Seneca
Sign up

Production of Alcohols

Chemistry: AQA A Level, topic 5.5.1

Types of Alcohols

Alcohols can be either primary, secondary, or tertiary.

Primary alcohols

Primary alcohols

  • A primary alcohol is one which has one carbon bonded to the carbon bonded to the functional group.
  • E.g. ethanol.
Secondary alcohols

Secondary alcohols

  • A secondary alcohol is one which has two carbons bonded to the carbon bonded to the functional group.
  • E.g. propan-2-ol.
Tertiary alcohols

Tertiary alcohols

  • A tertiary alcohol is one which has three carbons bonded to the carbon bonded to the functional group.
  • E.g. 2-methyl-propan-2-ol.

Alcohol Production

There are a variety of methods for synthesising alcohols. Different methods are used for industrial purposes and for drinking purposes.

Industrial alcohol production

Industrial alcohol production

  • Alcohols are produced industrially by hydration (adding water) of alkenes in the presence of an acid catalyst.
  • The most common catalyst is phosphoric (V) acid. It’s done at 300oC and 60 atm pressure.
  • The mechanism is via an electrophilic addition.
Hydration of ethene

Hydration of ethene

  • The mechanism above shows how:
  • The acid catalyst acts first to add to the alkene.
  • The lone pair on the oxygen then attacks the carbocation intermediate.
  • Finally, a proton is lost to give an alcohol and to reform the acid catalyst.
Fermentation

Fermentation

  • For some purposes, ethanol is produced by fermentation. This makes alcohol that is drinkable, but it can also be used for making biofuels.
  • Fermentation uses yeast to turn glucose into ethanol.
  • The reaction for it is:
  • C6H12O6→40oCYeast2C2H6O+2CO2C_6H_{12}O_6 \xrightarrow[{40^oC}]{\text{Yeast}}2C_2H_6O + 2CO_2
  • Ethanol is separated by distillation.
  • The process is cheap, but distillation adds extra cost.

Biofuels

Biofuels are fuels composed of grown organic matter.

Biofuel

Biofuel

  • Ethanol is produced by fermentation and then separated by fractional distillation.
  • This can then be used as a biofuel.
  • Biofuels are greener than petrol.
  • This is because they are renewable - we can grow more plants so they won't run out.
    • They are more sustainable.
  • They are carbon neutral (kind of). They release CO2 when they burn, but they absorb it when they grow.
    • These cancel each other out.
Problems

Problems

  • Switching to biofuels is problematic:
  • Car engines aren’t built to run on ethanol.
  • Land that's growing biofuel isn't producing food.
    • This could lead to food shortages.
Carbon neutrality

Carbon neutrality

  • Earlier, we said that biofuels are kind of carbon neutral.
  • It’s true that what they release in CO2, they absorb when they grow.
  • But this is not the full story:
  • You need fossil fuels to make crop fertilisers, to harvest crops, and to transport the biofuel.
  • All these processes make CO2, so it's not completely carbon neutral.

Test yourself on Production of Alcohols

  • Which of the following is a tertiary alcohol?

  • Which of the following are secondary alcohols?

  • The acid catalyst acts in what step of the electrophilic addition mechanism?

  • What is the catalyst used in the hydration of an alkene to give an alcohol?

  • Despite the fact that when biofuels are burnt they release CO2, we say that they are carbon neutral. Why is this?

Practise this topic for free