Grossmont Synthesis of Triphenylmethanol and the Trityl Carbocation Lab Report

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Synthesis of Triphenylmethanol and the Trityl Carbocation
Synthesis of Triphenylmethanol
and the Trityl Carbocation
Reactions of Carbonyl Compounds. Reactions of Organic Halides.
Preparation of Alcohols. Nucleophilic Addition. Organometallic
Compounds. Carbocations. Infrared Spectrometry.
Operations
OP-12
OP-7
OP-10
OP-11
OP-14
OP-15
OP-16
OP-17
OP-19
OP-24
OP-25
OP-26
OP-28
OP-33
OP-39
Excluding Water from Reaction Mixtures
Heating
Mixing
Addition of Reactants
Trapping Gases
Gravity Filtration (SS)
Vacuum Filtration
Centrifugation (µS)
Evaporation
Washing Liquids
Drying Liquids
Washing and Drying Solids
Recrystallization
Melting Point
Infrared Spectrometry (optional)
Before You Begin
1. Read the experiment and operation OP-12, read or review the other operations as necessary, and write an experimental plan.
2. Calculate the mass of 22.0 mmol (SS) or 4.00 mmol (µS) of magnesium,
the mass of 20.0 mmol (SS) or 3.80 mmol (µS) of benzophenone, the
mass and volume of 22.0 mmol (SS) or 4.00 mmol (µS) of bromobenzene, and the theoretical yield of triphenylmethanol. Calculate the theoretical yield of trityl fluoborate from 1.00 g (SS) or 0.100 g (µS) of
triphenylmethanol.
Scenario
The Complementary Colors Company manufactures synthetic dyes, including a number of triphenylmethane dyes. Gilda Lillie, product development
director for the company, would like to develop some new colors to
improve its market share in the dye industry. She has learned, for example,
that having two para-dimethylamino groups on two of the three benzene
rings in the parent structure yields a green dye (Malachite Green), while
having three of them yields a violet dye (Crystal Violet).
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EXPERIMENT
30
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NMe2
Me2N
C
+
Cl–
carbocation structure
NMe2
+
Me2N
C
Cl–
iminium ion structure
Two resonance structures for Malachite Green, a triphenylmethane dye
C
+
BF4–
triphenylmethyl (trityl)
fluoborate
The company’s technicians need to know how the kinds and positions of
substituents on the parent triphenylmethyl ring structure affect the color of
triphenylmethane dyes. For that, they need a sample of the unsubstituted
parent compound of these dyes, the triphenylmethyl (trityl) carbocation.
Your assignment is to prepare triphenylmethanol and convert it to trityl fluoborate, a salt that contains the trityl carbocation.
Applying Scientific Methodology
The main scientific problem—determining the color of trityl fluoborate—
will be solved when you prepare this substance.
The Colorful Career of the Triphenylmethanes
The synthesis of quinine was not accomplished until 1944—88 years after Perkin
had attempted it.
Reduced form of malachite green
NMe2
Me2N
C
H
Key Concept: When a substance cannot be represented satisfactorily by a
single structural formula, its actual
structure is regarded to be a composite
of two or more contributing structures.
During his Easter vacation from London’s Royal College of Chemistry,
18-year-old William Perkin was trying to synthesize quinine when he
came up with an unpromising red-brown solid that had none of the properties of quinine. Undeterred, Perkin used similar methods to synthesize
mauve, a light purple dye whose commercial success launched the synthetic
dye industry. The first triphenylmethane dye, fuchsin, was synthesized a
few years later, and it was followed soon after by Malachite Green, Crystal
Violet, and other triphenylmethane dyes. The race to develop commercially marketable dyes also stimulated research into the molecular basis
of color. Why, for example, is Malachite Green green and its reduced form
colorless?
