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Grignard Reagent Reactions

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					                Grignard Reagent / Reactions
                  Preparation of a Grignard Reagent
            (Phenylmagnesiumbromide) and reaction with
                 Carbon Dioxide to form Benzoic Acid
               through an Electrophilic Addition reaction



     References:
               Pavia - p. 303 – 309; 313 – 314
               Schornick
                http://classweb.gmu.edu/jschorni/chem318


9/28/2012                                                   1
            Grignard Reagent / Reactions
      Synthesis of Benzoic Acid – Electrophilic Addition reaction
       between a freshly prepared Grignard Reagent
       (Organomagnesium Halide) acting as the Nucleophilie (Lewis
       Base) and the Carbonyl group of Carbon Dioxide acting as the
       positively charged Electrophile
      The Grignard Reagent must be prepared and reacted with the
       solid Carbon Dioxide before the end of the day
        Sonication of mixture of Magnesium, Bromobenzene &
          Ether (Ether acts as solvent and Grignard stabilizer)
        Reaction between Grignard Reagent and Carbon Dioxide
        Hydrolysis of Acid Salt
        Product Separation – Liquid/Liquid Extraction
        Precipitation
        Vacuum Filtration
        Product Purification – Recrystallization
        Product Verification – Melting Point
9/28/2012                                                             2
                Grignard Reagent / Reactions
           Laboratory Report:
               Synthesis Experiment
                   Mass, Moles, Molar Ratio, Limiting Reagent,
                    Theoretical Yield
               Procedures
                   Title – Concise: Simple Distillation, Dry Sample, IR
                    Spectrum, etc.
                   Materials & Equipment (2 Columns in list (bullet)
                    form)
                    Note: include all reagents & principal equipment used
                   Description:
                       Descriptions must be detailed, but concise
                       Use list (bullet) form
                       Use your own words (don’t copy book)
9/28/2012                                                                   3
                Grignard Reagent / Reactions
           Laboratory Report: (Con’t)
               Results – Neat, logically designed template to present
                results
               Summary
                   Paragraph summarizing experimental observations,
                    computed results, and principal absorptions from IR
                    & NMR
               Analysis & Conclusions
                   Limiting reagent
                   Discuss the reaction in context with the results you
                    obtained; what evidence do you have to indicate
                    you got the intended product?
                   Yield – How could the experimental process be
                    improved?

9/28/2012                                                                  4
                Grignard Reagent / Reactions
           Background:
               Grignard Reagents, aka, Organomagnesium Halides
               Prepared by Reaction of an Organic Halide and
                Magnesium metal turnings, usually in an Ether solvent
               Relative Reactivity of Halides is important
                             RI > RBr > RCl > RF
               Very few Organomagnesium Fluorides are produced.
               Aryl Grignards are more easily prepared from Aryl
                Bromides and Aryl Iodides than Aryl Chlorides
               Grignard Reagents are seldom isolated, but they can
                be stabilized briefly in Ether solution and then used in
                the preparation of other compounds
               The preparation of a Grignard Reagent usually
                produces a high yield (85 – 95%)
9/28/2012                                                                  5
             Grignard Reagent / Reactions
           Reactions:   –             +        –


                              –         +        –


                               –            +    –

                              –             +        –

                                   –    +            –




9/28/2012                                                   6
                Grignard Reagent / Reactions
           Reactions (Con’t):
               The Alkyl portion of the Grignard reagent behaves as
                if it had the characteristics of a Carbanion – the
                carbon atom bears a formal negative charge
               The Magnesium portion bears the positive charge.
               Grignard Reagent, thus, acts as a Strong Base and is a
                strong Nucleophile that acts as a Lewis Base donating
                an Electron pair to a positively charged Electrophile
               Especially useful in Nucleophilic Addition reactions of
                the Carbonyl group, which has electrophilic character
                where the positive charge of the carbonyl carbon
                atom is attractive to the Grignard Nucleophile


9/28/2012                                                                 7
              Grignard Reagent / Reactions
           Reactions (Con’t):
                   Grignard & CO2 to produce Benzoic Acid




                Grignard & Ketone to Produce Tertiary Alcohol




9/28/2012                                                       8
              Grignard Reagent / Reactions
           Reactions (Con’t)
             Grignard & Aldehyde to Produce Secondary Alcohol




              Grignard & Formaldehyde to Produce Primary Alcohol




9/28/2012                                                          9
             Grignard Reagent / Reactions
           Grignard with Benzophenone to form
            Triphenylmethanol 
                                     




                                  
                                     




9/28/2012                                        10
              Grignard Reagent / Reactions
           Grignard with Dry Ice (Solid CO2 ) to form Benzoic Acid
            Salt, which is then hydrolyzed with HCL to form Benzoic
            Acid
                          Today’s Experiment




