Preparation of Triphenylmethanol
Grignard reagents are highly reactive organometallic reagents generated by
treating alkyl or aryl halides with magnesium metal in the presence of an anhydrous
The reaction proceeds like a strong base reaction.
R'-X + Mg [ R'-X ]- + Mg+ R' - MgX
This reagent was discovered by Victor Grignard around
A Grignard reagent is capable of reacting with many different substrates such as
carbonyl compounds, alkyl halides and other organometallic reagents.
The addition of Grignard reagents to carbonyl compounds such as aldehydes,
ketones, and esters yields the corresponding secondary and tertiary alcohols.
The Grignard reaction results in the formation of a carbon-carbon bond.
Grignard reagents are extremely strong bases that can react violently with hydroxylic
compounds such as water or alcohols:
In the laboratory, initiation of the Grignard reaction is sometimes very slow. Because
of the presence of air, a coating of magnesium oxide forms on the magnesium metal
To initiate the reaction, adding a small crystal of iodine or a drop of dibromoethane
can help to clean the surface of the magnesium.
Today's experiment is preparation of triphenylmethanol using a Grignard reagent.
MgBr + C OCH3 COMgBr
Br MgBr . 2Et2O
MgBr . 2Et2O + H3CO C H3CO C
C O MgBr MgBr . 2Et2O + BrMgOCH3
- + H3 O +
C O MgBr C O H
Water sensitive chemical reactions require:
1. thoroughly dried glassware (no visible water!)
2. rigorously dried, anhydrous grade reagents
3. heat gun dried glassware (eliminate invisible water!)
4. a drying tube packed with a drying agent (CaCl2)
5. a nitrogen or argon line supplying a dry, inert atmosphere (not today)
NO washing glassware with water – today! Use acetone if necessary.
Et2O (diethyl ether) is extremely flammable!!!!
Adequately grease all joints to avoid leakage of fumes.
Use only fresh (new) anhydrous diethyl ether.
The boiling point of diethyl ether is 35 ºC so DO NOT set max setting for the
temperature controller. USE 40 ºC water bath set on top of heating mantle.
Also, dry your 10 mL graduated cylinder and 100 mL Erlenmeyer flask with
the heat gun before use and tightly cover with aluminum foil.
Remove the magnesium oxide
from magnesium metal surface
using mortar and pestle.
No boiling chips with the Mg.
Assemble as shown on right, except need
to add drying tubes. Your TA will show you
how to assemble the drying tubes with CaCl2
as the drying agent.
USE WARM WATER BATH 40 ºC as heat
source, using the large beaker. Place the
bath on wire gauze on top of the heating Drying columns
mantle. Monitor the temperature during the are to be placed
at the opening
of the addition
DO NOT use Aluminum Foil around the funnel and the
Remove the heat source when you
open the reflux set up to the air.
Do not use the hot plate!!!
1. Grignard Reagent Preparation:
Dry the entire assembly from the A water-cooled
bottom to top with your heat gun.
ether (Et2O) vapors
Put drying tubes in place while the from escaping
equipment is hot. (AFTER HEATING)
TA provides 9 mL bromobenzene.
Then add bromobenzene and 50 mL
Et2O into the additional funnel.
Add just enough of the solution of
bromobenzene in ether to cover Mg then
let the solution sit for 5 min to initiate
reaction. Warm with hands. 100 mL RB flask
containing 2 grams
When the reaction begins (cloudy or
grayish), you will see bubbles in solution.
If no reaction begins after 5 minutes, go
get your TA.
1. Grignard Reagent Preparation:
If the reaction doesn’t start: TA adds 3-5 drops
dibromoethane or a couple of crystals I2.
Continue to monitor the reaction.
Once reaction begins, add the remaining solution
of bromobenzene in Et2O (NOT too slow and NOT
all at once) by steady dripping.
Maintain a gentle reflux for 30 minutes – the
applied heat will expedite the reaction.
Monitor the 40 ºC warm
water bath to avoid
overheating the system.
2. Triphenylmethanol Preparation:
Cool RMgX solution in ice bath for 5 min before
going to the next step.
You will use the existing set-up for the next step.
DO NOT EXPOSE SOLUTION TO AIR!!!
Add 5 g methyl benzoate and 15 mL Et2O into the
Slowly add from the funnel to the cooled solution.
Reflux for 30 min with 40 ºC warm water bath.
Prepare 25 grams crushed ice and 50 mL 10% H2SO4
then add them into your E.flask.
When you finish refluxing the reaction mixture for 30
min, pour it into that E.flask.
(O - is protonated and the bases are neutralized.)
4. Work-up of the Reaction:
Stir or swirl to dissolve lumps in your E.flask – you may use a stirring rod to gently
break up lumps.
Transfer the solution to a separatory funnel, then drain the aqueous layer
through the stopcock.
Keep the top layer (ether layer)
Might see fine layer between
top and bottom phases so it
should be removed through
stopcock (may be metal bits) Aqueous layer- Discard
Wash ether layer with saturated NaCl (1/3 volume of ether layer).
Dry ether layer over Na2SO4 then filter through cotton plug in glass funnel into
preweighed round bottomed flask.
Remove ether using rotary evaporation. Use house vacuum to remove last ether
SAVE small sample of crude product for TLC.
4. Work-up of the Reaction (Week 2):
Triturate residue in distilling flask with cold petroleum ether (not diethyl ether).
…to reduce to fine particles or powder by rubbing, grinding, bruising, or
Decant petroleum ether (SAVE small sample of ether for TLC).
Recrystallize from isopropyl alcohol to obtain the pure product.
5. Characterization of Your Product:
Perform TLC on the crude product, petroleum ether extract, and pure product
by using diethyl ether to dissolve all solids and using petroleum ether as your eluent.
Measure the mass and mp, and calculate % yield.
Grading is 50% for write up and 50% for results.