Grignard Reagent / Reactions References: Slayden – p. 99 - 102 Pavia – p. 303 – 309; 313 –...

Grignard Reagent / Reactions

References: Slayden – p. 99 - 102 Pavia – p. 303 – 309; 313 – 314 Schornick

http://classweb.gmu.edu/~jschorni/chem318

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

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

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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)

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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?

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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%)

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–

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+

+

+

+

Grignard Reagent / Reactions Reactions:

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– – +

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

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Grignard & Ketone to Produce Tertiary Alcohol

Grignard & CO2 to produce Benzoic Acid

Grignard Reagent / Reactions Reactions (Con’t):

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Grignard & Aldehyde to Produce Secondary Alcohol

Grignard & Formaldehyde to Produce Primary Alcohol

Grignard Reagent / Reactions Reactions (Con’t)

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Grignard Reagent / Reactions Grignard with Benzophenone to form

Triphenylmethanol

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Grignard Reagent / Reactions

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Grignard with Dry Ice (Solid CO2 ) to form Benzoic Acid Salt, which is then hydrolyzed with HCL to form Benzoic Acid

Today’s Experiment

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

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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 washing with NaOH

Note: The Biphenyl produced from excessive heat is not the same as the Hydrocarbon formed in an acid - base reaction above.

Grignard Reagent / Reactions Use as little heat as possible to avoid formation of Biphenyl

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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

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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

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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

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Grignard Benzoic Acid Procedure Week 2 - Hydrolysis & Purification of Benzoic Acid

Slowly add about 10 mL of 6 M HCl to the beaker containing the product

Stir and scrape contents of the beaker with the spatula until the solids and liquid form a uniform mixture

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 Methyl –t-Butyl Ether to the beaker Continue to stir and scrape the mixture with a spatula

If solids still persist in the solution, use a medicine dropper and add small additional amounts of Methyl-t-Butyl Ether and stir

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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.

Keep track of the volume of HCL added

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

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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

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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

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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 with5% Sodium Hydroxide

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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 transferredto the aqueous phase

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

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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

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Cool the solution to room temperature

Add 10.0 mL 6 M HCl and stir (a precipitate forms in the beaker)

Note: If little or no precipitate forms add an additional 3 ml HCL. If no precipitate forms inform instructor

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

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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 Benzoic 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

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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

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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

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Grignard Benzoic Acid ProcedureWeek 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

04/18/23 28

Grignard Benzoic Acid ProcedureThe 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

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Grignard Benzoic Acid ProcedureThe 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

04/18/23 30

Grignard Benzoic Acid ProcedureThe Experiment Was a Bust

Gently heat the NaOH solution, approximately 15 mL, to drive off any residual EtherCool the solution to room temperatureAdd 10 mL HCL, as before, and place in water-ice bathA precipitate should formSeparate the precipitate by vacuum filtrationWash product on filter with 5 mL cold waterRecrystallize the product from hot distilled waterVacuum FiltrationDry the ProductDetermine the Melting Point

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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.

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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

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BalancedEquation

Mol Wgt

Grams

Moles

CO2Mg Ether HCl MixedSalts

Grignard Benzoic Acid Procedure Use the following table form (handout) as a

substitute for the table in the Lab Manual Data Report.

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Hydrolysis ofBenzoic Acid Salt

Ether Layer Aqueous Layer

Benzoic Acid Biphenyl

Inorganic Salts (Discard)

Add 5 mL 5% NaOH

Biphenyl (Discard) Soluble Benzoic Acid Salt(Trace of Ether)

(Repeat 3 Times)

Heat on Hot Plate in Hood to Drive off Ether

Cool to Room TemperatureAdd 10 mL 6 M HCl

8 mL 6 M HCL20 mL Anhydrous Ether

Separatory Funnel (Shake Gently)

Aqueous Layer


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Grignard Reagent / Reactions References: Slayden – p. 99 - 102 Pavia – p. 303 – 309; 313 –...

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Transcript of Grignard Reagent / Reactions References: Slayden – p. 99 - 102 Pavia – p. 303 – 309; 313 –...