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Oxidation of Alcohols

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Last updated date: 23rd May 2024
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Identification of Primary, Secondary and Tertiary Alcohols

An organic compound which is the derivative of water is called Alcohol; when an alkyl or substituted alkyl group replaces a hydrogen atom in H²O, the formation of Alcohol is done.


This replaced alkyl group will be of various types: tertiary, primary, secondary- aromatic ring, open chain, cyclic ring, etc.


Identification of Primary, Secondary and Tertiary Alcohols

Alcohols are organic compounds, which are derivatives of water. When one of the hydrogen atoms in H2O (water molecule) gets replaced with an alkyl group or a substituted alkyl group, Alcohol is formed. 


R - OH

 

The replaced alkyl group can be Primary, Secondary or Tertiary - open chain, cyclic or the one with an aromatic ring.

 

Oxidation of Alcohol

One hydroxyl (-OH) group can be attached with an Alkane in Alcohols in a single bond. This bond can form different types of compounds like ketones and aldehydes. Any cab leaves the compounds at the time of chemical reactions, and thus, other formations are made. In this way, when aldehydes and ketones are converted from Alcohol, this procedure is called oxidation.

 

Before starting the oxidation process, it is essential to have a clear idea of oxidation mechanisms. Some catalysts like Ruthenium can be used for accelerating the process. In modern-day chemistry, oxidation is used in various fields.


Oxidation of Alcohols

Alcohols can contain more than one hydroxyl (-OH) group attached to an Alkane with a single bond. They hold the utmost importance in organic chemistry as they can be converted to different types of compounds, such as Aldehydes and Ketones. During the chemical reactions, either of both bonds (R-O or O-H) can leave the compound, leading to different formations. The process through which Alcohols are converted to either Aldehydes and Ketones, is called Oxidation.

 

Oxidising Alcohols to Aldehydes and Ketones is important in modern-day synthetic chemistry. These reactions are prompted through the presence of best oxidants/catalysts with compounds like Ruthenium. Before proceeding with the Oxidation, it is important to have a full understanding of all the mechanisms and inclusive factors.

 

Different Types of Alcohols

Oxidation and Reductions of Alcohols: Alcohols can be divided into three major categories based on the chemical groups associated with carbon atoms.

  • Primary Alcohol: If a carbon atom attached to the OH group can form a bond with a single carbon atom, this can be called primary Alcohol.

  • Secondary Alcohol: If a carbon atom attached to the OH group can form a bond with two carbon atoms, this can be called primary Alcohol. 

  • Tertiary Alcohol: If a carbon atom attached to the OH group can form a bond with three carbon atoms, this can be called primary Alcohol.


Every type of Alcohol has different chemical properties.


Different types of Alcohols - Oxidation and Reduction of Alcohols

Based on the chemical groups that are attached to the carbon atom, Alcohols are basically divided into three types:

Primary Alcohol: When a carbon atom linked with the OH group is bonded to only one carbon atom, it is known as primary Alcohol.

Secondary Alcohol: When a carbon atom of the OH group is bonded to two carbon atoms, it is known as secondary Alcohol.

Tertiary Alcohol: When a carbon atom of the OH group is bonded to three carbon atoms, it is known as tertiary Alcohol.

 

Each of these three Alcohol types has different physical and chemical properties.

 

Oxidation of Alcohols Mechanism

When primary Alcohol is oxidised into aldehyde and tertiary Alcohol into ketone, the Oxidation is based on various substituents. Here the hydrogen atom shall be on the carbonyl carbon to accelerate the process.

 

Oxidation of Alcohols Mechanism

The catalytic Oxidation of primary Alcohol into aldehyde and Oxidation of secondary Alcohol/Oxidation of tertiary Alcohol into ketone is important in various synthetic chemical industries. The result of Oxidation is based on the types of substituents that are used against the carbonyl carbon. To initiate the Oxidation reaction, the hydrogen atom shall be on the carbonyl carbon. The catalysts used in the Oxidation reactions are Sodium Solutions, or Potassium Dichromate acidified with Dilute Sulphuric Acid. During the process, the orange solution with Dichromate ions is reduced to a green solution with Chromium ions.

 

Oxidation of Alcohols to Aldehydes and Ketones

  • Alcohol to Aldehyde: When primary Alcohol is converted to aldehyde, acidified Potassium Dichromate solution is used as a reagent. In this process, the oxygen atom of the catalyst group attaches to a carbon atom. 

  • Alcohol to Ketone: A ketone can be formed with the Oxidation of secondary Alcohol. But the obtained ketone cannot go through the Oxidation purpose, unlike aldehyde.


Oxidation of Alcohols to Aldehydes and Ketones

Preparation of Aldehydes and Ketones from Alcohol needs a suitable catalyst to make the procedure easier.

 

Alcohol to Aldehyde

An Aldehyde is usually formed by oxidizing the primary Alcohol. The reagent used in the Oxidation of primary alcohol to carboxylic acid during the Alcohol to aldehyde reaction is acidified Potassium Dichromate solution. The result occurs when the oxygen atom of the catalyst eliminates the hydrogen atom from the -OH group and attaches a carbon atom to it.

 

Alcohol to Ketone

The preparation of Aldehydes and Ketones by Oxidation of Alcohol is almost the same. By oxidizing the secondary Alcohol, one can obtain a Ketone. For example, if you're heating a Propanol with Sodium solution/Potassium Dichromate acidified solution, eventually, you will get Propanone.  The obtained Ketones can't be oxidized further because it would then involve a lot of energy to break the C-C bond, unlike Aldehyde.

