Grossmont College Preparation of Triphenylmethanol Grignard Reagent Lab Report

1. Please show your calculations for obtaining your percent yield

2. A. Analyze your NMR using the grinard NMR attachment below. In my NMR I have included the chemical shifts of each peak, and the integration underneath it as well. B. You also have to include an explanation matching up the peaks in your NMR spectra to that of the product, TRIPHENYLMETHANOL using the structure. C. Explain why you think you matched up the peaks properly based on chemical shift, integration, and splitting pattern. If you have extra peaks in your NMR that do not match up to any proton peaks in your product, explain where they could have come from.

PREPARATION OF TRIPHENYLMETHANOL:
An Example of a Grignard Reaction
Br
MgBr
+
MgBr
2
Et2 O
Mg
O
+
OH
OCH3
Et2 O
H+
C
Reference: Roberts, Gilbert, Rodewald and Wingrove Chapter 22.2B, pages 555-558.
Solomons and Fryhle 10th Ed. Organomagnesium and Organolithium Compounds, Formation and
Structure, Chapter 12.6-12.8C, pages 562-573.
General Concepts
Grignard reagents are one of the most important organometallic reagents used in organic
chemistry, and are generally prepared from organic halides and magnesium metal. In order to
successfully prepare the organomagnesium compound, all water must first be removed from the
apparatus. The glassware used in this reaction should be cleaned and allowed to air-dry the week
before starting this experiment. This is a long experiment, and so your preparation is key. In order
to finish in an acceptable amount of time, you must know exactly what you need to do before
coming to lab. The formation of the Grignard reagent is inhibited by both water and oxygen. After
assembling the apparatus, flame dry to remove adsorbed water, starting from the bottom and
working slowly to the top. The drying tubes on the top of the condenser and addition funnel protect
from readsorption of water. The diethyl ether solvent protects the Grignard reagent by displacing the
oxygen due to it high vapor pressure. The boiling point of ether is 35 °C. It is important that the
glassware joints are lightly greased and thoroughly mated to prevent readsorption of water and the
loss of the ether solvent. Sometimes it is necessary to active the magnesium turnings to assist the
formation of the Grignard reagent by adding a small amount of I2. Iodine reacts with the magnesium
oxide layer which exposes the more reactive metal which readily reacts with bromobenzene.
Warming the reaction mixture briefly may also help start the reaction. The evolution of small
bubbles in addition to the boiling of the solvent and a color change is evidence that the Grignard
reagent is being formed. The reaction will become cloudy and tan to brown. At this point, 15 mL of
additional ether is added. The mechanism for the formation of the Grignard reagent has some free
radical character. While the high initial concentration of reagents aid formation of the
phenylmagnesium bromide, it also increases the amount of biphenyl formed from the coupling of the
relatively stable phenyl radicals. The additional ether slows the rate of formation of the biphenyl to
a larger extent than it reduces the formation of the Grignard reagent. If your reaction hasn’t started
after 15-20 minutes, see your TA. Add the bromobenzene dropwise to maintain gentle reflux of the
solvent. The majority of magnesium turnings will be consumed after 30-45 minutes and the rate of
reflux will slow.
The Grignard reagent is used immediately after its formation. The heat of reaction with the
methyl benzoate is controlled by its rate of addition. If the reflux rate increases too much, use a cold
water bath to slow the reaction. You will notice different color changes at this point which do not
affect the reaction, and may be different from your neighbor. Once the reaction mixture has cooled,
label your round bottomed flask and stopper it with a greased glass stopper, placing it in a beaker for
storage until the following laboratory period.
