Grignard Reagents and Their Reactions

Making Grignard Reagents

• Starting Material: 1-bromo butane
• Reagent: Magnesium metal
• Reaction:
  • Magnesium inserts between the carbon and bromine atom, forming a Grignard reagent.
  • Structure: Carbon (nucleophilic, partial negative charge) - Mg (positive charge) - Br (negatively charged).

Grignard Reagent Reactivity

Reaction with Aldehydes

• Example: Reacting methyl magnesium bromide with an aldehyde.
• Steps:
  1. Nucleophilic attack on carbonyl carbon, breaking pi bonds.
  2. Formation of an alkoxide ion (O with a negative charge).
  3. Protonation with H3O+ leads to a secondary alcohol.

Reaction with Ketones

• Example: Cyclopentanone with methyl magnesium bromide.
• Steps:
  1. Grignard reagent attacks carbonyl carbon from the back.
  2. Forms an alkoxide ion.
  3. Protonation gives a tertiary alcohol.

Reaction with Aromatic Compounds

• Example: Bromobenzene with Mg, then CO2, then H3O+.
• Steps:
  1. Formation of phenyl magnesium bromide.
  2. Reaction with CO2 leads to benzoate.
  3. Final protonation gives benzoic acid.

Reaction with Esters

• Example: Ethyl magnesium bromide with an ester.
• Key Point: Esters allow two R groups to be added.
• Steps:
  1. First Grignard addition gives a tetrahedral intermediate.
  2. Methoxide ion leaves, forming a ketone.
  3. Grignard reacts with the ketone to yield a tertiary alcohol.

Identifying Reagents for Given Reactants

Example 1: Aldehyde to Secondary Alcohol

• Reactant: Aldehyde
• Product: Secondary alcohol
• Reagents Needed: 1 equivalent of Grignard reagent (alkyl group), then H3O+.

Example 2: Acid Chloride to Tertiary Alcohol

• Reactant: Acid chloride
• Product: Tertiary alcohol
• Reagents Needed: 2 equivalents of Grignard reagent (propyl groups), then H3O+.

Additional Reaction Example

1-Bromo Butane with Ethylene Oxide

• Steps:
  1. React 1-bromo butane with magnesium to form butyl magnesium bromide.
  2. React with ethylene oxide.
  3. Protonation with H3O+ yields 1-hexanol (6 carbon primary alcohol).

Conclusion

• Grignard reagents are vital in organic synthesis, allowing for the formation of alcohols from various carbonyl compounds.
• Understanding the reaction mechanisms and products is crucial for applying Grignard chemistry.