The chromophore of a compound is the part of its molecule over which
electrons can be delocalized and which is responsible for its absorption of
ultraviolet or visible light. Triphenylmethane dyes come in all colors of the
rainbow, from the red of rosaniline through Malachite Green and Victoria
Blue to Crystal Violet. The chromophores responsible for these colors
appear to be nitrogen-substituted triphenylmethyl (trityl) cations, but
they are not true carbocations because most of their positive charge is
distributed to the nitrogen atoms, as in the iminium ion form of Malachite
Green shown previously. According to resonance theory, the iminium ion
and carbocation forms are contributing structures of a resonance hybrid that
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Synthesis of Triphenylmethanol and the Trityl Carbocation
has some characteristics of each. The electron delocalization suggested by
such structures is responsible for their colors. As a rule, compounds with extensive chromophores that allow electron delocalization over many atoms
tend to be colored; the longer the chromophore, the higher the wavelength of
light it absorbs. The reduced form of Malachite Green is not colored because
the saturated carbon that connects the rings prevents electron delocalization
over the three-ring system.
Although it is nearly a trillion times less stable than Crystal Violet, the
triphenylmethyl cation is unusually stable for a true carbocation. When
protected from atmospheric moisture, trityl salts will keep almost indefinitely. The cation owes this unusual stability to delocalization of the positive
charge about its three benzene rings. The cation is shaped somewhat like a
propeller with the “blades” (benzene rings) pitched at a 32° angle, because
steric interference between the ortho hydrogen atoms makes a planar configuration impossible.
Shape of triphenylmethyl cation
+
Understanding the Experiment
The year 1900 featured two milestones in organic chemistry: Moses Gomberg
announced his discovery of the trityl free radical (see Experiment 25), and
Victor Grignard reported his discovery of Grignard reagents. Just a year
later, trityl carbocations were being prepared from triphenylmethanol,
which is most easily synthesized using a Grignard reagent.
Grignard’s original procedure for preparing a Grignard reagent was as
follows: Approximately a mole of magnesium metal was placed in a dry,
two-necked, round-bottom flask fitted with a reflux condenser and addition
funnel. A mole of the organic halide was dissolved in diethyl ether, and
50 mL of this solution was added to the magnesium. When a white turbidity
appeared at the metal surface and effervescence began, more ether was
added in portions, with cooling, followed by drop-by-drop addition of the
remainder of the halide/ether solution. The reaction was brought to completion by heating under reflux in a hot-water bath.
Although extensive studies of the reaction have led to some modifications of the reaction conditions, essentially the same method is used today to
make most Grignard reagents. It is important that the reagents and apparatus be very dry because water not only reacts with Grignard reagents but
also inhibits their formation. In a study using butyl bromide, it was found
that the induction period (the time between the combination of reactants
and the start of a noticeable reaction) for forming the Grignard reagent was
7 12 minutes using sodium-dried diethyl ether, 20 minutes using commercial
absolute diethyl ether, and 2 hours using diethyl ether half saturated with
271
One resonance structure of the
triphenylmethyl cation
+
C
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Part II
Trituration of a solid involves rubbing
and grinding it, usually in the presence
of a solvent.
Preparation of carbocations using
fluoboric acid
ROH
Page 272
+ HBF4
+ BF4–
R+BF4– + H2O
ROH2+
Correlated Laboratory Experiments
water. It is apparent that careful drying of the reaction apparatus and
reagents saves time in the long run; it should increase the yield of the desired
product as well. An especially effective drying technique is to add the magnesium to the reaction flask and heat it (well away from any container of
diethyl ether!) in a soft, blue Bunsen burner flame for 2–3 minutes, then set
it on a cork ring or in a beaker to cool. If the bromobenzene/ether solution is
then added, and the magnesium pieces are rubbed with a stirring rod for
about 10 seconds, the reaction should start almost immediately.
The type and quantity of reagents and solvents used are also important.
When magnesium metal is exposed to air, it forms a thin film of magnesium
oxide on its surface. For the standard scale synthesis, you will crush dry magnesium turnings with a glass rod to remove some of the oxide film and provide a fresh surface for reaction. For the microscale synthesis, you will
scrape a length of magnesium ribbon and then cut it into small pieces. Commercial anhydrous diethyl ether is suitable for most routine preparations,
but the optimum quantity of ether depends on the kind of Grignard reagent
being prepared. One study showed that the highest yields of phenylmagnesium bromide were obtained with 5 moles of diethyl ether per mole of
bromobenzene.
