Alcohols revision notes

Alcohols in Chemistry

Ethanol Production

Ethanol can be produced through two main methods: fermentation and hydration of ethene.

• Fermentation: This is a biological process where sugars are converted into ethanol and carbon dioxide by yeast. The overall equation is:

C₆H₁₂O₆ (aq) → 2 C₂H₅OH (aq) + 2 CO₂ (g)

• Hydration of Ethene: This is a chemical process where ethene reacts with steam in the presence of a catalyst (usually phosphoric acid) to form ethanol. The equation is:

C₂H₄ (g) + H₂O (g) → C₂H₅OH (aq)

Combustion of Alcohols

Alcohols undergo combustion reactions to produce carbon dioxide and water. The general equation for the complete combustion of an alcohol (R-OH) is:

CₙH₂ₙ₊₁OH + (3n + 1)/2 O₂ → n CO₂ + (n + 1) H₂O

Oxidation of Alcohols

• Primary Alcohols: These can be oxidized to aldehydes and then to carboxylic acids. For example, ethanol can be oxidized to ethanal (acetaldehyde) and then to acetic acid.
• Secondary Alcohols: These can be oxidized to ketones. For instance, isopropanol can be oxidized to acetone.
• Tertiary Alcohols: These do not oxidize easily due to the lack of a hydrogen atom on the carbon bearing the hydroxyl group.

Elimination of Water from Alcohols

Alcohols can undergo dehydration (elimination of water) to form alkenes. This reaction typically requires an acid catalyst and heat. For example:

C₂H₅OH → C₂H₄ + H₂O

Practical Testing for Alcohols

In organic functional-group analysis, alcohols can be tested using the Lucas test, where the alcohol is treated with Lucas reagent (HCl and ZnCl₂). The formation of an alkyl chloride indicates the presence of an alcohol.