Grignard Reaction - Triphenylmethanol

Last update: 22 August 2005

Dr. Glenn Nomura

Reference:
Williamson, Macroscale and Microscale Organic Experiments, 4th edition, Houghton Mifflin: New York, 2003,
Experiment 38.4, (1/2 X) macroscale Phenylmagnesium Bromide Grignard reagent, p. 472
Experiment 38.5, (1/2 X) macroscale Triphenylmethanol from Methyl Benzoate, p. 474

Equations:

Br MgBr

1. Mg anhyd. Et2O

bromobenzene
Mg, 24.31 phenylmagnesium bromide
MW 157.02 (used directly in next step)
bp 156oC
d 1.491

MgBr
O H3O
2. 2 C OCH3 Ph3COMgBr (C6H5)3COH

triphenylmethanol
methylbenzoate
MW 260.36
MW 136.15
mp 164.2oC
bp 198-199oC

Materials:
1. bromobenzene, C6H5Br [108-86-1], 4.5 mL (6.71 g, 0.0427 mol), d 1.491
2. Mg, [7439-95-4], 1.0 g (0.0411 g-atom)
3. methylbenzoate, C6H5COOCH3 [93-58-3], 2.5 mL (2.72 g, 0.0200 mol), d 1.088
4. diethyl ether, Et2O, anhydrous, (CH3CH2)2O [60-29-7], FW 74.12, bp 34.6C, d 0.706
5. diethyl ether, Et2O, general grade (non-anhydrous) for extraction work up
6. ligroin [8032-32-4], bp 60 -80C, d 0.656, 15 mL
7. 10% H2SO4, 25 mL

Procedure:
Preparation of the Phenylmagnesium Bromide (phenyl Grignard reagent)
This reaction should be performed in the hood, and all precautions should be taken to perform the reaction
under dry conditions. Into a clean dry (ungreased) 50 mL round bottom flask is placed 1.0 g (0.0411 g-atom) of
magnesium turnings. If convenient, the flask and Mg can be oven dried together for 30 minutes or longer at around
125°C. Refer to Fig. 38.6 on p 473 of Williamson 4/e. The 50 mL round bottom flask is removed from the oven
and while still hot is connected to a greased clean dry condenser with a cotton ball placed at the top joint to reduce
moisture exchange as the glassware cools, or alternatively attach a drying tube filled with fresh Drierite or CaCl 2
using an adapter from the Williamson kit (Note A). *2nd Reminder: grease all joints thoroughly to prevent
seizing of joints and install Keck clips on every joint.

Addition of Phenyl Magnesium Bromide to Methyl Benzoate.

Prepare a cold-water bath (OK to share one bath per hood; icy cold not necessary). Clamp the flask and
condenser assembly by the body of the condenser (not by the flask joint). Allow for space beneath the flask for
manually raising and lowering the cold water bath. The instructor will next add about 10 mL of dry Et2O and 4.5
Exp 29.5 Alkylation of m-Xylene 2
mL (or 6.71 g) of bromobenzene. Since the Grignard reaction normally will not start on its own, the instructor will
come to your hood to press on the Mg turnings with a flattened glass rod. A white turbidity will develop as the
Grignard reagent first begins to form. The exothermic formation of Grignard will cause the Et2O to reflux rapidly.
Allow the reflux to continue at a rapid boil until the Mg has nearly disappeared. The reaction solution will darken to
amber as the concentration of PhMgBr builds. Cool the flask with the cold bath intermittently for 1-2 seconds at a
time to control the reflux rate. As the auto refluxing begins to slow down, heat the flask with a hot water bath drawn
from the sink for another 5-10 minutes. At this point install a clean dry addition funnel at the top of the condenser.
Either you or the instructor should add 2.5 mL (ca. 2.5 g) of methyl benzoate and 10 mL of dry Et2O. Agitate the
solution to mix the contents. Open the stopcock and drip about ¼ the volume of methyl benzoate solution into the
reaction with cooling from a cold bath. The methyl benzoate and PhMgBr react within seconds, causing the Et2O to
reflux. Drip the remaining solution in at a rapid rate with continued cooling. Note the color transitions of the
reaction as methyl benzoate is added. Heat the reaction for an additional 5-10 minutes from a warm water bath.

Work Up
Quenching the Reaction: Into a large beaker, add ca 15 g (ca. 30 mL) of crushed ice and 25 mL of 10%
H2SO4. Detach the reaction flask from the condenser. Use a glass rod and swirling motions to loosen the thick
white Mg salt complex. While the white emulsion is still fluid, pour the entire contents of the flask in a single
motion into the H2SO4 and ice mixture. Complete the transfer by pouring some of the H2SO4 and Et2O back to the
flask with swirling. The H2SO4 hydrolyzes the white Mg alkoxide product complex and any remaining unreacted
Mg metal turnings within about 10 minutes.

