Oxidation of Ethanol

• Functional Group Transformation: During chemical oxidation, the $-CH_2OH$ group of ethanol is converted into a $-COOH$ (carboxyl) group, which is the defining feature of carboxylic acids.

• The Role of the Oxidising Agent: Substances like potassium dichromate(VI) provide the necessary oxygen atoms for the reaction. In chemical equations, this agent is often represented as $[O]$.

• Energy Release in Combustion: Complete oxidation through burning breaks all $C-C$ and $C-H$ bonds, releasing significant thermal energy, which explains ethanol's use as a clean-burning fuel.

1. Combustion

• Reaction: $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$
• Observations: Ethanol burns with a clear, blue, non-sooty flame. This method is used in spirit burners for consistent heat without odor.

2. Aerobic (Microbial) Oxidation

• Mechanism: Bacteria such as Acetobacter use atmospheric oxygen to metabolize ethanol into ethanoic acid.
• Real-world Context: This is the process responsible for wine turning into vinegar when exposed to air for several days.

3. Chemical Oxidation (Laboratory)

• Reagents: Ethanol is heated with potassium dichromate(VI) dissolved in dilute sulfuric acid.
• Technique: The mixture is typically heated under reflux (using a condenser) to ensure the volatile ethanol has time to react without escaping.

• Comparison of Oxidation Pathways:

• Balancing Equations: When writing the combustion equation for ethanol, always remember to account for the single oxygen atom already present in the ethanol molecule when totaling the oxygen on the left side.

• Color Change Identification: Be prepared to describe the transition of potassium dichromate(VI) from orange (the $Cr^{6+}$ state) to green (the $Cr^{3+}$ state) as positive proof of oxidation.

• Condition Specificity: Distinguish clearly between the catalysts used for ethanol manufacture (e.g., phosphoric acid) and the reagents used for its oxidation (potassium dichromate).

• Forgetting Water: Students often omit the water molecule produced during chemical oxidation ($CH_3CH_2OH + [O] \rightarrow CH_3COOH + H_2O$). Remember that hydrogen atoms are removed as part of the process.

• Confusing Products: Do not confuse microbial oxidation with fermentation. Fermentation produces ethanol, while microbial oxidation destroys ethanol to produce ethanoic acid.

• Flame Color: Avoid stating that ethanol burns with a yellow or smoky flame; it is distinctively clean and blue.

• Carboxylic Acids: Oxidation of ethanol is the primary way to produce ethanoic acid, which is essential in the food industry as vinegar and in the chemical industry as a precursor to esters.

• Breathalyzer Tests: Historically, the color change from orange to green in dichromate crystals was the chemical basis for early breathalyzer technology to detect alcohol levels.