In this experiment, you will prepare phenylmagnesium bromide by
adding a solution of bromobenzene in anhydrous diethyl ether to magnesium metal. The Grignard reaction can usually be started by rubbing or
crushing the magnesium with a stirring rod while cupping the reaction flask
in the palm of the hand to warm the ether. The onset of the reaction is signaled by the formation of small bubbles on the surface of the magnesium
accompanied by the appearance of a cloudy precipitate. You can sometimes
jump-start a balky Grignard reaction by adding a small amount of previously
prepared Grignard reagent to the reaction mixture; consult your instructor
for help.
Using a high concentration of bromobenzene helps to get the reaction
started, but it can promote the formation of an undesirable by-product,
biphenyl, through a side reaction on the metal’s surface. For this reason, the
bromobenzene solution is diluted by adding diethyl ether as soon as the
reaction gets under way. Phenylmagnesium bromide reacts rapidly with water
to form benzene and more slowly with oxygen to form a magnesium salt of
phenol. Therefore, the reaction apparatus must be protected from moisture,
and the Grignard reagent should be used shortly after it is prepared.
When benzophenone is added to the Grignard reagent, a magnesium
salt of triphenylmethanol precipitates from the reaction mixture, which
usually turns pink during the addition. This salt is converted to triphenylmethanol by water, and dilute hydrochloric acid is added to dissolve the
basic magnesium salts that form along with the product. The ether solution
containing triphenylmethanol is washed to remove impurities, and the
solvent is evaporated. The crude product is then triturated with hexanes or
petroleum ether to remove biphenyl, and purified by recrystallization.
Carbocations can be prepared by mixing alcohols with a strong acid
such as fluoboric acid (tetrafluoroboric acid); the fluoborate anion is a very
weak nucleophile that doesn’t react with the resulting carbocation. You will
prepare trityl fluoborate by the reaction of triphenylmethanol with 48%
fluoboric acid. The water in the aqueous fluoboric acid solution, as well as
that produced during the carbocation-forming reaction, could prevent or
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Synthesis of Triphenylmethanol and the Trityl Carbocation
reverse the reaction. Acetic anhydride is added to consume the water by the
following reaction:
O O
O
CH3COCCH3
+ H2O
2CH3COH
acetic
anhydride
acetic
acid
Of the chemicals used in part A, diethyl ether is not considered toxic to
aquatic organisms and does not persist for long in either air or water; bromobenzene is toxic to aquatic organisms and should not be released into
the environment; and benzophenone is harmful to aquatic organisms. In
water, acetic anhydride hydrolyzes to acetic acid, which readily breaks
down to form carbon dioxide and water. Little information is available
about the environmental effects of fluoboric acid, but it is likely to be toxic
to aquatic organisms.
Reactions and Properties
A
Br
+ Mg
ether
MgBr
phenylmagnesium bromide
bromobenzene
O
MgBr
+
C
COMgBr
benzophenone
COMgBr
+ H2O
H+
COH
+ Mg(OH)Br
triphenylmethanol
B
COH
+ HBF4
Ac2O
C+BF4–
trityl fluoborate
+ H2O
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Table 30.1 Physical properties
bromobenzene
magnesium
diethyl ether
benzophenone
triphenylmethanol
fluoboric acid (48%)
acetic anhydride
trityl fluoborate
mol wt
mp
157.0
24.3
74.1
182.2
260.3
87.8
102.1
330.1
-31
– 116
48
164
-73
bp
156
1.495
34.5
306
380
0.714
140
Note: mp and bp are in °C; density is in g/mL.
Bromobenzene
Benzophenone
Br
O
C
Figure 30.1 IR spectra of the starting materials
d
3064.8
1578.4
1474.3
1068.7
999.5
902.6
733.9
683.8
457.1
1659.8
1598.4
1447.3
1277.4
941.4
763.4
698.2
638.8
407.3
1.41
1.082
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275
DIRECTIONS
A. Preparation of Triphenylmethanol
If possible, clean and pre-dry the glassware needed for the reaction before
the lab period to reduce the drying time.