The following week, the triphenylmethoxide salt is protonated with 6 M H2SO4 to yield the
triphenylmethanol product. This protonation reaction is sufficiently exothermic to boil ether and the
foam containing the product will spill out of the round-bottomed flask onto the bench top. To avoid
this, carefully transfer as much of the ether solution containing the solid triphenylmethoxide salt to a
500 mL beaker containing 40 mL of 6 M H2SO4 and a handful of ice. There will be solids left in the
round bottomed flask in addition to solids in the beaker. The material left in the round bottomed
flask is transferred by scraping with your spatula, dissolving it in fresh acid or dissolving it with
additional ether. All of the solid material in the beaker and the round bottomed flask will dissolve. If
it doesn’t dissolve in acid, add more ether. If it doesn’t dissolve in ether, add more acid. Break up
chunks with your spatula. Your organic layer must not be larger than about 60 mL. This organic
ether layer is washed with 3 M H2SO4 to remove magnesium salts, followed by washes with
saturated sodium chloride solution, testing each individual sodium chloride wash until they are no
longer acidic by litmus paper. The saturated salt solution minimizes solubility of the organic product
in the water layer. The organic ether layer is transferred to a 125 mL Erlenmeyer flask and dried
with anhydrous sodium sulfate. This is a granular drying agent also clumps-up when hydrated and
the cloudy solution becomes clear when dry, similar to drying with MgSO4. Remove the drying
agent by decanting, rinsing the flask and drying agent with a minimum amount of additional ether.
The ether is carefully evaporated with a stream of compressed air in your hood. The product is
recrystallized from a cyclohexane/absolute ethanol mixture. The product sometimes takes weeks to
crystallize, depending on its initial purity. The higher your yield, generally the higher the purity and
the easier it is to recrystallize.
DO IT SAFELY
1. When flame-drying the reaction apparatus, avoid excessive heating near the ground glass joints
and of the Teflon stopcock of the addition funnel.
2. Diethyl ether is extremely flammable and volatile. There must not be any no open flames in
the entire laboratory while handling the ether. Ether, and its vapors can easily travel several feet
along the bench top or the floor to a source of ignition.
3. Do not keep open containers of ether at your laboratory bench. Estimate the total volume of ether
needed measure it out in the dispensing hood, and return it to your work area in a stoppered
container.
4. Do not stopper flasks containing ether too tightly. Excessive pressure build-up may result
because of temperature changes creating a fire hazard.
5. Do not store ether in your laboratory locker. All reaction flasks containing must be labeled with
your name and class section. Your laboratory instructor will collect and store these until the next
laboratory period.
6. Lubricate and mate all ground-glass joints to prevent escape of ether during the reactions or
distillations.
7. The formation of the Grignard reagents are exothermic. Have an ice-water bath prepared in the
event that the reaction must be reduced, as measured by an very rapid rate of reflux.
Experimental Procedure
FLAME DRYING THE APPARATUS
You will not be able to wash your glassware the same day you flame dry it, because flame
drying is not effective on wet glassware. All glassware must be washed the week prior. Place a stir
bar in a 250-mL round-bottomed flask and add your parallel tube (“u-shaped”, Claisen) adapter to
which are attached a reflux condenser (use the shortest one you have) and your separatory funnel.
See figure on the next page. Position the funnel directly above the round bottomed flask and connect
the condenser on the side arm of the Claisen adapter as shown in the drawing. Place a small amount
of glass wool at the bottom of each drying tube to support the T.H.E. silica desiccant, filling the
drying tube about 1/3 – 1/2 full and securing the desiccant with a little more glass wool. Add 85
mmol of magnesium turnings to the round bottomed flask. Attach the drying tubes to the top of the
condenser and the separatory funnel. Flame dry the assembled apparatus, starting from the bottom
and working slowly to the top, driving any moisture vapor toward the drying tubes. The drying is
done the first time the week before the actual preparation of the Grignard reagent. Stopper the tops
of the drying tubes and store the now dry apparatus in your locker, in one piece if possible. It may
be necessary to remove the condenser from the Claisen connecting tube and stopper the open joints
if the apparatus does not fit.
PREPARATION OF THE GRIGNARD REAGENT
The week that the Grignard reagent is prepared, reassemble and flame dry the apparatus once
again. Allow the apparatus to cool to room temperature and be sure that there are NO flames
anywhere in the laboratory before continuing. No one in the lab may continue flame drying once
the ether is provided, because ether vapor is extremely flammable. It is important to flame dry
quickly so there will be time to finish the experiment. Exchange two clean, dry vials for samples of
bromobenzene and methyl benzoate. Make sure you obtain the correct two samples, and weigh them
in their respective vials. If directed by your TA, briefly remove the separatory funnel and add a
crystal or two of iodine to the magnesium turnings. Often it is not necessary to add iodine, however,
and it can obscure the initiation of the Grignard reaction since it will turn the solution purple.
