Oxidation of Alcohols, Aldehydes and Ketones, Identification, Questions (For CBSE, ICSE, IAS, NET, NRA 2022)

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Identification of Alcohols

Certain tests are carried out for the identification of primary, secondary and tertiary alcohols. Some of these tests are:

Lucas Test

  • Lucas test is based on the difference in reactivity of primary, secondary and tertiary alcohols with hydrogen chloride. In the Lucas test, the alcohol is treated with Lucas reagent (concentrated HCl and ) . Turbidity is produced as halides of the substituted alcohol are immiscible in Lucas reagent. The time taken to achieve turbidity is noted and the following observations are made:
    • In the case of a primary alcohol, turbidity is not produced at room temperature. However, on heating, an oily layer is formed.
    • In the case of a secondary alcohol, an oily layer is produced in 5 - 6 minutes. Thus, the reaction takes some time to produce turbidity.
    • In the case of tertiary alcohol, turbidity is immediately produced as halides are easily formed.
  • Thus, the rate of formation of turbidity upon the reaction of an alcohol with Lucas reagent helps us in the identification of primary, secondary and tertiary alcohol.

Lucas Test Reaction

  • Lucas test is based on the difference in reactivity of primary, secondary and tertiary alcohols with hydrogen chloride. In the Lucas test, the alcohol is treated with Lucas reagent (concentrated ) . Turbidity is produced as halides of the substituted alcohol are immiscible in Lucas reagent. The time taken to achieve turbidity is noted and the following observations are made:
    • In the case of a primary alcohol, turbidity is not produced at room temperature. However, on heating, an oily layer is formed.
    • In the case of a secondary alcohol, an oily layer is produced in 5 - 6 minutes. Thus, the reaction takes some time to produce turbidity.
    • In the case of tertiary alcohol, turbidity is immediately produced as halides are easily formed.
  • Thus, the rate of formation of turbidity upon the reaction of an alcohol with Lucas reagent helps us in the identification of primary, secondary and tertiary alcohol.
Lucas Test Reaction

Oxidation Test

  • In the oxidation test, the alcohols are oxidized with sodium dichromate (Na2Cr2O7) . The rate of oxidation varies between primary, secondary and tertiary alcohol. On the basis of their oxidation rates, alcohols can be distinguished as:
    • Primary alcohol gets easily oxidized to an aldehyde and can further be oxidized to carboxylic acids too.
    • Secondary alcohol gets easily oxidized to ketone but further oxidation is not possible.
    • Tertiary alcohol doesn՚t get oxidized in the presence of sodium dichromate.

Questions

What is Oxidation of Ethanol?

Answer:

  • Alcohol oxidation is oxidation with respect to the conversion of hydrogen. The alcohol is oxidised as a result of hydrogen degradation. In hydrocarbon chemistry, oxidation and reduction in hydrogen transfer are common.
  • Ethanol is oxidized to form the aldehyde ethanal by sodium dichromate () acidified in dilute sulphuric acid.

Why Are Tertiary Alcohols Not Oxidized?

Answer:

  • Acidified sodium or potassium dichromate (VI) solution does not oxidize tertiary alcohols. No reaction whatsoever occurs.
  • There՚s no hydrogen atom bound to the carbon in tertiary alcohols. In order to set up the carbon-oxygen double bond, you need to be able to eliminate those two unique hydrogen atoms.

What Do Secondary Alcohols Oxidised To?

Answer:

  • A significant oxidization reaction in organic chemistry is the oxidation of secondary alcohols to ketones. It is converted to a ketone as a secondary alcohol is oxidized.
  • Along with the hydrogen bound to the second carbon, the hydrogen from the hydroxyl group is lost.

Can Alcohols be Oxidized?

Answer:

  • In organic chemistry, the oxidation of alcohol is an important reaction. To form aldehydes and carboxylic acids, primary alcohols can be oxidised; secondary alcohols can be oxidised to deliver ketones.
  • Tertiary alcohol, on the other hand, cannot be oxidised without breaking the C-C bonds of the molecule.

How Does Oxidation of Alcohols Work?

Answer:

  • Depending on the reaction conditions, primary alcohols may be oxidised into either aldehydes or carboxylic acids.
  • As carboxylic acids are formed, the alcohol is first oxidised into an aldehyde and then further oxidised into the acid.

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