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UTSC – Chemistry Lab Grignard Reaction Experiment


The grignard reaction is used to prepare
the grignard reagent which can then be used to make new carbon-carbon bonds. In this video we will be discussing how you can prepare the grignard reagent and the
various techniques involved. First collect the necessary apparatus as shown here. You will need a posture pipette, 2
graduated cylinders a mortar and pestle a 50 mil round bottom flask a condenser
drying tube and a wing boat. The reagents that you will need to obtain
will be dry ether magnesium turnings and the halogen compounds you will be
working with. Once all the glassware is collected
ensure that all glassware is properly cleaned before starting the experiment
as the grignard reaction is highly sensitive to impurities. It is recommended that you use a test tube brush when you’re cleaning the glasswear. Next, rinse all glassware with acetone after
washing. Ensure that there is absolutely no water remaining in or on the glasswear as the grignard reaction is very sensitive to moisture. It will not
proceed if it comes into contact with any amount of water no matter how small. Once you have rinsed the glasswear with
acetone invert them onto paper towels and set them aside to dry for roughly 10
minutes to remove any residual acetone which can cause unfavorable side
reactions. Refer to your lab manual for a
description of the side reactions that can occur if the preparation of the
grignard reaction is contaminated with water. While the glassware is drying set
up a hot plate on the bench outside the fumehood since the ether must be kept
away from heat sources. Heat a 200 mil beaker of water on the hot plate until it is warm. Do not heat until boiling. Use a wing
boat to obtain the required mass of magnesium turnings. Transfer the weighted out magnesium shavings into the mortar for grinding. Thoroughly grind the magnesium turnings
with the mortar and pestle. This is an important step as it exposes
more surface area of the magnesium for the reaction to take place. This is what your ground up magnesium
turning should resemble. Attach the 50 mil round bottom flask to the retort
stand and place the magnesium turnings inside. Here is a condenser. The bottom
pipe is connected to the tap and the upper pipe is connected to a ring clamp
which is placed on the sink. Ensure that there is a steady flow of water. Here is a
bottle of dry ether. This will be utilized as our solvent. To
ensure that the ether is dry look at the sodium ribbon and ensure that it shines.
To prepare a feeding solution add the required amount of dry ether and halogen
compounds separately using different pipettes within a graduated cylinder. Here is a stock solution along with the
feeding solution. Prepare the stock solution using the required amount of
dry ether and halogen compounds solutions. Ensure that you cork the
graduated cylinder so that the solution does not evaporate. Once you have both solutions prepared
transfer the stock solution into the round bottom flask and quickly attach
the condenser to it. At this point you should start the water flow. Here’s the experimental setup that will
be used for the preparation of the grignard reaction. The round bottom flask will be the site
of reaction the condenser will ensure that all the
wall tiles can be reused in the mixture and the drying tube has been employed
such that no moisture is incorporated in the reaction. While working on any
experiment please ensure that the fume hood is
placed at the maximum height of 18 inches. As per lab safety requirements a height
over 18 inches is restricted and it’s not considered a safe practice. The grinard
reaction is a self-sustaining reaction once it is initiated by a source of heat. Bear in mind that the heat source
must be subtle. The source should be warm enough to initiate the reaction but
also ensure that it does not produce any side reactions. Initiate the reaction by
using the heat of your palm. If you cannot start the reaction using
your palm you can employ a warm water bath to
initiate the reaction. This is a warmer source of heat and
provides a uniform distribution as well. The reaction mixture now appears to
bubble indicating the transfer of heat. The beaker is removed when the reaction is proceeding vigorously. See whether the reaction is self-sustaining or rather
the solution turns from colorless to grey. Once you remove the warm water bath and observe that the reaction is self-sustaining dry the bottom of the
flask using a paper napkin. The solution should appear colorless at this point.
Here is a bottle of iodine crystals. If you’re unable to initiate the
reaction using the previous techniques mentioned these crystals can be used to
initiate the reaction. The outing replaces the hay light from
our hay light compound and this facilitates the reaction to initiate. Obtain the crystals using a scupula so that the tip of the scapula is covered. Transfer the iodine crystals to the
reaction mixture. Immediately add the condenser to the round bottom flask and
try to initiate the reaction using your hands or a warm water bath. See how the
solution has turned brown. If the reaction could not be initiated
by using the techniques mentioned see your TA or course coordinator for
further help. Use the warm water bath to initiate the
reaction. After 15 to 20 seconds remove the water bath and see whether the
reaction is self-sustaining or not. If it is begin to add the feeding solution to
the reaction mixture. Once you remove the flask and observed that the reaction is
self-sustaining dry the bottom of the flask using a paper napkin. Use a clean and new posture pipette to
obtain the feeding solution. The feeding solution will be added
dropwise so you should fill the pipette only
slightly above the tip. To add the feeding solution into the reaction
mixture use either your left or right hand to remove the drying tube as shown.
Now using your other hand add the solution dropwise. The solution should be
added slowly so that the reaction proceeds in the desired manner. Once the feeding solution has been added immediately re-attach the drying tube to
the condenser using both hands. Use one hand to hold the condenser and the other
to fix the drying tube. Using both hands allows stability in inserting the drawing
tube. Keep on adding the solution dropwise until the feeding solution is over. Wait for the reaction to proceed for 10
to 15 minutes after that. At the end of the reaction the solution is expected to
appear milky white or gray. Disassemble the apparatus and cart the round bottom
flask and let it cool at room temperature for about five minutes

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