Bromobenzene causes eye and skin irritation, and inhalation, ingestion, or
skin absorption may be harmful. Avoid contact with the liquid, and do not
breathe its vapors.
Diethyl ether is extremely flammable and may be harmful if inhaled. Do
not breathe its vapors, and keep it away from flames and hot surfaces.
Magnesium can cause dangerous fires if ignited; keep it away from flames
and hot surfaces.
Petroleum ether is extremely flammable and can be harmful if inhaled or
absorbed through the skin. Avoid inhalation and prolonged contact, and
keep it away from flames and hot surfaces.
Safety Notes
2
2
4
0
0
3
bromobenzene
diethyl ether
1
2
0
Standard Scale
Reaction of Bromobenzene with Magnesium. It is essential that all
apparatus used during this reaction step be clean and scrupulously dried. Clean
the following items and dry them in a 110°C oven for at least 30 minutes:
100-mL round-bottom flask, Claisen adapter, West condenser, separatory–
addition funnel, glass stopper, thermometer adapter (remove the rubber
connector), drying tube filled with calcium chloride [OP-12], flat-bottomed
stirring rod, and 50-mL Erlenmeyer flask. Meanwhile, weigh 22.0 mmol of
clean, dry magnesium turnings, and leave them in the round-bottom flask
for the last 5 minutes of drying. As soon as the glassware is cool enough to
handle, assemble an apparatus for addition under reflux [OP-11], inserting the drying tube in the top of the reflux condenser. Weigh 22.0 mmol
of dry bromobenzene into the dried Erlenmeyer flask and dissolve it
in 5.0 mL of anhydrous diethyl ether. Then transfer this solution to the
separatory–addition funnel and stopper it, placing a strip of filter paper
between the stopper and the neck of the funnel. Add the bromobenzene
solution all at once to the reaction flask, and replace it in the addition funnel by 6.5 mL of anhydrous diethyl ether. Detach the flask momentarily,
hold it in the palm of your hand to warm the ether, and carefully (don’t
punch a hole in the flask!) crush and rub the magnesium turnings with the
flat end of your stirring rod for at least 30 seconds; then reattach the funnel
to the reaction apparatus.
Observe the reaction mixture closely for evidence of a reaction, such as
cloudiness and the evolution of bubbles from the magnesium surface. If the
reaction doesn’t begin within 5 minutes or so, detach the flask and rub the
magnesium turnings with your stirring rod as before; if the reaction still
doesn’t start, consult your instructor. When the reaction mixture begins to
boil quite vigorously without external heating, add the ether, drop by drop,
1
1
W
2
magnesium
Stop and Think: What might happen if the reaction apparatus isn’t
completely dry?
Alternatively, heat the flask containing
the magnesium pieces as described in
“Understanding the Experiment.”
Take Care! Keep diethyl ether away
from flames and hot surfaces.
Stop and Think: What is the purpose of the filter paper?
If the reaction starts but then stops, add
some more ether as described next.
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at a rate just sufficient to keep the reaction mixture boiling. When all of the
ether has been added, let the reaction continue until the boiling has nearly
stopped, and then use a heating mantle or a warm-water bath to heat the
reaction mixture under gentle reflux [OP-7] for another 10–15 minutes. The
reflux ring of condensing ether should be in the lower third of the condenser. If a significant amount of ether evaporates, thereby reducing its
volume in the reaction flask, replace it with fresh anhydrous ether. Don’t
stop at this point because the phenylmagnesium bromide solution will not
keep for long.
Stop and Think: What is the limiting reactant in this synthesis?
Observe and Note: What happens
during the addition?
Take Care! A gas may be evolved;
vent as necessary.
Waste Disposal: Unless your instructor directs otherwise, wash the
aqueous layers down the drain.
Place the recovered diethyl ether
and the filtrate in designated solvent
recovery containers.
Waste Disposal: Place the filtrate
in a designated solvent recovery
container.
