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