Producing Alcohols (CIE A Level Chemistry)

• 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

Electrophilic addition of alkenes

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

 

The electrophilic addition of ethene using water

Electrophilic addition of steam to alkenes forms alcohols

Oxidation of alkenes

• Cold, dilute KMnO₄ is a mild oxidising agent and oxidises alkenes
• The C-C double bond is not fully broken and a diol is formed
  • A diol is a compound with two hydroxy, -OH, groups

 Oxidation of ethene using cold, dilute KMnO₄

The oxidation of alkenes using cold, dilute KMnO₄ forms a diol

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 bromoethane using NaOH

The nucleophilic substitution of halogenoalkanes using NaOH forms 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

 The reduction of ethanal and propanone

The reduction of aldehydes and ketones forms primary and secondary alcohols respectively

Reduction of carboxylic acids

• Similarly, carboxylic acids are reduced by LiAlH₄ to primary alcohols
• NaBH₄ is not used as it is not a strong enough reducing agent for this reaction

 

The reduction of ethanoic acid

The reduction of carboxylic acids forms 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

 

Acidic and alkaline hydrolysis of ethyl ethanoate

The hydrolysis of esters forms alcohols

Alcohol production reactions table

Reaction Type	Reagents	Conditions	Product
Electrophilic addition	Alkene Steam	Heat  H3PO4 catalyst	Alcohol
Oxidation	Alkenes Cold, dilute KMnO4	Shaking of reagents	Diol
Nucleophilic substitution	Halogenoalkanes Aqueous NaOH	Heat	Alcohol
Reduction	Aldehydes / ketones	LiAlH4 or NaBH4	Primary / secondary alcohol
Reduction	Carboxylic acid	LiAlH4	Primary alcohol
Hydrolysis	Esters	Dilute acid or alkali Heat	Alcohol