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

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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 rightwards arrow with straight H subscript 2 straight O on top RCH2OH + Mg(OH)I

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

CH3CH2MgI  + HCHO begin mathsize 14px style rightwards arrow with H subscript 2 O on top end style 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  rightwards arrow with straight H subscript 2 straight O on top 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 rightwards arrow with straight H subscript 2 straight O on top CH3CH2C(CH3)(OH)CH3 + Mg(OH)I

Reactions of Grignard reagents with carbon dioxide 

  • This reaction will produce a carboxylic acid

CH3CH2MgI  + CO2 rightwards arrow with straight H subscript 2 straight O on top CH3CH2COOH + Mg(OH)I

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Richard

Author: Richard

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Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.