* [this is a potential stopping point for continuation the next lab period; the instructor will advise the class]

Separatory Funnel Extraction: If Et2O has evaporated from the reaction at this point or upon standing
between lab periods, add more Et2O back (visual estimate OK). Transfer everything quantitatively to a separatory
funnel using small portions of additional Et2O to complete the transfer. Shake the separatory funnel carefully with
frequent venting of gases especially at the early stage of shaking (instructor will demonstrate). Draw off the
aqueous layer into a beaker and extract the Et2O with a second portion of dilute H2SO4 and draw off the aqueous
layer. Shake the Et2O with saturated NaCl (ca. 1/3 the volume of Et2O layer) to remove some of the water dissolved
in the Et2O. Pour the Et2O out the top of the separatory funnel (Note B) into a clean dry 125 mL Erlenmeyer flask.
Crystallization by Concentrating the Solution: Add dessicating agent provided in lab, such as, CaCl2, or
Na2SO4, or MgSO4, (ca. 1/5 the volume of the Et2O layer, Note C). Transfer the Et2O to a tared Erlenmeyer flask
(Note D). Add 15 mL of ligroin to the Et2O, cover the Erlenmeyer flask loosely with a piece of foil, add a glass rod
rather than a boiling chip for ebullation, and warm the flask gently on a hot plate to concentrate the solution. Be
careful to monitor the setting of the hot plate to avoid splattering. The Et2O (bp 34C) distills at a faster rate than
ligroin (bp 60 - 80C). As the solution becomes more concentrated in ligroin, the triphenylmethanol product, which
is less soluble in ligroin, will begin to separate. If the solution becomes cloudy (“cloud point”) then remove the
flask from the hot plate and set it on a paper towel on your bench with an inverted beaker covering it to protect it
from drafts. If no cloud point develops, then remove the flask from the hot plate when the solution has reduced to
ca. 20 mL (visual estimate) (Note E).

Isolation Steps
Buchner Funnel Filtration: You may perform this vacuum filtration in the hood or on the open bench
(although hood is preferred due to the organic vapors). *Remember to clamp the filter flask. Use the correct
sized paper for your Buchner funnel and wet the paper with ligroin just before pouring the crystal mixture through.
Also stir the crystals with a glass rod to create a suspension prior to pouring the crystal mixture through the Buchner
funnel. Dry the crystals thoroughly and determine the % yield. Finally, determine the melting point of your product
and compare the result to the literature value. Submit the product in a labeled vial.

Summary of Final Steps:

1. Weigh the triphenylmethanol and determine the yield
2. Determine the mp of the recrystallized triphenylmethanol product.
3. Submit the product in a vial labeled with your name and experiment name “Grignard”.
Exp 29.5 Alkylation of m-Xylene 3

Notes:
A. Fresh indicating Drierite is anhydrous CaSO4 impregnated with a blue moisture indicator
that turns pink upon absorbing moisture. The instructor can show the class how the drying
tube can be connected to the pre-assembled hot glassware so that dry air enters the
glassware during cooling.
B. The Et2O is poured out the top of the separatory funnel to minimize collection of water
droplets together with the Et2O. Residual water droplets exist on the inside walls of the
separatory funnel and Teflon valve area. You should first dry the ground glass joint of the
separatory funnel with a paper towel before pouring out the top.
C. As always, cover the mouth of the Erlenmeyer flask with aluminum foil or Parafilm when
drying. An open vessel reabsorbs moisture on standing.
D. If the Et2O is cloudy, this would likely be due to fine particles of dissicant in suspension, in
which case the Et2O should be filtered. If the Et2O is clear, then carefully decant the Et2O
leaving the dessicant behind; usually the dessicant is clumped from moisture absorption at the
bottom of the glassware and is therefore easy to separate. Wash the dessicant with a small
portion of Et2O to complete the transfer.
E. Crystallization techniques are an art. Generally, the best crystal quality is obtained when the
hot solution is allowed to cool slowly as opposed to immersing the hot flask in an ice bath
right away, which would result in a powdery separation “crashing out” of product. You may
encounter supersaturation of your solution upon standing. In this case, the flask may cool
without formation of crystals. By gently immersing a glass rod into the solution and
scratching the glass surfaces, you can induce nucleation (the instructor will help you with
this if you wish).

Waste & Clean Up:

Into Mr. White: All aqueous washings; organic residues; and acetone rinses of dirty equipment

Prep Room Instructions:

For a class of 24 students

1. bromobenzene 150 mL
2. methyl benzoate 50 mL
3. magnesium turnings 24 g
4. Et2O, anhydrous 1 L (fresh bottle each class for ensured dryness of solvent)
5. Et2O, general solvent 1L
6. Drierite or CaCl2, anhy. general supply; used for drying tubes
7. sat. NaCl (brine solution) 1L
8. dessicant 1st choice: Na2SO4; 2nd choice CaCl2 or MgSO4
9. boiling chips bottle
10. foil roll
11. cotton small quantity for covering joint openings
12. filter papers 42 mm & 5.6 cm sizes for Buchner funnels; plus larger sizes, eg. 12
cm