Reaction of Phenylmagnesium Bromide with Benzophenone. Dissolve
20.0 mmol of benzophenone in 10 ml of anhydrous diethyl ether in a dry
Erlenmeyer flask, and place it in the separatory–addition funnel. When the
reaction mixture has cooled so that the ether is no longer boiling, add this
solution, drop by drop, to the reaction mixture with shaking or magnetic
stirring [OP-10]. The solution should be added fast enough to keep the
ether boiling gently without external heating. When the addition is complete, use a heating mantle or warm-water bath to heat the reaction mixture
under gentle reflux for another 15 minutes. (If you stop after the addition
and allow the reaction mixture to stand overnight or longer, this heating
period can be omitted.)
After the reaction mixture has cooled to room temperature, add 5.0 mL
of water, drop by drop, through the separatory–addition funnel while shaking or stirring, and then add 15 mL of 5% (1.4 M) hydrochloric acid. Wait
for the reaction to subside, and continue stirring or shaking until most or all
of the white solid has dissolved (some magnesium may remain undissolved). If any undissolved white solid remains, detach the reaction flask
from the apparatus and use a spatula to break up the solid; then shake the
flask, adding enough solvent-grade (not anhydrous) diethyl ether or 5%
HCl (or both) as needed to dissolve all of the solid. There should be at least
20 mL of ether in the reaction mixture at this time; if necessary, add solventgrade diethyl ether to replace any ether that evaporated.
Separation. If undissolved magnesium is present, remove it by gravity
filtration [OP-15] through glass wool, washing the magnesium and glass
wool with a small amount of solvent-grade diethyl ether. Transfer the reaction mixture to a separatory funnel, shake gently to mix the layers thoroughly, then drain and discard the aqueous layer. Carefully wash [OP-24]
the ether layer with 15 mL of aqueous 5% sodium bicarbonate. Then wash
it with 15 mL of saturated aqueous sodium chloride. Dry [OP-25] the ether
solution over anhydrous sodium sulfate or magnesium sulfate. Evaporate
[OP-19] the ether under vacuum, using a cold trap, to leave a solid residue.
Add 10 mL of hexanes (or high-boiling petroleum ether) to the solid
residue, and then use a spatula or flat-bottomed stirring rod to triturate
(see OP-26a) the solid in this solvent for 2–3 minutes. Collect the product
by vacuum filtration [OP-16], wash it [OP-26a] with the fresh solvent, and
let it air-dry on the filter.
Purification and Analysis. Recrystallize [OP-28] the crude triphenylmethanol from a 2:1 mixture of hexanes (or high-boiling petroleum ether)
with absolute ethanol. Triphenylmethanol crystals form slowly, so allow
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30 minutes or more for crystallization. Dry [OP-26b] the purified triphenylmethanol, weigh it, and measure its melting point [OP-33]. If requested,
obtain an infrared spectrum [OP-39] of the product.
Microscale
Reaction of Bromobenzene with Magnesium. It is essential that all apparatus used during this reaction step be clean and scrupulously dried. Clean
the following items, place them in a labeled beaker, and dry them in a 110°C
oven for at least 30 minutes: 10-mL round-bottom flask, Claisen adapter,
water-cooled condenser, drying tube containing calcium chloride [OP-12],
3-mL conical vial, 4-dram screw-cap vial, calibrated Pasteur pipet, 10-mL
graduated cylinder, glass stirring rod, flat-bladed microspatula, and forceps.
Keep the dried vials capped when not in use so they will stay dry. Also, obtain a clean, dry syringe and other parts (caps, septum) that you will need for
the apparatus; do not dry the septum or plastic syringe in the oven. Unless it
is already cut to length, cut a strip of magnesium ribbon to the length suggested by your instructor (usually ~ 5 cm). While holding it with a forceps,
scrape both sides of the ribbon thoroughly with a microspatula to remove
the oxide coating and reveal a shiny surface; avoid touching the ribbon with
your fingers. Cut it into pieces 1–2 mm in length, weigh about 4.00 mmol of
the magnesium pieces, and leave them in your 10-mL round-bottom flask
for the last 5 minutes of drying. As soon as the glassware is cool enough to
handle, assemble an apparatus for addition under reflux [OP-11], using a
water-cooled condenser and a clean, dry septum. Attach the drying tube
to the top of the condenser before you start running water through the
condenser.
