Edexcel A Level Chemistry:复习笔记7.6.3 Increasing 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]

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)CH2 + 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