Preparation of Triphenylmethanol via the Grignard Reaction

Introduction

• Experiment: Preparing triphenylmethanol using the Grignard reaction.
• Context: Common undergraduate organic chemistry experiment.
• Challenge: Grignard reaction is sensitive to moisture.

Materials and Chemicals

• Starting Materials:
  • Bromobenzene (alkyl halide/aryl halide)
  • Magnesium
  • Ethylbenzoate (electrophile)
• Solvents:
  • Diethyl ether
• Reagents:
  • Sodium-dried diethyl ether
  • Iodine

Procedure

Preparation of Reagents

• Bromobenzene was dried over calcium chloride and distilled.
• Diethyl ether was fractionated and dried over sodium.
• Magnesium turnings (2.43g, 100mmol) crushed to expose fresh surfaces.

Flame Drying Glassware

• Removed moisture using a propane torch.
• Air drawn through the system to remove adsorbed water.

Initial Reaction Setup

• Used iodine to aid in Grignard formation.
• Subliming iodine onto magnesium to increase reactivity.

Reaction Steps

1. Formation of Grignard Reagent:
   • Magnesium reacts with bromobenzene in ether to form phenylmagnesium bromide.
2. Reaction with Ethylbenzoate:
   • Phenylmagnesium bromide reacts with ethylbenzoate to form benzophenone, then a second equivalent forms triphenylmethanol.

Workup and Purification

• Acidic Workup: Used 0.5 molar sulfuric acid to protonate alkoxides.
• Separation: Used separatory funnel to separate organic and aqueous layers.
• Washing: Washed with sulfuric acid and saturated sodium chloride brine.
• Drying: Used magnesium sulfate to dry the organic phase.
• Recrystallization: Used hexanes to recrystallize the triphenylmethanol.

Yield and Purity

• Final Yield: 5.84 grams of triphenylmethanol.
• Melting Point:
  • First crop: 158-160°C (close to literature 160-163°C).
  • Second crop: Melting point increased after recrystallization.
• Overall Yield: 45% yield, which is low compared to literature values (89-93%).

Mechanism of Reaction

Grignard Formation

• Radical process involving single electron transfer (SET) from magnesium to bromobenzene.
• Possible side reactions: Formation of biphenyl via dimers.

Reaction with Carbonyl

• Carbanion attacks carbonyl to form intermediate.
• Ejection of leaving group (ethoxide) forms benzophenone.
• Second reaction with Grignard forms triphenylmethanol.

Side Reactions and Impurities

• Biphenyl Formation: Often occurs due to transition metal contaminants leading to coupling reactions.
• Grignard Reagent's Reactivity: Reacts with moisture, oxygen, and CO2 reducing yield.

Conclusion

• Successful preparation of triphenylmethanol in non-ideal conditions.
• Noted difficulties in drying solvents and maintaining an inert atmosphere.
• Grignard reaction is complex and sensitive to conditions.