Table of Contents
Oxidation of Alcohols to Aldehydes and Ketones
Alcohols are a group of compounds containing one, two or more hydroxyl (-OH) groups that are attached to the alkane of a single bond. These compounds have a general formula -of OH. They have primary importance in the field of organic chemistry as they can be changed or converted to different types and types of compounds such as Aldehydes and Ketones, etc. The reactions with alcohol are two different categories. These Reactions can leave the R-O bond or even they can leave O-H bond.
The alcohols are converted to aldehydes and ketones by the process of oxidation. This is one of the most important reactions in the field of organic chemistry.
Oxidizing alcohols to aldehydes and ketones are one of the vital reactions in the field of synthetic organic chemistry. These reactions occur in the presence of catalysts and the best oxidants required for these conversions have high valent ruthenium acting as the catalyst for this kind of reaction. It is very much important to have complete knowledge and also understanding the factors and mechanisms of the oxidation reactions influencing them.
1. Mechanism of conversion Alcohols into Aldehydes and Ketones
The catalytic conversion of the primary type of alcohols into aldehydes and the secondary form of alcohols into ketones are important in the preparation of various synthetic intermediates in organic chemistry.
The result of the oxidation reaction of the alcohols depends on the types of substituents used on the carbonyl carbon. For the oxidation reaction to take place, a hydrogen atom needs to be present on the carbonyl carbon.
The oxidizing agents or the catalysts used in these types of reactions are normally the solutions of sodium or also potassium dichromate(VI) which is acidified with the dilute sulphuric acid. In the process of oxidation, the orange solution which contains ions of dichromate(VI) is reduced to the green solution which contains chromium(III) ions.
2. Making of Aldehydes
The preparation of Aldehydes is by oxidizing the primary alcohols. The aldehyde which is produced can be oxidized further to the carboxylic acids by the use of acidified potassium dichromate(VI) solution that is used as an oxidizing agent. The net effect occurs as the oxygen atom of the oxidizing agent eliminates the hydrogen atom from the hydroxyl (-OH) group of alcohol and also one carbon atom attached to it.
Here, R and R’ are the alkyl groups or hydrogen. If these groups contain the hydrogen atom, you will get aldehyde. These aldehydes are obtained from the primary alcohols.
3. Making of Ketones
The preparation of Ketones is done by the oxidation of secondary alcohols. Consider, for example, heating the secondary alcohol propan-2-ol with the sodium or even potassium dichromate(VI) solution which is acidified with the dilute sulphuric acid, then the ketone called propanone formed.
The occurring reaction is as shown below-
The Ketones obtained cannot be further oxidized because this reaction would involve the breaking up of C–C bond, requiring too much energy.
What are the Different Types of Alcohol?
On the basis of chemical groups attached to the carbon atom, alcohols are divided into three categories:
- Primary alcohol: When the carbon atom attached to the hydroxyl group is bonded to only one carbon atom such type of alcohol is known as primary alcohol.
- Secondary alcohol: When it is bonded to two carbon atoms such type of alcohol is known as secondary alcohol.
- Tertiary alcohol: When it is bonded to three carbon atoms such type of alcohol is known as tertiary alcohol.
Each of the three types of alcohol (primary, secondary and tertiary alcohol) exhibits different physical and chemical properties.
Identification of Alcohols
Certain tests are carried out for the identification of primary, secondary and tertiary alcohols. Some of these tests are:
1. 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 ZnCl2). 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.
2. 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.
Thus, the rate of oxidation upon oxidation with sodium dichromate helps us in the identification of primary, secondary and tertiary alcohol.