Chem 334 Lab Lecture

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Chem 334 Lab Lecture

• Expt. 17.1A
• Microscale Wittig Synthesis of
• trans-9-(2-phenylethenyl)anthracene

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Georg Wittig The Wittig Reaction
Nobel Prize 1979
"for development of the use of phosphorus-containing compounds into important reagents in organic synthesis"
 
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Purpose

• Convert an aldehyde into an alkene
• Determine the stereochemistry of the product
  (cis- or trans-geometry)
• Identify the product using melting point and TLC
• Relate color with structure

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Generic Wittig Reaction
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Phosphorus Ylides

• Prepared from triphenylphosphine and an
  unhindered alkyl halide. (What kind of reaction?)
• A strong base then abstracts a hydrogen from the
  carbon attached to phosphorus.

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Wittig Mechanism

• The negative C on ylide attacks the positive C of
  carbonyl to form a betaine.
• Oxygen combines with phosphine to form the
  phosphine oxide.

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Reaction
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Formation of Ylide
benzyl triphenyl- phosphonium chloride
ylide
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Pre-lab Preparation

• Read Background and Expt. 17.1A.
• Prepare Notebook
• Title, purpose, procedural reference
• Changes to procedure
• Use a 5-mL conical vial

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Pre-lab Preparation

• Prepare Notebook
• Table of reagents products
• For each reactant, solvent, and product, include
  structure, molecular weight, melting point or
  boiling point, and hazards
• Balanced Reaction
• Flow Chart

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In Lab

• Perform experiment
• Add reagents and solvent to 5-mL conical vial.
• Dissolution of solids is not necessary.
• Stir for 30 minutes.
• Extract and dry
• Evaporate solvent.
• Recrystallize from 2-propanol.

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In Lab

• Perform experiment
• TLC
• Spot
• 9-anthraldehyde
• triphenylphenylphosphine oxide
• trans-9-(2-phenylethenyl)-anthracene
• crude product
• recrystallized product.
• Develop in CH2Cl2
• 10 mL in a 400 mL beaker

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In Lab

• Experimental considerations
• NaOH instead of CH3(CH2)3Li
• Steric hinderance
• Interpret TLC plate
• Color
• Yield
• Purity

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After Lab

• Melting point
• Calculate error.
• Calculate theoretical and percent yield.
• Calculate Rf values.
• Determine stereochemistry of product based upon
  TLC and melting point.

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