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