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  Transformation of alcohols and arenes, Chemistry tutorial

Introduction:

Primary and secondary alcohols are simply oxidized via a variety of reagents, comprising chromium reagents, permanganate, nitric acid and even household bleach (that is, NaOCl, sodium hypochlorite). The choice of reagent based on the amount and value of the alcohol. We make use of cheap oxidants for large-scale oxidations of simple, economical alcohols. We make use of the most efficient and selective reagents, apart from cost, for delicate and valuable alcohols.

Alcohols are oxidized to a diversity of carbonyl compounds. Primary alcohols are oxidized to either aldehydes or carboxylic acids via replacing one or two C-H bonds via C-O bonds. Secondary alcohols are oxidized to ketones by substituting the only C-H bond in the molecule through a C-O bond. Tertiary alcohols don't have H atom on the carbon atom bearing the OH functional group therefore they are not simply oxidized.

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Fig: Transformation of alcohols to carbonyls

Oxidation of primary alcohols:

The Primary alcohols can be oxidized to either aldehydes or further to carboxylic acids based on the reaction conditions. In aqueous media, the carboxylic acid is generally the main product. The direct oxidation of primary alcohols to carboxylic acids generally proceeds via the corresponding aldehyde that is transformed through an aldehyde hydrate via reacting with water before it can be further oxidized to the carboxylic acid. 

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Fig: Primary alcohols oxidation

Acquiring the aldehyde is often hard; as most of the oxidizing agents are strong adequate to oxidize primary alcohols as well oxidize aldehydes. Chromic acid usually oxidizes a primary alcohol all the way to the carboxylic acid. Aldehydes made from oxidation of primary alcohols by employing Cr(VI) reagents are generally further oxidized to the carboxylic acids;  this 'over-oxidation' is a practical problem. We can prevent this through distilling the intermediate aldehyde from the reaction mixture as it forms before it is oxidized further. This process is merely successful for aldehydes of adequately low molecular weight. In this manner, 1- butanol provides butanal in 50% yield.

CH3CH2CH2CH2OH + H2Cr2O7 → CH3CH2CH2CHO

  1-Butanol                                         Butanal

Only aldehydes which boil considerably beneath 100oC can be conveniently made in this way. As this efficiently limits the process to the production of few aldehydes, it is not a significant synthetic method. The other special oxidants have developed that helped to circumvent this trouble. By employing modified Cr(VI) reagents which is illustrated in the following paragraph. 

A common reagent which selectively oxidizes a primary alcohol to an aldehyde (and no further) is pyridinium chlorochromate, (PCC) or pyridinium dichromate (PDC). Such reagents, (PCC) or (PDC) that are employed in dichloromethane, let the oxidation to be stopped at intermediate aldehyde.

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Fig: Oxidization of primary alcohol to an aldehyde

Oxidation of secondary alcohols:

As ketones are more stable to general oxidation as compare to aldehydes, chromic acid oxidations are more significant for secondary alcohols. In one common method a 20% excess of sodium dichromate is added to the aqueous mixture of the alcohol and a stoichiometric amount of acid.

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Fig: Secondary alcohols oxidation

A particularly convenient oxidizing agent is Jones reagent, a solution of chromic acid in the dilute sulphuric acid. The secondary alcohol in acetone solution is 'titrated' by the reagent by stirring at 15 to 20oC. Oxidation is fast and efficient. The green chromic salts separate from the reaction mixture as a heavy sludge; the supernatant liquid comprises mostly of an acetone solution of the product ketone.

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Fig: Cyclooctanol to Cyclooctanone

Chromic (VI) oxidations are acknowledged to proceed by the manner of a chromate ester of the alcohol. Whenever the alcohol consists of one or more hydrogen linked to the carbinol position, a base-catalyzed elimination takes place, resulting the aldehyde or ketone and a chromium (iv) species. The total effect of such two consecutive reactions is oxidation of the alcohol and reduction of the chromium.

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Fig: Oxidation of alcohol and reduction of chromium

Under conditions like these, tertiary alcohols don't usually react, however under proper conditions; the chromate ester can be isolated.

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Fig: Isolation of chromate ester

As there is no carbinol proton to remove in the case of a tertiary alcohol, these esters are stable. Whenever the chromate ester is treated by surplus water, simple hydrolysis takes place by the regeneration of tertiary alcohol and chromic acid.

More vigorous oxidizing conditions yield in cleavage of C-C bonds. Aqueous nitric acid is such a reagent. Oxidation all the manner to carboxylic acid is normal result. These oxidations appear to proceed by means of the intermediate ketone that undergoes further oxidation.

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Fig: Oxidation all the way to carboxylic acid

Rather than oxidation, direct dehydrogenation can be achieved by different catalysts and conditions. The reaction is of industrial interest however is not much employed in the laboratory as the specialized equipment and conditions needed. Catalysts comprise copper metal, copper chromite or copper-chromium oxides prepared in special manners. The illustrations of dehydrogenation are as follows:

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Fig: Examples of dehydrogenation

Oxidation of tertiary alcohols:

Tertiary alcohols are not oxidized via acidified sodium or potassium dichromate (VI) solution. There is no reaction whatsoever. Whenever we look at what is happening by primary and secondary alcohols, we will observe that the oxidising agent is eliminating the hydrogen from the -OH group, and hydrogen from the carbon atom linked to the -OH. Tertiary alcohols do not encompass a hydrogen atom linked to that carbon. We require being capable to eliminate such two particular hydrogen atoms in order to set up the carbon-oxygen double bond.

Transformation of arenes:

The benzene ring is instead stable to oxidizing agents, and under suitable conditions side-chain alkyl groups are oxidized rather.

Sodium dichromate in the aqueous sulphuric acid or acetic acid is a general laboratory method; however aqueous nitric acid and potassium permanganate have as well been employed.

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Fig: Transformation of arenes

The detailed reactions method via which such oxidations takes place are complex. They comprise many intermediates comprising chromate and permanganate esters; however they as well appear to comprise the intermediate benzyl cation.

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Fig: Reaction mechanisms by which oxidations are complex

As we have observed, this carbocation is relatively stable due to conjugation of the positive charge by the benzene ring. Reaction by water results benzyl alcohol that can oxidize further. Larger side chains can as well be oxidized totally so long as there is one benzylic hydrogen for the initial oxidation. The cleavage reactions of larger side chains most likely comprise the formation of an intermediate alkene.

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Fig: Cleavage reactions of larger side chains

The more widespread oxidation needed in such reactions often yields in lower yields in such a way that they are not as helpful for laboratory preparation as they are for structural identification. Whenever there is no benzylic hydrogen, the side chain resists oxidation. For illustration, vigorous conditions are needed for the oxidation of t-butylbenzene and the product is trimethylacetic acid, the product of oxidation of benzene ring.

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Fig: Oxidation of t-butylbenzene

Oxidation of the side-chain methyl groups is the significant industrial route to aromatic carboxylic acids. The most significant oxidizing agent for these reactions is air.

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Fig: Oxidation of the side-chain methyl group

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