Lecture Notes: Preparation of Triphenylmethanol via Grignard Reaction

Introduction

• Experiment: Preparing triphenylmethanol using the Grignard reaction.
• Significance: Common experiment in undergraduate organic chemistry, noted for sensitivity to moisture.
• Reactants:
  • Bromobenzene
  • Magnesium
  • Ethyl benzoate
• Solvent: Very dry diethyl ether or tetrahydrofuran.

Grignard Reaction Overview

• Organometallic Reagent: Phenylmagnesium bromide.
  • Acts as a carbanion equivalent and strong nucleophile.
• Reaction Process:
  • Step 1: Bromobenzene reacts with magnesium to form phenylmagnesium bromide.
  • Step 2: Phenylmagnesium bromide reacts with ethyl benzoate to form benzophenone.
  • Step 3: Second equivalent of phenylmagnesium bromide reacts to form the alkoxide of triphenylmethanol.
  • Step 4: Acidic workup converts the alkoxide to triphenylmethanol.

Procedure

• Preparation:

  • All reagents must be distilled and dried.
  • Bromobenzene dried over calcium chloride and distilled.
  • Ether fractionated from starting fluid, dried over sodium, and distilled.

• Glassware Preparation:

  • Flame dry the apparatus using a propane torch to remove moisture.
  • Use a vacuum pump and calcium chloride drying tube to remove moist air.

• Magnesium Preparation:

  • Magnesium turnings (2.43g, 100mmol).
  • Surface oxide layer removed by crushing.
  • Iodine added to promote reaction start.

• Reaction Setup:

  • Mix magnesium with iodine in a flask.
  • Sublime iodine onto magnesium to increase reactivity.
  • Use ether without ignition sources due to explosion risk.
  • Gradual addition of bromobenzene in ether to magnesium.

• Reflux Conditions:

  • Maintain reaction and apply warm water bath after boiling stops to ensure Grignard formation.

Reaction Workup

• Addition of Electrophile:

  • Slowly add ethyl benzoate dissolved in ether to Grignard reagent.

• Ending Reaction:

  • Reaction mixture becomes biphasic.
  • Add to sulfuric acid solution to neutralize and separate layers.

• Purification:

  • Use separatory funnel to remove aqueous layer containing inorganic salts.
  • Wash organic layer with sulfuric acid and brine to remove impurities.
  • Dry with magnesium sulfate and filter.
  • Recrystallization using hexanes to purify product.

Results

• Final Product: 5.84 grams of triphenylmethanol.
• Yield: 45%, noted as low, potential improvements identified.
• Purity: Initial melting point tests indicate high purity in first crop of crystals.

Reaction Mechanism

• Formation of Grignard Reagent:

  • Single electron transfer (SET) mechanism involving bromobenzene and magnesium.
  • Possibility of biphenyl impurity through dimerization.

• Reaction with Carbonyl:

  • Attack of phenylmagnesium bromide on carbonyl carbon of ethyl benzoate.
  • Formation of benzophenone and subsequent reaction to form triphenylmethanol.

Side Reactions and Impurities

• Biphenyl Formation: Not primarily through direct reaction with bromobenzene.
• Transition Metal Contaminants: May catalyze coupling reactions leading to biphenyl impurities.

Conclusion

• Challenges: Managing moisture and purity of reactants, achieving high yield.
• Achievements: Successful execution of Grignard reaction in non-lab environment.

References

• Referenced paper: "An Improved Preparation of a Grignard Reagent" from JChemEd, Volume 64, page 179.
• Mention of OrgSyn preparation of triphenylmethanol ("triphenylcarbinol").

The lecture provided in-depth insights into the Grignard reaction's complexities and practical challenges faced during experimental procedures.