Oxidation of Alcohols

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.

(image will be uploaded soon)

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. 

Oxidizing 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 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. 

(image will be uploaded soon)

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. 

(image will be uploaded soon)

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 -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.

(image will be uploaded soon)

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. 

(image will be uploaded soon)

  • 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 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 are 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 to 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.

FAQ (Frequently Asked Questions)

1. Which Alcohol is Readily Oxidized?

Primary alcohol gets oxidized to an aldehyde, and further is also oxidized to carboxylic acids using a catalyst. Secondary alcohol only gets oxidized to Ketones as they can't be further oxidized. Tertiary alcohols don't get oxidized at all.

2. Why can't Tertiary Alcohols be Oxidized?

Tertiary alcohols are resistant to oxidation because of the absence of hydrogen atom in the OH group. Instead, a carbon atom carries the OH group, and it is attached to other carbon atoms. Therefore, they can't be oxidized.

3. Why can't Ketones be Oxidized 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.