Experiment 18:

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Experiment 18
THE GRIGNARD REACTION
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Objectives

• To synthesize a 3o alcohol from an alkyl halide
  and a ketone using a Grignard reaction.
• To purify product using a liquid extraction
  method.
• To determine purity using GC analysis.
• To characterize starting materials and products
  using IR, 1H-NMR, and 13C-NMR spectra.

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Before coming to lab

• Review the following techniques
• Extraction
• Drying organic solvents with MgSO4
• Preparing GC samples
• You will be expected to perform these will little
  review, as you learned these techniques in the
  first semester lab.

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CHEMICAL EQUATION
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MAKING THE GRIGNARD REAGENT

• Organic halides react with magnesium metal in
  ether or THF to yield an organomagnesium halide
  RMgX
• ETHER
• R-X Mg -------------gt R-Mg-X
• or THF
• Where R 1o, 2o, or 3o alkyl, aryl or
  alkenyl
• X Cl, Br, I
• The C-Mg bond is a highly polar covalent bond.
  The carbon atom is both nucleophilic and basic
  making it very reactive with a wide variety of
  E.

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WHEN THE GRIGNARD REAGENT MEETS WATER

• Since the carbon atom of a Grignard reagent is so
  nucleophilic and basic, it reacts with proton
  donors (Brönsted acids) such as H2O, ROH, RCOOH,
  RNH2 to yield hydrocarbons.
• This makes it extremely important to keep the
  reaction flask and solvent completely dry of
  water.

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MECHANISM
2. Nucleophilic addition of butyl group to
acetone produces a tetrahedral intermediate
1. Lewis acid Mg2 forms an acid-base complex
with basic oxygen atom of acetone making the
carbonyl group a better acceptor.
3.which reacts with water when HCl is added,
undergoing hydrolysis to form the product alcohol.
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EXPERIMENTAL PROCEDURE(Synthesis of Grignard
reagent)

• Place Mg turnings in 50mL flask and place a CaSO4
  tube in the top IMMEDIATELY!
• Clamp flask to ring stand and set up remainder of
  reflux with addition apparatus.
• Start reaction with a small amount of
  n-butylbromide and ether by scratching Mg surface
  and stirring with glass rod.
• Add remaining n-butylbromide and ether to sep
  funnel.
• Add to reaction mixture dropwise.

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EXPERIMENTAL PROCEDURE(Synthesis of alcohol
product)

• Add acetone/ether to sep funnel.
• Add to reaction mixture dropwise.
• Cool reaction flask in water bath.
• Transfer cooled liquid to 125 mL Erlenmeyer
  flask.
• Add an ice cube and NH4Cl.

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EXPERIMENTAL PROCEDURE(Purification)

• Set up an extraction apparatus.
• Transfer the liquid from the flask to the funnel.
• Extract with 5HCl, 10 NaHCO3, and Sat. NaCl.
• Transfer organic layer to a 50 mL flask and dry
  over MgSO4.
• Transfer dried organic liquid to a beaker.
• Submit a GC sample.

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Table 18.1
Compound GC Retention Times (min) GC Retention Times (min) Adjusted Area
Compound Standard Sample Adjusted Area
methanol ---
acetone
n-butyl bromide
2-methyl-2-hexanol
octane ---
The product is only pure if the sample
chromatogram indicates presence of GC solvent and
expected product ONLY.
Record retention times for ALL peaks in standard
chromatogram!
Record retention times for any peaks present in
sample chromatogram!
Calculate AA for REACTANTS AND PRODUCT only!
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Product Analysis(IR Spectroscopy)
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Table 18.2
Functional Group Base Values acetone n-butyl bromide 2-methyl-2- hexanol
Functional Group Frequency (cm-1) Frequency (cm-1) Frequency (cm-1) Frequency (cm-1)
OH stretch 2500-3300
sp3 CH stretch 2800-3000
CO stretch 1680-1740
C-O stretch 1000-1200
C-Br stretch 500-700

• IR spectra available on p. 155 of the lab
  manual!
• In discussion, only refer to frequencies which
  indicate a CONVERSION to product!

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Product Analysis(1H-NMR Spectroscopy)


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Product Analysis(13C-NMR Spectroscopy)

NMR solvent

NMR solvent
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Table 18.3
n-butylbromide (reactant)
2-methyl-2-hexanol (product)

• Enter chemical shifts ONLY based on the spectra
  on page 154.
• In discussion, only refer to frequencies which
  indicate a CONVERSION to product!

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SAFETY CONCERNS

• Diethyl Ether is EXTREMELY flammable. Use
  extreme caution at all times!
• Be sure to wear goggles at ALL times during this
  experiment!
• GLOVES are available upon request!

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WASTE MANAGEMENT

• Unreacted magnesium should be rinsed with water
  and placed in Recovered Magnesium (Grignard)
  container.
• Aqueous extracts and washes Aqueous Waste
  (Grignard) container.
• Crude Alcohol product Transfer to bottle labeled
  Grignard product (2-methyl-2-hexanol, Student
  Prep.

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CLEANING

• Rinse condenser and Claisen adapter with wash
  acetone only.
• Clean all other glassware with soap, water,
  brush, and rinse with wash acetone.
• Leave all ground glass jointware in lab hood for
  inspection. DO NOT place any in lab drawer.
• DO NOT return any glassware to lab drawer dirty
  or wet.