Store about 6 mL of anhydrous diethyl ether in the dry 4-dram vial
for later use. Weigh about 4.00 mmol of dry bromobenzene into the 3-mL
conical vial, and dissolve it in 1.0 mL of anhydrous diethyl ether from
your storage vial. Use your syringe to add all of the bromobenzene solution to the reaction flask through the septum. Detach the flask momentarily, hold it in the palm of your hand to warm the ether, and carefully
(don’t punch a hole in the flask!) rub the magnesium pieces with the end
of your stirring rod for at least 30 seconds. Then reattach the flask to the
reaction apparatus.
Observe the reaction mixture closely for evidence of a reaction, such as
cloudiness and the evolution of bubbles from the magnesium surface. If the
reaction doesn’t begin within 5 minutes or so, detach the flask and rub the
magnesium with your stirring rod as before; if the reaction still doesn’t start,
consult your instructor. Transfer 2.0 mL of ether from your storage vial to
the conical vial that contained the bromobenzene. When the reaction mixture begins to boil quite vigorously without external heating, use your
syringe to add the ether, drop by drop, at a rate just sufficient to keep the
reaction mixture boiling. When all the ether has been added, let the reaction continue until the boiling has nearly stopped; then use an ~40°C water
bath to heat the reaction mixture under gentle reflux [OP-7] for another
10–15 minutes. The reflux ring of condensing ether should be in the lower
third of the condenser. If a significant amount of ether evaporates, thereby
reducing its volume in the reaction flask, replace it with fresh anhydrous
Stop and Think: What might happen if the reaction apparatus isn’t
completely dry?
Alternatively, heat the flask containing
the magnesium pieces as described in
“Understanding the Experiment.”
Take Care! Keep diethyl ether away
from flames and hot surfaces.
If the reaction starts but then stops, add
some more ether as described next.
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ether. Do not stop at this point because the phenylmagnesium bromide
solution will not keep for long.
Stop and Think: What is the limiting reactant in this synthesis?
Observe and Note: What happens
during the addition?
Take Care! A gas may be evolved;
vent as necessary.
Waste Disposal: Unless your instructor directs otherwise, wash the
aqueous layers down the drain.
Place any recovered diethyl ether
and the filtrate in designated solvent
recovery containers.
Waste Disposal: Place the filtrate
in a designated solvent recovery
container.
Reaction of Phenylmagnesium Bromide with Benzophenone. Dissolve
3.80 mmol of benzophenone in 2.0 ml of anhydrous diethyl ether in your dry
3-mL conical vial. When the reaction mixture has cooled so that the ether is
no longer boiling, remove the septum cap and drop a dry stir bar into the
reaction flask; then replace the cap. Start the stirrer and use your syringe to
add [OP-10] the benzophenone solution at a rate sufficient to keep the reaction mixture boiling gently without external heating. When the addition
is complete, use 0.5 mL of anhydrous diethyl ether to rinse the vial that contained the benzophenone solution, and add it to the reaction mixture. Use a
warm-water bath to heat the reaction mixture under gentle reflux for
another 10 minutes; it may become so thick that the stirrer stops moving, in
which case you can stop the stirring motor. (If you stop after the addition
and allow the reaction mixture to stand overnight or longer, this heating
period can be omitted.)
After the reaction mixture has cooled to room temperature, use your
syringe to add 1.0 mL of water, drop by drop, to the reaction flask, and wait
for any reaction to subside. Add 2.0 mL of 5% (1.4 M) hydrochloric acid
while stirring or shaking, then stir until the reaction subsides and most or all
of the white solid has dissolved (some magnesium may remain undissolved). If undissolved white solid remains, detach the reaction flask from
the apparatus and use a spatula to break up the solid; then shake the flask,
adding 1–2 mL of solvent-grade (not anhydrous) diethyl ether or 5% HCl
(or both) as needed to dissolve all of the solid. Transfer the reaction mixture
to a 15-mL centrifuge tube, using a small amount of solvent-grade ether for
the transfer; leave the stir bar and any undissolved magnesium behind.
