Alcohol Production (AQA AS Chemistry): Revision Note

Alcohol Production

• Alcohols are compounds that contain at least one hydroxy (-OH) group

• The general formula of alcohols is C_nH_2n+1OH

• Alcohols can be prepared by a wide range of chemical reactions

Fermentation

• Fermentation is a batch process, where sugar or starch is dissolved in water and yeast is added

• The mixture is then kept at between 15 and 35°C with the absence of oxygen for a few days

• Enzymes within the yeast undergo anaerobic respiration to break the sugar down into ethanol and carbon dioxide

C₆H₁₂O₆ $\stackrel{\mathrm{enzymes}}{\to }$ 2CO₂ + 2C₂H₅OH

• If the temperature is too low the reaction rate is low

• If the temperature is too high the enzymes are denatured

• The enzymes in yeast are killed off once the concentration of alcohol reaches around 15%

Electrophilic addition to alkenes

• When hot steam is reacted with an alkene, using concentrated phosphoric(VI) acid (H₃PO₄) or sulfuric acid as a catalyst, electrophilic addition takes place to form an alcohol

Electrophilic addition of steam to alkenes to form an alcohol

Nucleophilic substitution of halogenoalkanes

• The halide atom in halogenoalkanes can be substituted when heated with aqueous NaOH in a nucleophilic substitution reaction

Nucleophilic substitution of halogenoalkanes using NaOH to form an alcohol

Reduction of aldehyde & ketones

• Aldehydes and ketones can be reduced by reducing agents such as NaBH₄ or LiAlH₄ 

• Aldehydes are reduced to primary alcohols

  • The carbon attached to the hydroxy group is bonded to one other alkyl group

• Ketones are reduced to secondary alcohols

  • The carbon attached to the hydroxy group is bonded to two other alkyl groups

Reduction of aldehydes and ketones to form primary and secondary alcohols

Reduction of carboxylic acids

• Similarly, carboxylic acids are reduced by NaBH₄ or LiAlH₄ to primary alcohols

• Carboxylic acids can also be reduced by H₂ using a nickel catalyst and heat

Reduction of carboxylic acids to primary alcohols

Alcohol Production Reactions Table

* Hydrolysis of ester is included for completeness but is A Level content

Biofuels

• A biofuel is a fuel produced from biological material (biomass), such as plant matter

• Ethanol produced by fermentation can be used as a biofuel

Carbon neutrality of ethanol

• A carbon-neutral fuel produces no net emissions of CO₂ to the atmosphere

  • The amount of CO₂ absorbed equals the amount released

• Three equations are used to justify the claim that bioethanol is carbon neutral:

1. Photosynthesis (CO₂ absorbed by the crop):

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

2. Fermentation (CO₂ released):

C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

3. Combustion (CO₂ released):

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

• Photosynthesis absorbs 6 mol CO₂

• Fermentation and combustion together release 6 mol CO₂

• Net CO₂ = 0

Why the carbon-neutral claim is not valid

• Fossil fuels are burned to transport the crop or the ethanol, releasing additional CO₂

• Fractional distillation is required to purify the ethanol

  • The energy used often comes from burning fossil fuels

• Fossil fuels power the farming machinery used to grow and harvest the crop

Environmental and ethical issues

• Forests may be cleared to create farmland → deforestation and loss of biodiversity/habitat

• Crops grown for biofuel compete with food production for agricultural land