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

Hydrolysis of ester

• Esters are made by a condensation reaction between an alcohol and a carboxylic acid

• When an ester is heated with dilute acid or alkali, hydrolysis will take place and the carboxylic acid and alcohol will be reformed

Hydrolysis of esters to form alcohols

Alcohol Production Reactions Table