Alcohols

General properties of alcohols

• General formula: C_nH_2n+2O

• Alcohols can be classified as primary (1º), secondary (2º) or tertiary (3ºº depending on how many R groups (carbon chains) the carbon with the OH is attached to

• Chain and position isomers based on position of OH or length of carbon chain

• Functional group isomers of alcohols are ethers R—O—R

Physical properties of alcohols

• Form hydrogen bonds between molecules so have high melting and boiling points

• Bigger alcohols have more electrons and stronger van der waals forces between the molecules, requiring more energy and giving them a higher bp

• Branched alcohols have lower bp as they have fewer points of contact between the molecules- so the van der waals forces are weaker and less energy is needed

• Alcohols are soluble in water due to hydrogen bonding

what is the industrial preparation of alcohols

• Hydration of alkenes

• Reagent: H₂O_(g) (steam)

• Conditions: H₃PO₄ catalyst, high temperature and high pressure

• The starfish does the box splits and and OH is added to one leg and H to the other

• Position isomers can be formed based on the position of OH

Hydration mechanism

• H⁺ is taken by the double bond

• Starfish does the splits and the H is added (be mindful of the position)

• Water with micky mouse ears (curly arrow from lone pair on water to carbocation)

• Arrow from hydrogen on water to oxygen carbocation

• Product and H⁺ ions form

• H⁺ role : catalyst

Uses of ethanol

• Chemical feedstock (used to make other chemicals)

• A solvent

• Alcoholic drinks

What are the two methods for the industrial preparation of ethanol?

• Fermentation

• Hydration of ethene

Process of fermentation

• Carbohydrates from crops are broken down into sugars

• Sugars converted to ethanol through anaerobic respiration

• Ethanol removed from the reaction mixture by fractional distillation

Anaerobic respiration

Reagents: C₆H₁₂O₆, yeast

Conditions: 35-40ºC, aqueous conditions, anaerobic conditions

Equation: C₆H₁₂O_6(aq) —→ 2C₂H₅OH_(aq) + 2CO_2(g)

• Air is kept out of the reaction to prevent oxidation

• Temp of 35ºC is used to increase reaction rate without denaturing enzyme (yeast)

Fractional distillation

1. Used to separate liquids that have similar boiling points

2. Heat the flask (vaporising the ethanol and water)

3. Vapours pass up fraction in column so water and ethanol will separate

4. Water will condense back into flask as has higher bpt than ethanol

5. Observe thermometer and keep the temp at or just below bpt of ethanol

6. Ethanol vapour passes into condenser where water in outer jacket cools vapour which condenses into a liquid which is collected

Comparison of the two processes of the preparation of ethanol

• Hydration uses crude oil which is a finite resource, fermentation uses sugar which is renewable

• In hydration the product is pure, in fermentation it is impure and requires distillation

• Hydration fast rate of reaction, fermentation slow rate of reaction

• Very high energy requirements in hydration, very low in fermentation

• Hydration continuous process, fermentation batch process

• Hydration higher atom economy (100%) than fermentation

Biofuel definition

A fuel derived or produced from renewable biological sources

When is ethanol being called a biofuel?

When it is made by fermentation

What is a carbon neutral fuel?

One which uses the same amount of carbon dioxide from the atmosphere in its production as is released into the atmosphere when it is used

Why is it argued that bioethanol is not carbon neutral?

• However, fossil fuels need to be burned to power machinery used to make fertilisers, harvest crops and refine and transport bioethanol

• Burning fuel to power machinery produces CO₂

Disadvantage to use of crops for production of fuel

• Land used to grow crops for fuel cannot be used to grow food

• Fertilisers added to soil to increase biofuel crops production

• Deforestation to create land for biofuel crops to grow