9/28/2012                                                             11
                Grignard Reagent / Reactions
           Preparation of Benzoic Acid:
               Experimental Precautions
                   The Grignard reaction is very reactive. It must be
                    used in a reaction as soon as it is created.
                   Moisture must be avoided. Water acts as an Acid in
                    this reaction; thus the acid proton competes for the
                    Magnesium in an acid – base reaction.
                    Therefore, moisture could result in the formation of
                    a Hydrocarbon – R-H
                R:MgX + H:OH  R-H             +    HO-    + Mg+2 + X-
                Stronger     Stronger    Weaker     Weaker
                   Base        Acid       Acid       Base
                   Anhydrous Ether is used to stabilize the Grignard

9/28/2012                                                                  12
            Grignard Reagent / Reactions
               Use as little heat as possible to avoid formation of
                Biphenyl



        Note: The Biphenyl produced from excessive heat is not
        the same as the Hydrocarbon formed in an acid - base
        reaction above.
               The Organic Ether layer contains Benzoic Acid,
                Biphenyl and Inorganic Salts.




               The Benzoic Acid is separated from the Biphenyl by
                converting it to an aqueous soluble form by
9/28/2012
                washing with NaOH                                      13
            Grignard Benzoic Acid Procedure
           Procedure (Ultrasonic Bath Version)
                     NOTE: Water/Moisture must be avoided:
                           “All glassware must be “dry”
               Weigh a little more than 0.5g of Magnesium (Mg) (MW –
                24.31) to the nearest 0.001 g and place the Magnesium
                in a large oven-dried Test Tube or Centrifuge Tube
               Obtain a vial of Bromobenzene (MW – 157.01) and
                weigh it to the nearest 0.001g
               Transfer the Bromobenzene to the test tube containing
                the Magnesium filings
               Reweigh the empty vile to obtain the net mass of
                Bromobenzene


9/28/2012                                                               14
            Grignard Benzoic Acid Procedure
           Procedure (Ultrasonic Bath Version) (Con’t)
               Add about 10 mL of Anhydrous Ether to the test tube
                containing the Bromobenzene and Magnesium
               Add a very small piece of “Iodine” to the mixture
               Add a cotton ball to the top of the test or loosely
                screw a cap on the Centrifuge tube (if used)
               Place the tube in the Sonication Bath to stimulate the
                reaction. The solution will begin to turn brownish/gray
                as the reaction proceeds
               After about 5 minutes in the Sonication bath, remove
                the tube and gently swirl the mixture to insure
                maximum exposure of the Magnesium filings to the
                Bromobenzene

9/28/2012                                                                 15
            Grignard Benzoic Acid Procedure
           Procedure (Ultrasonic Bath Version) (Con’t)

               Replace the tube in the Sonication bath for about 40
                minutes or so

               Remaining in the Hood, remove the tube containing
                the Grignard reaction mixture from the Sonication
                Bath

               Pour the solution over approximately 10 g of Dry Ice
                (solid CO2) in a labeled 150 mL beaker. Leave the
                unreacted Magnesium particles in the Centrifuge tube

               Rinse the tube with a few mL of Ether and add the
                Ether to the beaker containing the product

               Store labeled beaker containing your product in the
                hood until next lab session
9/28/2012                                                              16
            Grignard Benzoic Acid Procedure
      Week 2 - Hydrolysis & Purification of Benzoic Acid
               Slowly add about 8 mL of 6 M HCl to the beaker
                containing the product
               The mixture may show some reaction activity, such as
                frothing and increased temperature, which might
                include the reaction between any remaining
                Magnesium and the HCl to evolve Hydrogen
               add 10 mL anhydrous Ether to the beaker
               Stir the mixture with a spatula
               Stir and scrape the mixture with the spatula until the
                solids completely dissolve
               If solids still persist in the solution, use a medicine
                dropper and add small additional amounts of Ether
                and stir
9/28/2012                                                                 17
            Grignard Benzoic Acid Procedure
           Week 2 - Hydrolysis & Purification of Benzoic Acid

               If some solids are still insoluble in either the ether
                layer or the aqueous layer, add small amounts of the
                6 M HCl and stir

               There should be two distinct clear layers in the beaker
                 Note: The Benzoic Acid (and any by-product
                       Biphenyl) is soluble in the Ether
                       The inorganic compounds (MgX2) are soluble
                       in the aqueous phase




9/28/2012                                                                 18
            Grignard Benzoic Acid Procedure
           Week Two - Hydrolysis & Purification of Benzoic Acid

               Liquid/Liquid Extraction:

