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Oxidation of Alcohols

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The hydroxyl group of alcohol weakens the $\alpha$-C-H bond. Alcohols having a hydroxyl group form hydrogen bonds with polar compounds such as hydroperoxides, ketones etc. Alcohols are polar compounds. They have dipole moment and this influences their reactivity in reactions with polar peroxyl radicals.

Oxidation is normally defined as the loss of electrons from an element and reduction is defined as the gain of electrons. If the oxidation state of an element is known both before and after a reaction takes place.

Alcohol to Aldehyde

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The oxidation of different alcohols is an important reaction in organic chemistry. It links alcohols with aldehydes, ketones and carboxylic acid shown below.

Oxidation of Different Alcohols
  • Aldehydes and ketones both contain the carbonyl group C=O.
  • Carboxylic acids contain the carboxyl group COOH.
The stages of oxidation are shown below.

hydroxyl group C-OH $\rightarrow$ Carbonyl group C=O $\rightarrow$ Carboxyl group COOH

Primary alcohol by heating and distilling the first oxidation product immediately, oxidation can be stopped at the aldehyde stage.

Primary Alcohols to Carboxylic Acids

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A primary alcohol can be oxidized to an aldehyde and then to a carboxylic acid. The stages of oxidation of primary alcohol are shown below.

Oxidation of Primary Alcohol

This oxidation is carried out using oxidizing agent. the oxidizing agent used is a mixture of concentrated sulfuric acid and potassium dichromate.

Secondary Alcohols to Ketones

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Oxidation of alcohols occurs when the organic molecule reacts with oxygen or a reagent containing oxygen. Secondary alcohol on oxidation produces ketones.

For example, propan-2-ol on oxidation gives propanone. The reaction is shown below.

Oxidation of Secondary Alcohol

Unlike aldehydes, ketones are resistant to further oxidation. They cannot usually be oxidized by the air but can be oxidized with powerful reagents like potassium dichromate(VI) mixed with concentrated sulfuric acid, with the breaking of the carbon-carbon chain.
Aldehydes and ketones are prepared primarily by the oxidation of the corresponding alcohol. Tertiary alcohols cannot undergo oxidation reactions.

Oxidation of Tertiary Alcohol

But tertiary alcohols are oxidized by using a strong oxidizing agents like acidified KMnO4, acidified dichromate or nitric acid. Oxidation is carried out by eliminating the water molecule from the adjacent carbon atoms.

Tertiary Alcohol Oxidation

Tertiary alcohols are not oxidized under neutral or alkaline conditions, but undergo oxidation in acidic conditions presumably through the alkene formed under the acidic conditions to a mixture of aldehydes, ketones and acids.
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