Oxidation and Dehydration of Alcohol - SS2 Chemistry Lesson Note
Oxidation and dehydration are important reactions that alcohols can undergo, leading to the formation of various products. These reactions play a significant role in organic synthesis, as well as in industrial and biological processes.
Oxidation of Alcohols:
Oxidation of alcohols involves the loss of hydrogen and/or gain of oxygen, resulting in the formation of different functional groups. The extent of oxidation depends on the nature of the alcohol and the oxidising agent used. Alcohols can be classified into three types based on their oxidation potential: primary, secondary, and tertiary alcohols.
a. Primary Alcohols:
Primary alcohols can undergo both partial and complete oxidation to form aldehydes and carboxylic acids, respectively.
● Partial Oxidation: Primary alcohols are oxidised by mild oxidising agents, such as pyridinium chlorochromate (PCC) or Collins reagent, to form aldehydes. The reaction proceeds through the formation of an aldehyde intermediate before further oxidation occurs.
● Complete Oxidation: Strong oxidising agents, such as potassium dichromate (K2Cr2O7) or acidic potassium permanganate (KMnO4), can oxidise primary alcohols to carboxylic acids. This reaction involves the loss of both hydrogen atoms from the alcohol group.
b. Secondary Alcohols:
Secondary alcohols can be oxidised to ketones, but they cannot be further oxidised to carboxylic acids under normal conditions. Mild oxidising agents, such as chromic acid (H2CrO4) or Jones reagent, are commonly used for this purpose.
c. Tertiary Alcohols:
Tertiary alcohols are resistant to oxidation due to the absence of a hydrogen atom attached to the carbon bearing the hydroxyl group. As a result, they do not undergo oxidation under typical conditions.
Applications of Alcohol Oxidation:
● Production of Aldehydes and Ketones: Partial oxidation of alcohols is essential in synthesising aldehydes and ketones, which serve as intermediates for various organic compounds.
● Oxidation Reactions in Biological Systems: Oxidation of alcohol occurs naturally in biological systems through enzymatic reactions, such as alcohol dehydrogenases and aldehyde dehydrogenases.
Dehydration of Alcohols:
Dehydration of alcohol involves the removal of a water molecule, resulting in the formation of an alkene. The reaction typically occurs under acidic conditions, where the acid acts as a catalyst. Dehydration reactions are particularly important for converting alcohols into alkenes, which are valuable building blocks in organic synthesis.
Mechanism of Dehydration:
The dehydration of alcohol follows an E1 or E2 mechanism, depending on the nature of the alcohol and the reaction conditions.
● E1 Mechanism: In the E1 mechanism (unimolecular elimination), the reaction proceeds through a carbocation intermediate. The hydroxyl group is protonated, leading to the formation of a carbocation, followed by the loss of a water molecule to generate the alkene.
● E2 Mechanism: In the E2 mechanism (bimolecular elimination), the reaction occurs in a concerted manner. A strong base abstracts a proton from a carbon adjacent to the hydroxyl group, and the resulting alkoxide ion eliminates a water molecule to form the alkene.
Applications of Alcohol Dehydration:
● Synthesis of Alkenes: Dehydration reactions are important for producing alkenes from alcohols, providing a versatile method for the formation of carbon-carbon double bonds.
● Industrial Processes: Dehydration of alcohol plays a role in various industrial processes, including the production of gasoline, plastics, and rubber.
The oxidation and dehydration of alcohols are fundamental reactions in organic chemistry. Oxidation converts alcohols into aldehydes, ketones, or carboxylic acids, depending on the type of alcohol and oxidising agent used. Dehydration reactions eliminate a water molecule, leading to the formation of alkenes. Understanding the mechanisms, products, and applications of these reactions allows chemists to manipulate and utilise alcohols effectively in synthesis and industrial processes.