcomes of diols. This is partially because it can cleave the C-C bond of the diol.

As aqueous KMnO4 is purple, this reaction is frequently employed as a qualitative test for the presence of an alkene: a dilute solution of permanganate is added to the sample of unknown compound; whenever the color is discharged, the test is taken as positive. The formation of a grey-black precipitate of manganese dioxide verifies the analysis.

Ozone (O3):

a) Preparation:

Ozone, O3, is the allotrope of oxygen. This is a highly reactive molecule which is produced via passing a stream of oxygen over a high voltage electric discharge. This is possible to smell ozone in the atmosphere after a lightning storm whenever the lightning has struck close to.

b) Application in functional group transformation:

The reaction between ozone (O3) and an alkene comprises direct addition of O3 across the double bond to provide an unstable intermediate which decomposes to the ozonide intermediate in a process termed as ozonolysis, an alkene is treated by ozone to produce intermediates termed as ozonides that are reduced directly, usually by zinc metal in acetic acid, to outcome aldehydes or ketones, based on the substituents linked to the double bond of the initial alkene. 

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Fig: Preparation of Ozone

It will be noted that an aromatic ring is resistant to ozone. The value of ozonolysis lies in the structural insight it affords a chemist who is trying to find out the recognition of an unknown compound.

Peroxy-acids or Peracids (RCO3H):

The number of peracids encompassing the general formula, RCO3H has been employed for the oxidation of organic compounds. A few common peracids are: peracetic acid (CH3CO3H), trifluoroacetic acid (CF3CO3H), perbenzoic acid (PhCO3H) and m-chloroperbenzoic acid (m-ClC6H4CO3H).

a) Preparation:

Peroxycarboxylic acids (or peracids) are generally made up in situby the reactions of carboxylic acids by hydrogen peroxide (H2O2) as illustrated for the peracetic acid. 

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Fig: Peroxy-acids or Peracids

Most of the peroxy-acids are instead unstable and usually have to be made freshly before use. Performic and peracetic acids, for illustration, are often made in situ and not isolated by action of hydrogen peroxide on the carboxylic acid. The Epoxidation by peroxy-acids are highly stereoselective and occur via cis addition to the double bond of alkene.

b) Application in functional group transformation:

i) Oxidation of alkenes:

 Oxidation of the alkenes by peroxy-acids gives increase to Epoxides (oxiranes) or to trans-1,2-diols, based on the experimental condition. The number of peroxy-acids have been employed in the past, comprising perbenzoic, performic and peracetic acid, however such have now been largely superseded, for the formation of Epoxides at any rate, via m-chloroperbenzoic acid; it is commercially available and is an outstanding reagent for the epoxidation of alkenes. This is more stable than the peroxy-acids and has even been employed at an elevated temperature (90oC) to affect the epoxidation of the unreactive alkenes.

ii) Oxidation of ketones:

On oxidation by peroxy-acids, ketones are transformed to esters or lactones. This reaction was introduced in the year 1899 via Baeyer and Villiger. Better results are obtained by organic peroxy-acids like perbenzoic acid, peracetic acid and trifluoroperacetic acid; however in practice nowadays most reactions are affected by m-chloroperbenzoic acid. This is more stable as compare to the other acids that generally have to be prepared instantly before use, and is commercially available. The reaction takes place under mild conditions and has been broadly employed both in degradative work and in the synthesis. This is applicable to open chain and cyclic ketones and to aromatic ketones and has been employed to form a variety of steroidal and terpenoid lactones, and also medium and large ring lactones which are or else hard to obtain. It as well gives a route to alcohols from ketones, via hydrolysis of esters formed, and of hydroxyl-acids from cyclic ketones by the manner of lactones; lithium aluminium hydride reduction of the lactones provides diols by a defined disposition of the two hydroxyl groups.

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