Measure the vertical length of the ether column in the centrifuge tube and,
as necessary, add enough solvent-grade diethyl ether to make the column
about 4 cm deep. Then shake gently to mix the layers thoroughly. If an
emulsion forms at this or any other stage, try using a wooden applicator
stick to break it up and, if necessary, centrifuge [OP-17] the mixture for a
few minutes.
Separation. Remove and discard the aqueous layer, and cautiously wash
[OP-24] the ether layer with 3 mL of aqueous 5% sodium bicarbonate.
Then wash it with 3 mL of saturated aqueous sodium chloride. Dry [OP-25]
the ether solution using anhydrous sodium sulfate or magnesium sulfate.
Evaporate [OP-19] the ether to leave a solid residue. Add 2.0 mL of hexanes
(or high-boiling petroleum ether) to the solid, and use a flat-bladed
microspatula to triturate (see OP-26a) the solid in this solvent for 2–3 minutes.
Collect the product by vacuum filtration [OP-16], wash it [OP-26a] with the
fresh solvent, and let it air-dry on the filter.
Purification and Analysis. Recrystallize [OP-28] the crude triphenylmethanol from a 2:1 mixture of hexanes (or high-boiling petroleum ether)
with absolute ethanol. Triphenylmethanol crystals form slowly, so allow
30 minutes or more for complete crystallization. Dry [OP-26b] the purified
triphenylmethanol, weigh it, measure its melting point [OP-33], and turn it
in to your instructor. If requested, obtain an infrared spectrum [OP-39] of
the product.
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B. Preparation of Trityl Fluoborate
Acetic anhydride can cause severe damage to skin and eyes, its vapors are
very harmful if inhaled, and it reacts violently with water. Use gloves and
a hood; avoid contact with the liquid, do not breathe its vapors, and keep
it away from water.
Fluoboric acid is poisonous and corrosive, its solutions can cause severe
damage to skin and eyes, and its vapors irritate the respiratory system.
Use gloves and a hood; avoid contact with the acid solution, and do not
breathe its vapors.
Safety Notes
2
3
3
W
2
acetic anhydride
3
fluoboric
acid
Standard Scale
Under the hood, mix about 1.00 g of triphenylmethanol with 7.0 mL of
acetic anhydride in a small, dry Erlenmeyer flask. Carefully add 1.0 mL
of 48% fluoboric acid, and swirl to dissolve the solid. Stopper the flask and
let the mixture stand for about 15 minutes; then cool it in ice until crystallization is complete. Collect the product by vacuum filtration [OP-16] on a
Hirsch funnel, wash it [OP-26a] with cold anhydrous diethyl ether, and airdry it on the filter. Weigh the dry trityl fluoborate in a dry tared vial.
Take Care! Wear gloves, avoid
contact with acetic anhydride and
fluoboric acid, and do not breathe
their vapors.
Observe and Note: What color is
the product?
Waste Disposal: Place the filtrate
in a designated waste container.
Microscale
Under the hood, mix 0.100 g of triphenylmethanol with 0.70 mL of acetic
anhydride in a small, dry test tube. Carefully add 0.10 mL of 48% fluoboric
acid, and swirl to dissolve the solid. Stopper the test tube and let the mixture stand for about 15 minutes; then cool it in ice until crystallization
is complete. Collect the product by vacuum filtration [OP-16], wash it
[OP-26a] with cold anhydrous diethyl ether, and air-dry it on the filter.
Weigh the trityl fluoborate in a dry tared vial.
Exercises
1. (a) Write a balanced equation for the coupling reaction of bromobenzene on the metal surface to form biphenyl. (b) Write balanced equations for the reactions of phenylmagnesium bromide and trityl
fluoborate with water.
2. If you obtained an IR spectrum of triphenylmethanol, compare it with
the spectra in Figure 30.1 and describe the evidence indicating that the
expected reaction has taken place. Interpret your spectrum as completely as you can.