Prepare a solution of your sample of bromobenzene (it should be around 90 mmol) in 20 mL of
anhydrous ether in your separatory funnel (first be sure that the stopcock is closed!). The easiest
way to accomplish this is to weigh your vial of bromobenzene, transfer it to your separatory funnel,
and then weigh the empty vial. Also prepare a cold water bath in case it is needed. Check that water
is running through the condenser. Have your TA approve your setup before you begin.
Allow just enough bromobenzene/ether solution to drain from
Stopper Here
the funnel to cover the magnesium turnings, and make sure the
for Storage
solution is stirring well. Add more bromobenzene/ether solution in
small portions of about 2-3 mL and heat gently with your hotplate
until the solution comes to a boil. After the solution has boiled for
a minute or so you should notice a color change to tan or brown
and the solution should turn cloudy. If this does not happen, add
more of the bromobenzene/ether solution and continue heating
until the change in appearance is observed. This indicates the
Grignard reagent has started to form. If this does not happen
within 10 minutes, it may be necessary to add some iodine or
activated magnesium turnings to get the reaction started. Most
often this happens because the glassware was inadequately dried.
Once the solution’s appearance changes turn off the heat, and
continue the addition of the bromobenzene/ether solution dropwise
until it has all been added. The solution should boil on its own as
Clamp Here
you complete this process. If the boiling stops, heat briefly until
the solution boils again, and increase the rate of the
bromobenzene/ether
addition.
Once
the
whole
bromobenzene/ether solution has been added, add about 20 mL of
ether to the separatory funnel, and let it run into your reaction in a
slow stream. While this dilutes the reaction mixture and slows the
formation of the Grignard reagent, it also reduces the formation of
Water In
the main biphenyl sideproduct. If the solution boils too rapidly
cool it briefly with a cool water bath. If the volume of the solution
decreases dramatically at any point add additional ether in the
same way, and check to make sure your glassware is properly
sealed.
Clamp Here
The reaction mixture will normally have a tan to brown color
with a chalky appearance. Most of the magnesium will have
consumed, although residual bits of magnesium will remain. After
complete addition of both the bromobenzene/ether solution and
additional ether, the solution should continue to boil on its own
without heating, and then it will eventually stop boiling on its own. Once this happens, heat briefly
to a boil again, and then turn off the heat and let the boiling stop again. This ensures complete
formation of the Grignard reagent. Once the reaction has finally stopped boiling on its own for the
second time, cool it in a cool water bath. Once it is near room temperature it will be time to add the
methyl benzoate.
REACTION OF THE GRIGNARD REAGENT: PREPARATION OF TRIPHENYLMETHANOL
While the phenylmagnesium bromide solution is cooling to room temperature, dissolve 35
mmol of methyl benzoate in about 15 mL of anhydrous diethyl ether in your separatory funnel,
making sure your stopcock is closed. Make sure your solution is stirring rapidly, and begin a
dropwise addition of the solution of methyl benzoate to the room temperature solution of the
Grignard reagent. This reaction is exothermic; control the rate of reaction by slowing the addition
rate or by occasional cooling of the reaction flask with a cool water bath as necessary. The reaction
will boil. As long as the boiling is not excessive this is fine. Stirring of the mixture may become
more difficult during addition. The formation of a white, red or orange solid is an indication that the
reaction is proceeding normally. After the addition is complete, and the reaction is no longer boiling
and has cooled to room temperature without the use of a water bath, stopper the flask with a
greased glass stopper, secure it with a keck clip, and place the flask in a beaker with an appropriate
label. It is important that the reaction is finished before you remove and seal the flask, both to
maximize product formation and so the reaction doesn’t start boiling the solvent again in its sealed
container. If you do not grease your glass stopper it will be difficult to remove later. The laboratory
instructor will collect and store these mixtures in a hood until the next laboratory period.*
The solid product formed in the above step is the triphenylmethoxide magnesium bromide salt.