                   Transfer the mixture to a 125 mL Separatory funnel

                   Rinse sides of beaker with additional ether and add
                    to Separatory funnel

                   Insert stopper and secure it with your finger

                   Shake the mixture gently, invert funnel, point stem
                    up and away from your face and slowly open
                    stopcock to vent any gas

                   Allow the layers to separate (remove stopper)

                   Discard the aqueous (lower) phase into a separate
                    waste beaker and put aside
9/28/2012                                                                 19
            Grignard Benzoic Acid Procedure
           Week Two - Hydrolysis & Purification of Benzoic Acid

             Liquid / Liquid Extraction with Distilled Water (Con’t)

              ● Add 5 mL distilled water to the organic solution

              ● Shake the mixture gently, invert the funnel, and
                slowly open stopcock to vent any gas

              ● Allow the layers to separate (remove stopper)

              ● Discard aqueous phase

              ● Repeat the liquid / liquid extraction with an
                additional 5 mL of distilled water



9/28/2012                                                               20
            Grignard Benzoic Acid Procedure
           Week Two - Hydrolysis & Purification of Benzoic Acid
               Liquid / Liquid Extraction with Sodium Hydroxide
                   To the organic layer, which contains the Benzoic
                    Acid and by-product Biphenyl, add 5 mL 5% NaOH
                   Shake the mixture gently, invert funnel pointing
                    away from your face, and slowly open stopcock to
                    vent any gas
                   Allow the layers to separate (remove stopper)
                   Drain aqueous (bottom) layer into clean, dry 100 ml
                    beaker
                   Repeat liquid/liquid extraction with another 5 ml
                    5% Sodium Hydroxide, draining bottom layer into
                    your beaker
                   Repeat liquid/liquid extraction a 3rd time with
                    5% Sodium Hydroxide
9/28/2012                                                                 21
            Grignard Benzoic Acid Procedure
     Week Two - Hydrolysis & Purification of Benzoic Acid

               Liquid / Liquid Extraction with Sodium Hydroxide (con’t)

            Note: The Sodium Hydroxide converts the Benzoic
                  Acid to the water-soluble Sodium Benzoate salt

                      The Sodium Benzoate is thus transferred
                               to the aqueous phase

                Discard the organic Ether layer into the waste jar in
                 the hood




9/28/2012                                                                  22
            Grignard Benzoic Acid Procedure
      Week Two - Hydrolysis & Purification of Benzoic Acid
               Remove Excess Ether
                    Gently heat the combined aqueous extracts (about
                     15 mL) containing the Benzoic Acid salt on a hot
                     plate (set setting at 3-4) until formation of small
                     bubbles (Ether) ceases. This removes any residual
                     Ether, which is slightly soluble in water
                    Note: Don’t overheat; heat just enough to boil off
                          Ether




9/28/2012                                                                  23
            Grignard Benzoic Acid Procedure
           Week Two - Hydrolysis & Purification of Benzoic Acid
               Cool the solution to room temperature
               Add 10.0 mL 6 M HCl and stir (a precipitate forms in
                the beaker)
               Cool mixture in water/ice bath
               Vacuum Filtration
                   Vacuum filter the product using a Buchner Funnel
                   Wash crystals on the filter 2 times with 5 mL cold
                    distilled water




9/28/2012                                                                24
            Grignard Benzoic Acid Procedure
           Week Two - Hydrolysis & Purification of Benzoic Acid
               Recrystallize product from hot (near boiling) water
                   Transfer solid from Buckner Funnel to 100-150 mL
                    beaker using cold Distilled Water to insure complete
                    transfer
                   Add an additional 15 mL of distilled water to
                    mixture
                   Heat solution to just below boiling point, swirling
                    the mixture occasionally until all solids go into
                    solution
                Note: The Benzoid Acid solution gives off a strong
                      pungent vapor; avoid proximity to nose
                   Check with instructor if you feel additional Distilled
                    Water is needed to effect total solution
9/28/2012                                                                    25
            Grignard Benzoic Acid Procedure
           Week Two - Hydrolysis & Purification of Benzoic Acid
               Recrystallize Product (Con’t)
                   Allow solution to cool slowly to room temperature
                    effecting recrystallization of the purified product
                    (Soluble impurities remain in solution
                   If the cooled solution does not appear to have
                    sufficient water to suspend the purified crystals as a
                    slurry, add 10 ml of Distilled Water, redissolve the
                    mixture, and cool again slowly to room temperature
                   After mixture has reached room temperature, place
                    the beaker in an ice/water bath for about a minute