3. Describe and explain the possible effect on your results of the following
experimental errors or variations. (a) You used solvent-grade (not anhydrous) diethyl ether for the reaction in part A. (b) After adding the
bromobenzene solution in part A, you forgot to add anhydrous diethyl
ether to the reaction mixture. (c) You used diethyl ether, rather than
petroleum ether, to remove biphenyl from the crude triphenylmethanol.
Take Care! Wear gloves, avoid
contact with acetic acid and fluoboric acid, and do not breathe their
vapors.
Observe and Note: What color is
the product?
Waste Disposal: Place the filtrate
in a designated waste container.
M31_LEHM3752_02_SE_C30.QXD
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280
2:13 PM
Page 280
Part II
O
COCH2CH3
ethyl benzoate
Correlated Laboratory Experiments
4. (a) Calculate the atom economy and reaction efficiency of your synthesis. (b) Describe some green features of your synthesis, and any that
aren’t so green.
5. Following the format in Appendix V, construct a flow diagram for the
synthesis of triphenylmethanol (part A).
6. The reaction of phenylmagnesium bromide with benzophenone to
form the salt of triphenylmethanol is an example of nucleophilic addition; its reaction with ethyl benzoate to yield the same product involves
nucleophilic substitution followed by a nucleophilic addition step.
Write reasonable mechanisms for both reactions.
7. Outline a synthetic pathway for preparing each of the following
compounds, using the Grignard reaction and starting with benzene
or toluene: (a) 1,1-diphenylethanol; (b) 1,2-diphenylethanol; (c)
2,2-diphenylethanol; (d) 2,3-diphenyl-2-butanol.
8. Excluding alternative Kekulé structures for the benzene rings, (a) draw
all possible resonance structures for the trityl cation; (b) draw all possible resonance structures for Malachite Green.
9. One possible by-product from the triphenylmethanol synthesis is ethoxytriphenylmethane. Tell how and when it might form, and give an equation
and a mechanism for the reaction.
Other Things You Can Do
(Starred items require your instructor’s permission.)
*1. Record the ultraviolet–visible spectrum (200–600 nm) of trityl fluoborate in dry acetone.
*2. Dissolve a small amount of trityl fluoborate in dry methanol, and
record your observations. Dissolve about 0.1 g of trityl fluoborate in
1 mL of dry acetone; then add a solution of sodium iodide in dry acetone (0.1 g NaI in 1 mL acetone), drop by drop, until no more changes
are observed. Write balanced equations to explain your observations.
*3. Prepare the fluorescent dye fluorescein as described in Minilab 26.
4. Write a research paper about the structures, properties, and applications
of Grignard reagents, starting with sources listed in the Bibliography.
Experiment 30: Synthesis of Triphenylmethanol (from Lehman text, 2nd edition)
Read the entire lab experiment from the Lehman text (pages 269 – 280) to obtain a more
complete understanding of Experiment 30 (Note: The full experiment is provided on Canvas).
The experimental data and results below are based on the Standard Scale procedure for Part A
(Preparation of Triphenylmethanol).
Data for Procedure:
Reagent masses used:
Magnesium: 0.54 g
bromobenzene: 3.46 g
benzophenone: 3.64 g
Results:
Final mass of triphenylmethanol: 4.16 g
Melting point range for triphenylmethanol: 162.8 – 164.1 ºC
The IR spectrum for triphenylmethanol is provided on Canvas.
Lab Report Instructions:
1. Title Page (This section should contain the experiment title, your name, and chemistry course
number)
2. Analysis:
•
Calculate the theoretical yield and % yield of triphenylmethanol using the data and
results given above (Show calculations).
•
Provide a table with important IR peaks for your product. The table should have two
columns labeled bond type and wavenumber (cm-1). Refer to the IR spectrum for
triphenylmethanol that is provided on Canvas.
•
Provide answers to Exercises 1a, 2, 3 and 6 from Experiment 30 in the Lehman text (For
the mechanisms in Exercise 6 you can draw them by hand or use a chemistry drawing
software program).
*Note: Your report should be uploaded to Canvas in pdf or Microsoft Word format.