Chemistry Unit 2 - Alcohols

Alcohols

Functional Group = -OH 

General Formula = CnH2n+1OH

Types of Alcohol

Primary Alcohol - The carbon atom directly bonded to the -OH group is also bonded to atleast 2 Hydrogen atoms.

Secondary Alcohol - The carbon atom directly bonded to the -OH group is also bonded to 1 Hydrogen atom.

Tertiary Alcohol - The carbon atom directly bonded to the -OH group is bonded to no hydrogen atoms.

Strongest type of intermolecular force between molecules of alcohols? (e.g. methanol)

• Hydrogen Bonds
  • This is because the oxygen atom has a lone pair of electrons and is highly electronegative. The O-H bond is highly polar. The H atom is δ+ and forms a hydrogen bond with the lone pair of electrons on an Oxygen atom in a neighbouring molecule.
    • This increases the boiling point of alcohols (e.g. of methanol compared to methane) because the hydrogen bonds are stronger than the Van der Waals forces in alkanes, and so more energy is required to overcome them.

How does this affect the solubility of alcohols?

• Alcohols (e.g. methanol) are soluble in water as they can make hydrogen bonds with the water molecules. (A diol or triol would be even more soluble as they can form more hydrogen bonds per molecule.)
  • However, longer chain alcohols are immiscible with water as the longer the chain, the longer the hydrocarbon skeleton, meaning a larger proportion of the molecule is non-polar, meaning only a small proportion can hydrogen bond with the water.

Two methods - which is used depends on the desired purity of the ethanol and the availability of the raw materials required in the country of manufacture.

Fermentation of Sugars

• Sugars (e.g. glucose), which come from crops like sugar cane, are fermented by enzymes in yeast, producing ethanol and carbon dioxide.
• This process is carried out in the absence of air (anaerobic conditions)
  • C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂
• _{Process is slow at low temperatures, but enzymes are denatured if temperature is too high, therefore optimum temperature of 35}°C is used.
• When ethanol content reaches 15%, the enzyme is unable to function and fermentation stops, and the ethanol in aqueous solution removed by fractional distillation.

The ethanol produced by this process is for human consumption.

It can also be used as fuel, mixed with petrol and called gasohol. This is done in countries like Brazil, where there is an abundant supply of the raw material required - sugar cane.