9/28/2012                                                                    26
            Grignard Benzoic Acid Procedure
           Week Two - Hydrolysis & Purification of Benzoic Acid
               2nd Vacuum Filtration
                   Clean and dry the Buckner Funnel, add a new filter,
                    and weigh it to the nearest 0.001g
                ● Assemble the Buckner Funnel and vacuum filter the
                  product
                ● Wash crystals on the filter with 5 mL cold Distilled
                  Water
                ● Dry the product in a pre-weighed weighing tray in
                  the class drawer until next week




9/28/2012                                                                 27
            Grignard Benzoic Acid Procedure
     Week Two - Hydrolysis & Purification of Benzoic Acid
               Weigh the dried product & weighing tray to the
                nearest 0.001g
               Compute the mass of product (Benzoic Acid) by
                difference
               Determine the % yield
               Determine the melting point of the product
                (MP of Benzoic Acid – 122.4oC)
                 NOTE: If little or no yield was obtained, see next
                       slide




9/28/2012                                                             28
            Grignard Benzoic Acid Procedure
                          The Experiment was a Bust!!
           Occasionally students get little or no yield for the
            Grignard / Benzoic Acid experiment
           The principal reason for this is the failure of the original
            reaction between Bromobenzene and Magnesium to
            form the Grignard Reagent
           There are two main indications of a failed reaction:
               No precipitate forms upon the addition of the 6 M HCL
                to the aqueous NaOH washings
               The precipitate largely disappears after the first
                vacuum filtration



9/28/2012                                                                  29
            Grignard Benzoic Acid Procedure
                            The Experiment Was a Bust
               In order to give the students full exposure to the
                complete procedure, the following steps should be
                performed:
                   Weigh out to the nearest 0.001 g approximately 3
                    grams of Benzoic Acid supplied by the instructor
                   Dissolve the Benzoic Acid in 15 mL Anhydrous Ethyl
                    Ether.
                   Wash the Ether phase 3 times with 5 mL 5% NaOH
                    as before combining the 3 aqueous washings in a
                    beaker
                   Discard the Ether


9/28/2012                                                                30
            Grignard Benzoic Acid Procedure
                         The Experiment Was a Bust
                Gently heat the NaOH solution, approximately 15
                 mL, to drive off any residual Ether
                Cool the solution to room temperature
                Add 10 mL HCL, as before, and place in water-ice
                 bath
                A precipitate should form
                Separate the precipitate by vacuum filtration
                Wash product on filter with 5 mL cold water
                Recrystallize the product from hot distilled water
                Vacuum Filtration
                Dry the Product
                Determine the Melting Point
9/28/2012                                                             31
            Grignard Benzoic Acid Procedure
     Note: Lab procedure sequence vs. Lab report procedure
       sequence
               The writing of the laboratory report does not always
                follow the exact physical sequence of procedures used
                in the lab
               The measurement of the Mass of a reagent may be
                immediately followed by an apparatus set-up or
                transfer of the sample to a piece of apparatus
               This same process could be repeated with the second
                or a third reagent used in the experiment
               In your report, however, all determinations of Mass
                are followed immediately by all calculations that can
                be based on the Mass values, i.e. moles, molar ratios,
                limiting reagent, theoretical yield, etc.

9/28/2012                                                                32
            Grignard Benzoic Acid Procedure
               In the Grignard experiment the following calculations
                are carried out before any procedure dealing with the
                mixing of the reagents and subsequent processing of
                the reaction mixture
                   Mass of first reagent
                   Mass of second reagent (HCL – Vol & Density)
                   Moles of both reagents (1 procedure)
                   Set up Balanced Stoichiometric Equation
                   Determine Molar Ratio
                   Determine Limiting Reagent
                   Compute Theoretical Yield


9/28/2012                                                               33
            Grignard Benzoic Acid Procedure
            Use the following table form (handout) as a substitute
             for the table in the Lab Manual Data Report.

            Balanced                                           Mixed
                            Mg   Ether        CO2   HCl
            Equation                                           Salts

            Mol Wgt
            Grams
            Moles




9/28/2012                                                              34
             Grignard Benzoic Acid Procedure
     Hydrolysis of
   Benzoic Acid Salt
                                         8 mL 6 M HCL
                                    20 mL Anhydrous Ether
                                Separatory Funnel (Shake Gently)
            Ether Layer                                                   Aqueous Layer

            Benzoic Acid                                           Inorganic Salts (Discard)
              Biphenyl

                                      Add 5 mL 5% NaOH
                                                                          Aqueous Layer
                                    (Repeat 3 Times)
    Biphenyl (Discard)                                               Soluble Benzoic Acid Salt
                                                                         (Trace of Ether)




                           Heat on Hot Plate in Hood to Drive off Ether
                                   Cool to Room Temperature
                                       Add 10 mL 6 M HCl



9/28/2012                                                                                        35

				
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