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The Grignard Synthesis Miniscale Synthesis of Triphenylmethanol from Ethyl Benzoate. Organic Chemistry Lab II, Spring 2010 Dr. Milkevitch March 29 & 31, 2010. Today’s Experiment. Take a look at some very important chemistry Prepare a Grignard Reagent

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the grignard synthesis miniscale synthesis of triphenylmethanol from ethyl benzoate

The Grignard SynthesisMiniscale Synthesis of Triphenylmethanol from Ethyl Benzoate

Organic Chemistry Lab II, Spring 2010

Dr. Milkevitch

March 29 & 31, 2010

today s experiment
Today’s Experiment
  • Take a look at some very important chemistry
  • Prepare a Grignard Reagent
    • Group of very important synthetic reagents
  • React it with an ester
    • Make an alcohol
  • Purpose:
    • Learn about organometallic reagents
      • Prepare one: phenylmagnesium bromide
    • Accomplish a reaction in low water conditions
    • Use the Grignard reagent to make an alcohol
background
Background
  • Organometallic Reagents
    • Extremely useful group of organic compounds
    • Allow for some interesting chemistry to take place
    • Their use:
      • Make carbon-carbon bonds
  • Grignard Reagents
    • Developed by Victor Grignard in the turn of the 20th century
    • Discovered:
      • Organic halides and magnesium form organomagnesium reagents
      • Called organometallic compounds
organometallic compounds
Organometallic Compounds
  • Contain metal-carbon bonds
  • Many examples in chemistry
  • In Organic Chemistry:
    • 2 main “classes” of organometallic compounds
      • Organomagnesium reagents (“Grignards”)
      • Organolithium reagents
  • Usefulness of these compounds
    • The carbon atoms is nuclophilic
    • Widely used to make new carbon-carbon bonds
    • Used to attach carbonyl groups of aldehydes, ketones, and esters
formation of the reagent
Formation of the Reagent
  • Formed by reaction of an alkyl halide with magnesium metal
    • Reaction commonly done in ether solvents
    • Under very low water conditions
result grignard reagent
Result: Grignard Reagent
  • Carbon-magnesium bonds are polar
    • Carbon atom has a partial negative charge
    • Makes the carbon nucleophilic
      • Its going to “look” for a positive charge
    • Capable of attacking electrophilic carbons
      • Such as carbons in carbonyl groups
function of the ether solvent
Function of the Ether Solvent
  • Grignard reagents
    • Commonly formed in ether solvents
  • They stabilize the Grignard reagent
    • Protect it from oxidation
  • Must use anhydrous solvents
    • Grignards are very sensitive reagents
    • Grignards are strong bases
    • Will react with any reagent with an acidic proton
      • Water
      • Alcohols
      • Carboxylic acids
    • Destroys the reagent
slide9

Reaction of Grignards: Today’s Experiment Part 1: Formation of the Grignard Reagent

procedure i
Procedure I
  • All glassware must be dry
    • Dried in the 120 deg C oven overnight
    • Already done!
      • RB flask, stir bar, condenser, drying tube 125 ml erlenmeyer flask, Claisen adaptor, grad cylinder,

several small erlenmeyer flasks

  • Remove from oven, allow to cool, assemble according to diagram on next slide
    • CAUTION: It will be hot!!!!!!!!
procedure ii
Procedure II
  • Allow glassware to cool before assembling
  • Weigh out 960 mg of Mg turnings
    • Place in RB flask with stir bar
  • Assemble apparatus
  • In the addition funnel
    • Place 4.2 ml of dry bromobenzene
    • 20 ml of anhydrous ethyl ether
    • Swirl to mix
  • Have a ice bath standing by to cool reaction if necessary
  • Add about 1 ml of bromobenzene/ ether to the RB flask
    • Open stopcock, let ~ 1 ml of bromobenzene /ether to go into the flask
  • Turn on cooling water in the condenser
  • Look for cloudiness/ bubbles on the metal surface
    • Indicates the reaction has begun!
  • If after about 5 min you don’t see anything
    • Add a small crystal of iodine
  • Wait another 5 min, if again no reaction occurs…..
procedure iii
Procedure III
  • Ask for assistance
    • I will come over and work some type of magic in the attempt to get your reaction started
    • I won’t tell you what I’m doing, because I’m not sure what I will do to get your reaction going
    • I’ll giggle it around and look at it a lot, and say “hmmmm” a great deal
    • And your reaction will probably, eventually start
    • We hope!
  • In all likelihood, it will eventually begin
  • Once the reaction has started, turn on the stirrer
  • Add the remainder of the bromobenzene/ ether solution dropwise to maintain a steady reflux
  • Addition should take ~ 45 min
  • When addition is done, add 3 ml of ether to the addition funnel
    • To rinse down any residual bromobenzene
    • Add this rinse to the RB flask
  • Fit the flask with a heating mantle
    • Reflux the mixture gently for 15 min
  • When the 15 min is over, most of the Mg should be gone
    • But its not likely
    • Proceed with the experiment
procedure iv
Procedure IV
  • Add 1.41 ml of ethyl benzoate to 10 ml of anhydrous ether
  • Place this in the addition funnel
  • Add the ethyl benzoate solution to the Grignard reagent at rate that will maintain a steady reflux
  • After all the ethyl benzoate has been added
    • Reflux the solution (with stirring) for 30 min
  • When complete, remove from heat and allow to cool
  • Parafilm flask and set it aside until the next lab period
  • DO NOT USE A GLASS STOPPER
  • Place flask inside a beaker
    • Place in the hood until next week