Increasing the Carbon Chain Length (Edexcel International A Level Chemistry): Revision Note

Increasing the Carbon Chain Length

• If we need to increase the chain length we can use Grignard reagents 

Preparing Grignard reagents 

• A halogenoalkane is dissolved in dry ether and reacted with magnesium to produce the reactive Grignard reagent

• If 1-iodoethane is reacted with magnesium, ethyl magnesium iodide is produced

• This is an example of a Grignard reagent 

CH₃CH₂I + Mg → CH₃CH₂MgI  

• These compounds are highly reactive and the alkyl group (R group) can be considered to have a negative charge, therefore it can behave as a nucleophile 

  • R^-[⁺MgI]

• Carbonyl compounds and carbon dioxide will react with Grignard reagents to give different products as part of a two-step reaction (specific reaction details and mechanisms are not required at this level)

  • An initial product is formed during an addition reaction

  • Dilute acid is then added to hydrolyse this initial product  

Reactions of Grignard with carbonyl compounds

• Grignard reagents react with methanal to produce a primary alcohol 

RMgI + HCHO RCH₂OH + Mg(OH)I

• For example, methanal can react with ethyl magnesium iodide to produce propanol

CH₃CH₂MgI  + HCHO CH₃CH₂CH₂OH + Mg(OH)I

• They also can react with longer chain aldehydes to produce secondary alcohols

• For example, ethanal can react with ethyl magnesium iodide to produce butan-2-ol

CH₃CH₂MgI  + CH₃CHO  CH₃CH₂CH(OH)CH₃ + Mg(OH)I

• Ketones are also able to react with Grignard reagents producing a tertiary alcohol

• For example, propanone can react with ethyl magnesium iodide to produce 2-methylbutan-2-ol 

CH₃CH₂MgI  + CH₃COCH₃ CH₃CH₂C(CH₃)(OH)CH₃ + Mg(OH)I

Reactions of Grignard reagents with carbon dioxide 

• This reaction will produce a carboxylic acid

CH₃CH₂MgI  + CO₂ CH₃CH₂COOH + Mg(OH)I