 

Identification test of Alcohol - Selective Oxidation of Alcohols

To identify primary, secondary, and tertiary Alcohol, you can do various tests. Some of them are:

  • Lucas Test: Based on the reactivity of primary, secondary, and tertiary Alcohol Lucas test is done. In this test concentrated mixture of HCl and ZnCl₂, or Lucas reagent is taken to test the Alcohol. The turbidites are formed for primary secondary and tertiary alcohol but at different times. By noting the time, can identify the type of Alcohol.

  • In primary Alcohol, the solution needs to be heated. After heating for some time, the oily layer is formed.

  • The secondary Alcohol takes almost 5-6 minutes to form the oily layer. In this case, some time is taken to create the turbidity.

  • In the case of tertiary Alcohol, turbidity is immediately formed, and it takes almost no time for the easy formation of aldehyde.


As we can see, based on the rate of turbidity formation and by noting the time of reaction with Lucas reagent, we can easily differentiate primary secondary and tertiary Alcohol.


Identification test of Alcohol - Selective Oxidation of Alcohols

A lot of tests are carried for the identification of primary, secondary, and tertiary Alcohols. Few of them are:

Lucas Test

Lucas test is based on the differences of reactivity in the primary, secondary, and tertiary Alcohols when confronted with Hydrogen Chloride. In this test, the Alcohol group is treated with a concentrated mixture of HCl and ZnCl₂, widely known as Lucas reagent. A lot of turbidites are usually formed because Alcohol is immiscible in the solution. The time taken for turbidity is noted to identify the group of Alcohol.

  • If it is a primary Alcohol, the turbidity can't be formed at room temperature. After heating the solution, an oily layer is only formed.

  • If it is a secondary Alcohol, the oily layer is formed in a span of 5-6 minutes. You have to wait for some time to let the turbidity form.

  • If it is a tertiary Alcohol, the turbidity is immediately formed because of the easy formation of halide.

 

Therefore, the rate of turbidity formation is based upon the reaction of an Alcohol with the Lucas reagent - which helps in the differentiation of primary, secondary, and tertiary Alcohol. 

  • Oxidation Test: In the Oxidation test, Sodium Dichromate (Na₂Cr₂O₇) is used to identify primary, secondary, and tertiary Alcohol. Noting their taste of Oxidation can differentiate the Alcohols. The process of identifying Alcohol is discussed below

  • Primary Alcohol: Primary Alcohol can be easily identified as oxidised to an aldehyde. This type of Alcohol can be formed into carboxylic acid as well.

  • Secondary Alcohol: It can be oxidized only once to a ketone. Unlike primary Alcohol, it can not make any further Oxidation. 

  • Tertiary Alcohol: This type of Alcohol can not be oxidized if any catalyst is present. 


Thus with Sodium Dichromate there, one can quickly identify types of Alcohol.


Oxidation Test

In an Oxidation test, the alcohol groups are confronted with Sodium Dichromate (Na₂Cr₂O₇). Based on the rate of Oxidation, the identification of primary, secondary, and tertiary alcohols is made. The Oxidation rates can be easily distinguished from each other.

  • Primary Alcohol: It gets easily oxidized to an aldehyde and can be further turned into carboxylic acids as well.

  • Secondary Alcohol: It gets easily oxidized to Ketone, but further Oxidation can't be done.

  • Tertiary Alcohol: It doesn't get oxidized in the presence of any catalyst.


Therefore, the rate of Oxidation with sodium dichromate helps in the identification of different alcohol groups.

FAQs on Oxidation of Alcohols

1. Which Alcohol is Readily Oxidised?

Among the primary, secondary, and tertiary alcohols, primary alcohol can be oxidized easily to an aldehyde. After that, primary alcohol can be turned into carboxylic acids using a catalyst. For the secondary alcohol, it can be oxidized only once. At that time, it can turn secondary alcohol into a ketone. But after that, no oxidation can be possible with secondary alcohol. In the case of tertiary alcohols, it doesn't get oxidized if any catalyst is present there. It is different from the primary and secondary alcohol in the case of oxidation.

2. Why can't Tertiary Alcohols be oxidized?

Hydrogen atoms are absent in the OH group. That is one of the reasons for which tertiary alcohols act as resistant to oxidation. In the OH group, a carbon atom is added in the place of a hydrogen atom. In primary alcohol, this carbon atom is bonded with one carbon atom; in secondary alcohol, a carbon atom is attached with two other carbon atoms. In tertiary alcohols, this carbon atom is connected with three other carbon atoms. Because of this carbon atom carried in the OH group, tertiary alcohol can not be oxidized.

3. Why can't Ketones be oxidized easily?

The reason why ketones can’t be oxidized easily is hidden in the formation of aldehydes. In aldehydes, a hydrogen atom is present; oxidation is becoming easier for the oxidizing agents because of this hydrogen atom. It accelerates the process. But in ketones, hydrogen atoms are absent. As a result, the molecule lacks a solid oxidizing agent to react. Because of this, the oxidation process becomes complex. If any hydrogen atom were present in the ketones, the molecule would find an oxidizing agent readily, and oxidation would be faster.

4. Why can't Ketones be Oxidised Easily?

Aldehydes contain a hydrogen atom, which makes it easier for the oxidizing agent to oxidize them. But, ketones lack the hydrogen atom, which makes the oxidation process difficult. Therefore, one would require a strong oxidizing agent to make them react.