Redox Reactions

What are Redox Reactions?

Redox reactions involve the transfer of electrons between substances, resulting in oxidation (loss of electrons) and reduction (gain of electrons). In organic chemistry, redox reactions are critical for the transformation of alcohols, aldehydes, and ketones. These reactions are fundamental in metabolic processes, industrial applications, and fuel combustion.

Oxidation of Alcohols

Alcohols can be oxidized into different compounds depending on their structure and the strength of the oxidizing agent.

• Common oxidizing agents include potassium permanganate ($KMnO₄$) and sodium dichromate ($Na₂Cr₂O₇$).

Primary Alcohols

Primary alcohols can be oxidized to aldehydes and then further oxidized to carboxylic acids.

Step 1: Oxidation to Aldehyde

Step 2: Oxidation to Carboxylic Acid

Secondary Alcohols

Secondary alcohols are oxidized to ketones. However, ketones are generally resistant to further oxidation under mild conditions.

Oxidation and Resistance of Ketones

While aldehydes are easily oxidized to carboxylic acids, ketones do not undergo oxidation as easily due to the lack of a hydrogen atom attached to the carbonyl carbon.

For example, propanone (acetone) cannot be easily oxidized to a higher oxidation state without breaking carbon-carbon bonds.

Ethanal Formation in Ethanol Metabolism

In the human body, ethanol is metabolized by the liver. The enzyme alcohol dehydrogenase catalyzes the oxidation of ethanol to ethanal (acetaldehyde). This is a critical step in alcohol metabolism:

$$\text{C₂H₅OH} → \text{CH₃CHO}$$

Ethanal is then further oxidized to ethanoic acid (acetic acid) by the enzyme aldehyde dehydrogenase:

$$\text{CH₃CHO} + H₂O → \text{CH₃COOH} + 2H⁺ + 2e⁻$$

The buildup of ethanal is responsible for some of the unpleasant effects of alcohol consumption, including hangovers.

Reduction of Carbonyl Compounds

Carbonyl compounds, such as aldehydes and ketones, can be reduced back to alcohol using a hydrogenation reaction.

This process requires a catalyst, typically $H₂$/$Ni$ (hydrogen gas and a nickel catalyst).

Combustion of Alcohols

Combustion is a type of redox reaction that occurs when a substance reacts with oxygen, producing energy in the form of heat and light.

Alcohols, like ethanol, undergo complete combustion to produce carbon dioxide and water:

$$\text{C₂H₅OH} + 3O₂ → 2CO₂ + 3H₂O$$

Alcohols as Motor Fuels

Ethanol is commonly used as a motor fuel or fuel additive because it burns cleanly and produces fewer emissions than gasoline.

In some countries, ethanol is mixed with petrol to create gasohol, which helps reduce carbon monoxide emissions and reliance on fossil fuels.

Non-flammability of Fully Halogenated Alkanes

Fully halogenated alkanes, such as carbon tetrachloride ($CCl₄$), are non-flammable.

• This is because the halogen atoms make the molecule stable and difficult to combust.
• These compounds are often used in fire extinguishers and flame retardants due to their ability to inhibit combustion reactions.