Alcohols (Cambridge (CIE) O Level Chemistry): Revision Note

Alcohols

• All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions

• Alcohols are a homologous series of compounds that have the general formula C_nH_2n+1OH

• They differ by one -CH₂ in the molecular formulae from one member to the next

Diagram showing the first three alcohols

• Ethanol (C₂H₅OH) is one of the most important alcohols

  • Ethanol can also be represented by its structural formula CH₃CH₂OH

• It is the type of alcohol found in alcoholic drinks such as wine and beer

• It is also used as fuel for cars and as a solvent

• Alcohols burn in excess oxygen and produce CO₂ and H₂O

• Ethanol undergoes complete combustion:

C₂H₅OH (l) + 3O₂ (g) → 2CO₂ (g) + 3H₂0 (l)

The Manufacture of Ethanol

• There are two methods used to manufacture ethanol:

  • The hydration of ethene with steam

  • The fermentation of glucose

• Both methods have advantages and disadvantages which are considered

Hydration of ethene

• A mixture of ethene and steam is passed over a hot catalyst of phosphoric acid at a temperature of approximately 300 °C

  • The pressure used is 60 atmospheres (6000 kPa)

  • The gaseous ethanol is then condensed into a liquid for use

A water molecule adds across the C=C in the hydration of ethene to produce ethanol

Fermentation of glucose

• Sugar or starch is dissolved in water and yeast is added

  • The mixture is then fermented between 25 and 35 °C with the absence of oxygen for a few days

  • Yeast contains enzymes that catalyse the break down of starch or sugar to glucose

  • If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured

  • The yeast respire anaerobically using the glucose to form ethanol and carbon dioxide:

C₆H₁₂O₆ → 2CO₂ + 2C₂H₅OH

• The yeast are killed off once the concentration of alcohol reaches around 15%, hence the reaction vessel is emptied and the process is started again

• This is the reason that ethanol production by fermentation is a batch process