Definition reactions reagents and product formation in 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 in Organic Chemistry
1. What is oxidation of alcohols in organic chemistry?
The oxidation of alcohols is a chemical reaction in which an alcohol loses hydrogen or gains oxygen to form a carbonyl compound such as an aldehyde, ketone, or carboxylic acid. In organic chemistry, oxidation typically involves:
- Removal of hydrogen (dehydrogenation)
- Addition of oxygen
- Increase in the oxidation state of carbon
For example, a primary alcohol like ethanol (CH3CH2OH) can be oxidized to ethanal (CH3CHO) and further to ethanoic acid (CH3COOH) under stronger oxidation conditions.
2. What happens when primary alcohols are oxidized?
When primary alcohols are oxidized, they first form an aldehyde and can be further oxidized to a carboxylic acid. The reaction occurs in two stages:
- Primary alcohol → Aldehyde
- Aldehyde → Carboxylic acid (with strong or excess oxidizing agent)
Example using acidified potassium dichromate:
CH3CH2OH + [O] → CH3CHO + H2O
Further oxidation:
CH3CHO + [O] → CH3COOH
3. How are secondary alcohols oxidized?
Secondary alcohols are oxidized to ketones and generally do not oxidize further under normal conditions. The carbon bearing the –OH group becomes a carbonyl group (C=O).
- Secondary alcohol → Ketone
- No further oxidation without breaking C–C bonds
Example:
(CH3)2CHOH + [O] → (CH3)2CO + H2O
Here, propan-2-ol is oxidized to propanone (acetone).
4. Why are tertiary alcohols resistant to oxidation?
Tertiary alcohols are resistant to oxidation because the carbon attached to the –OH group has no hydrogen atom available for removal. Oxidation of alcohols typically requires:
- A hydrogen atom on the carbon bearing the –OH group
- Formation of a carbonyl (C=O) bond
Since tertiary alcohols lack this hydrogen, oxidation would require breaking a C–C bond, which does not occur under mild oxidizing conditions.
5. What are common oxidizing agents used for oxidation of alcohols?
Common oxidizing agents for alcohol oxidation include acidified potassium dichromate (K2Cr2O7/H2SO4), potassium permanganate (KMnO4), and PCC (pyridinium chlorochromate). Their uses depend on the desired product:
- K2Cr2O7/H2SO4: Oxidizes primary alcohols to acids (under reflux)
- PCC: Oxidizes primary alcohols to aldehydes without further oxidation
- KMnO4: Strong oxidizing agent, often forms carboxylic acids
These reagents are widely used in laboratory organic chemistry.
6. What is the difference between oxidation of primary, secondary, and tertiary alcohols?
The difference in oxidation of alcohols depends on the number of alkyl groups attached to the carbon bearing the –OH group.
- Primary alcohol (1°): Oxidizes to aldehyde, then to carboxylic acid
- Secondary alcohol (2°): Oxidizes to ketone only
- Tertiary alcohol (3°): Does not oxidize under normal conditions
This difference is due to the presence or absence of hydrogen atoms on the carbon attached to the hydroxyl group.
7. How do you oxidize ethanol to ethanoic acid?
Ethanol is oxidized to ethanoic acid by heating it under reflux with acidified potassium dichromate (K2Cr2O7/H2SO4). The overall reaction occurs in two steps:
- CH3CH2OH + [O] → CH3CHO + H2O
- CH3CHO + [O] → CH3COOH
Reflux ensures the intermediate aldehyde is not lost and is further oxidized to ethanoic acid.
8. What is the role of reflux in oxidation of alcohols?
Reflux allows the reaction mixture to be heated without loss of volatile substances, ensuring complete oxidation of alcohols. During oxidation:
- The mixture is boiled
- Vapors condense and return to the flask
- The reaction proceeds to completion
For example, refluxing a primary alcohol with acidified dichromate promotes full conversion to a carboxylic acid instead of stopping at the aldehyde stage.
9. Can alcohols be oxidized to aldehydes without forming acids?
Yes, primary alcohols can be oxidized to aldehydes without further oxidation by using a mild oxidizing agent such as PCC (pyridinium chlorochromate) or by distillation during oxidation. Key points include:
- Use of mild or controlled oxidizing conditions
- Immediate removal of aldehyde by distillation
- Avoiding excess strong oxidizing agent
For example: CH3CH2OH + [O] → CH3CHO + H2O, stopping at ethanal.
10. What is the oxidation state change during oxidation of alcohols?
During oxidation of alcohols, the oxidation state of the carbon bonded to the –OH group increases. For example, in ethanol:
- Carbon in CH3CH2OH (–CH2OH carbon) has oxidation state –1
- In CH3CHO, it becomes +1
- In CH3COOH, it becomes +3
This increase in oxidation state confirms that the process is an oxidation reaction in organic chemistry.
