Here's some information for this week's experiment.
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Grignard Synthesis of Benzoic Acid (Expt. 25B, Pavia, et al)
Supplementary Information
This experiment requires you to pay very careful attention to
experimental detail. It is imperative that you read the entire protocol
and mentally rehearse the procedures you will carry out. Much of the
experiment must be conducted under anhydrous conditions using oven-dried
equipment. It is important that you plan ahead to orchestrate efficient
use of the equipment provided. Here are a few tips and highlights of the
procedure, more or less in the order in which they will be important to
you. This is not a step by step description of the procedure, however.
Use this information together with your lab text.
Read the procedure thoroughly and make a list of all the equipment that
must be dried in a lab oven.
If any of the glassware that must be dry is visibly wet when you receive
it, rinse it with acetone before placing it in the oven. Put the acetone
rinsings in the non-halogenated waste container.
Put all of the equipment that must be dry inside the 250 mL beaker
provided and place it in the oven for 20-30 minutes. If you are provided
a glass syringe (vs. plastic) for use during the experiment, dry it in the
oven with the plunger removed from the barrel. Do not place plastic
syringes in the oven. Label the beaker with your name.
You will be provided one 5-mL conical vial for the reaction, one
approximately 5 mL ordinary vial for storage of anhydrous ether, and one
approximately 3-mL ordinary vial for storage of the bromobenzene solution.
These vials must be dried in the oven along with the other equipment. The
two ordinary vials can be used later for storage of the reaction mixture
at the specified stopping point and for submission of the sample for
grading.
When you remove the equipment from the oven (after 20-30 minutes) assemble
the various items as soon as they are cool enough to handle (cap the
vials, assemble the syringe, build the reaction apparatus). Be sure to
put the drying tube on the reaction setup.
The most important factor contributing to a successful experiment is
extreme attention to exclusion of moisture from all sources (until the
hydrolysis step).
Examples of Pasteur pipets calibrated for 0.5 and 1.0 mL delivery volumes
will be displayed by the stockroom window. You can use these as guides to
prepare your own calibrated pipets.
When scraping and cutting the magnesium use tweezers to hold the strips.
Do not touch the magnesium with your fingers, due to the moisture on your
skin.
If you pay careful attention to the instructions and tips provided in the
published procedure, the experiment should go well for you. However, if
your Grignard reaction is slow to start, the most effective aid for
starting it is to gently grind the magnesium pieces while they are inside
the reaction apparatus, keeping them submerged in the ether and
bromobenzene solution. To do this, momentarily open the reaction vial and
grind the magnesium with an oven-dried glass stirring rod. BE CAREFUL to
not break the stirring rod in your hand.
If the volume of the reaction mixture decreases during the formation of
the Grignard reagent (by evaporation of ether during the heating/refluxing
of the mixture), add additional anhydrous ether to replace the quantity
lost, keeping the reaction volume no lower than the 1 mL mark on the
reaction vial.
If your Grignard reaction develops a white precipitate near the beginning
this is a bad sign, indicating moisture may have been introduced somehow.
A grayish green or brown mixture (with little or no precipitate) is the
desired appearance of the Grignard reaction. If the white precipitate is
substantial it may be necessary to start the experiment over.
Take care handling the dry ice needed for reaction with the
phenylmagnesium bromide. Contact with the skin can result in tissue
damage. Minimize the time the dry ice is exposed to the air after
crushing and before adding the Grignard reagent, since it will rapidly
condense moisture on its surface.
The first place to stop during the reaction is after the hydrolysis step
in the middle of page 275. If you take a break here, DO NOT store your
product in a conical reaction vial (we need to clean these and turn them
around for use by other students). Place your hydrolysis mixture in an
ordinary 5-mL screw cap vial and label it for storage. The vial does not
have to be dry for storage (since you've just added an aqueous solution
for the hydrolysis step).
Once you have the crude product, recrystallize it using a Craig tube, as
directed in the text.
Omit the spectroscopy section.
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Guess that's it. I'll provide a hard copy of this in class.
CBF
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Craig B. Fryhle, Ph.D. Office 206-535-8314 FAX 206-536-5055
Associate Professor Email fryhle@u.washington.edu
Department of Chemistry URL http://rainier.chem.plu.edu/fryhle.html
Pacific Lutheran University
Tacoma, Washington 98447 ^ ^ ^ ^ ^ ^
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