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The organic compounds that have a functional group -OH attached to the carbon atoms of alkyl groups are called alcohols.
- The oxidation of alcohols is a chemical reaction that converts alcohols into carbonyl compounds called aldehydes and ketones.
- It results in the formation of carbon-oxygen double bonds by the reduction of oxidizing agents.
Mechanism of Oxidation of Alcohols
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The oxidation of alcohol is an important step in organic chemistry. Alcohols are converted into aldehydes, ketones, or carboxylic acids in the presence of oxidizing agents or catalysts.
- Alcohols are oxidized in the presence of oxidizing agent sodium or potassium dichromate (VI).
- These agents are acidified in diluted sulphuric acid. Different reagents are used in the process of oxidation.
Formation of Aldehydes
The formation of Aldehydes is done by oxidizing the primary alcohols. These types of alcohol are bonded with one carbon atom.
- The oxidizing agent used is potassium dichromate (VI) which oxidizes primary alcohol into carboxylic acids.
- The oxygen atom of potassium dichromate (VI) removes the hydrogen atom from the OH group and is replaced with a carbon atom.
- Orange potassium dichromate ions (Cr2O72-) are reduced to green Cr3+ ions when oxidation of alcohol takes place.
Formation of Ketones
Ketones are formed by oxidizing the secondary alcohols. These types of alcohols are bonded with two carbon atoms.
- The oxidizing agent used is potassium permanganate (VII) or sodium dichromates in the presence of sulfuric acid.
- Potassium permanganate (MnO4-) is purple which is reduced to colorless Mn2+ ions after oxidation.
Types of Oxidation of Alcohols
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There are mainly three types of alcohols that undergo oxidation reactions. These alcohols are responsible for producing different products. We will be discussing about the types of oxidation of alcohols briefly:
- Primary Alcohols: Oxidation of primary alcohols results in the formation of aldehydes or carboxylic acids. These types of alcohols are attached to one carbon atom.
- Secondary Alcohols: Upon oxidation of secondary alcohols ketones are formed. They are attached to two carbon atoms.
- Tertiary Alcohols: Due to the absence of hydrogen atoms, tertiary alcohols do not undergo oxidation.
Things to Remember
- Oxidation of alcohol leads to the conversion of carbonyl compounds called aldehydes and ketones.
- They are the organic compounds having a general formula ROH, where R is the alkyl group.
- Primary alcohols are oxidized in the presence of oxidizing agent potassium dichromate (VI)
- Ethanol is used in the beverage, food, chemical, cosmetics, and healthcare industries.
- The removal of hydrogen atoms from the alkane results in alkyl group formation.
Sample Questions
Ques. Mention any few characteristics of Alcohol. (2 marks)
Ans. The characteristics of alcohol are mentioned below:
- Alcohols are acidic.
- They are colorless.
- Alcohols are flammable.
- They are water-soluble compounds.
Ques. What tests are carried out in the identification of alcohol? What reagents are used in both tests? (3 marks)
Ans. Alcohols are identified by carrying out two tests: the Lucas test and the Oxidation test.
- Lucas test is carried out on the basis of the reactivity of primary, secondary, and tertiary alcohols.
- The reagents used are a concentrated solution of hydrogen chloride and zinc chloride.
- In the oxidation test, the reagent used is sodium dichromate.
Ques. In the oxidation of alcohols, what are the oxidizing agents used? (2 marks)
Ans. There are various oxidizing agents used in the reduction of alcohols. A few of them are listed below:
- Potassium dichromate (VI)
- Chromic Acid
- Pyridinium dichromate
- Potassium permanganate (VII)
Ques. Differentiate between an aldehyde and a ketone. (2 marks)
Ans. The table below shows the difference between the two:
| Aldehyde | Ketone |
|---|---|
| These are chemical compounds with a general formula R-CHO. | These are chemical compounds with a general formula R-CO-R’. |
| Primary alcohols form aldehydes. | Secondary alcohols form ketones. |
| They occur as volatile compounds. | They occur as sugars. |
Ques. Give a comparative analysis of primary, secondary, and tertiary alcohols. (3 marks)
Ans. The table below shows a comparative analysis between the three:
| Primary Alcohols | Secondary Alcohols | Tertiary Alcohols |
|---|---|---|
| They are bonded to one carbon atom. | They are bonded to two carbon atoms. | They are bonded to three carbon atoms. |
| Undergoes oxidation by oxidizing agent potassium dichromate. | Undergoes oxidation by oxidizing agent potassium permanganate. | Does not undergo oxidation. |
| They are more acidic in nature. | They are the least acidic in nature. | They are the least acidic in nature. |
| Example: Ethanol, Methanol | Example: Propan-2-ol | Example: 2 methylpropan-2-ol |
Ques. Oxidation of tertiary alcohols does not undergo an oxidation reaction. Why? (2 marks)
Ans. Tertiary alcohols do not undergo oxidation because the hydroxyl group (-OH) bearing the carbon atom does not contain any hydrogen atoms attached to it.
- In an oxidation reaction, the removal of hydrogen atoms is necessary from the carbonyl group.
- It is due to this reason that oxidation does not occur in tertiary alcohols.
Ques. What is Jones Reagent? Is it used in the oxidation of alcohol? (2 marks)
Ans. Yes, Jones reagent is used in the production of carboxylic acids and ketones from the oxidation of primary and secondary alcohols.
- It is a solution of sodium or potassium dichromate and sulfuric acid mixed with water and acetone.
- Sometimes, chromium trichromate (CrO3) is also used in place of sodium or potassium dichromate.
Ques. What are carbonyl groups? What are examples of carbonyl compounds? (2 marks)
Ans. The functional groups where the carbon atom is bonded with an oxygen atom by double bonds are called carbonyl groups.
- Carboxylic acids (and their derivatives like amides, esters, halides, etc.), aldehydes, ketones, and quinones are called carbonyl compounds.
Ques. Mention some uses of Alcohol. (3 marks)
Ans. Alcohols are organic compounds that are used for various purposes. Some of them are:
- Alcohols are used in the beverage industry for the production of wine.
- They are used as a fuel. For example, ethanol is combusted to produce carbon dioxide.
- It is used as a disinfectant in hospitals, or in food industries to decrease the microbial load.
- They are used in the manufacturing of perfumes and beauty products.
Ques. Differentiate between oxidation and reduction. (2 marks)
Ans. The table below shows the difference between the oxidation and reduction reaction:
| Oxidation | Reduction |
|---|---|
| Process of losing electrons | Process of gaining electrons. |
| Losing of hydrogen takes place. | The gaining of hydrogen takes place. |
| Energy is released | Energy is stored |
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