The protonation of this reaction mixture gives the triphenylmethanol product. The heat of
acidification is sufficient to boil the ether which can foam and spill out of the flask with a significant
loss of product. Add 40 mL of 6 M sulfuric acid and a small handful of ice to a 500 mL Erlenmeyer
flask. Transfer as much of the reaction mixture as possible into this flask with swirling. Then add a
fresh batch of acid and, if necessary, additional ether directly into the flask containing the residual
reaction mixture. Break up the chunks with your spatula and combine this with the material in the
Erlenmeyer flask. Continue stirring, adding additional acid and ether until the heterogeneous
mixture is completely free of undissolved solids. If the solid does not dissolve in acid, it will
dissolve in ether. If it does not dissolve in ether, it will dissolve in acid. It is essential that all solid
material is dissolved before the aqueous washes, to prevent foaming or clogging the separatory
funnel. It is also essential that your ether layer is less than about 60 mL so it fits in the separatory
funnel. Transfer the entire mixture to a 125 mL separatory funnel, shake and drain the aqueous layer
as necessary to get the whole organic layer added to the funnel. Note that ether is less dense than
water therefore your organic layer will be on the top. Shake and vent the funnel often to relieve
pressure; at the end you should have your entire organic layer in the funnel, and will have drained
away the aqueous layer. If you used too much ether, you may need to split your organic layer in half
and do two of the following washing procedures. Wash the organic layer with about 20 mL of 3 M
sulfuric acid and discard the washings, then with 40-65 mL of saturated sodium chloride (brine)
solution. Test the pH of each individual brine wash and continue washing the organic layer with
fresh brine until the most recent brine wash is no longer acidic to litmus. These aqueous washes
should be disposed of in the bottle labeled “Aqueous Acidic Washes.” Dry the organic layer with
anhydrous sodium sulfate, and decant it to a clean, dry 125 mL Erlenmeyer flask. Rinse the drying
agent with a minimum amount of ether. Combine the ether layers in the 125 mL Erlenmeyer flask
and evaporate the ether with a gentle stream of air in your hood—the solution should not splash or
boil violently. If it does, decrease the air flow. Store the goopy yellow to brown solid uncapped in
your locker for next week.*
Add 10 mL of cyclohexane to the goop and crystals in the 125 mL Erlenmeyer flask and gently
swirl for 2 min, then decant the cyclohexane solution from the remaining solid material. If your solid
is powdery you may need to filter it to remove the cyclohexane, then add the solid back to the 125
mL Erlenmeyer flask. The solid is your crude product. To a separate Erlenmeyer flask add a few
boiling chips and fill it halfway with a 2:1 mixture of cyclohexane/ethanol. Bring the
cyclohexane/ethanol mixture to a boil, then add enough boiling solvent to your crystals to cover
them, and bring the solution covering your crystals to a boil too. Once both flasks are boiling, add
small portions of the boiling cyclohexane/ethanol mixture to the crystals flask, swirling in between
each addition and keeping both flasks boiling, just until the crystals have dissolved. Once the
crystals have dissolved remove the flask from the heat, cover it with a watch glass, and let it slowly
come to room temperature. If a minimum amount of boiling solvent was used, the pure product
crystals should slowly form as the flask comes to room temperature. If crystals do not form by the
time the flask is at room temperature, too much solvent was added and you will need to boil off a
portion of your solvent (usually 1/3 to 1/2 of the total volume), and then remove the flask from the
heat, cover it with a watch glass, and wait for it to cool to room temperature again. Once crystals
form, cool the flask to ice bath temperature and separate the crystals by vacuum filtration, washing
them with a minimum amount of ice cold 2:1 cyclohexane/ethanol. The crystals should be white or
clear. If they are yellow or brown the recrystallization must be attempted again. You may attempt to
get an additional crop of crystals from your mother liquor by reducing the volume by at least half by
boiling it and cooling it again to room temperature, to see if any additional crystals form. The mother
liquor is disposed of in the “Ether, Cyclohexane / Ethanol Mother Liquor” container. Determine the
melting point and yield of the product. The reported melting point of triphenylmethanol is 164 °C.
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