Q) Following the procedure described in the handout, a student obtained 3.5 g of
ID: 481917 • Letter: Q
Question
Q) Following the procedure described in the handout, a student obtained 3.5 g of triphenylmethanol. What is the limiting reagent in this procedure? Calculate the theoretical yield (in moles and in grams) and the %yield for this student’s synthesis
prosedures:
1. All glassware, including a 10mL and a 25 or 50mL graduated cylinder and a glass stirring rod, should be clean and should have been dried overnight in the drying oven. Remove any plastic or rubber parts from the apparatus before placing it in the oven. Set up a reflux apparatus as follows: attach a Claisen adapter to the neck of a 100mL round bottom flask. To the curved neck of the adapter attach a reflux condenser and to the straight neck attach an addition funnel. Fit a CaCl2 drying tube to the end of the condenser.
Why must the apparatus be rigorously dry?
All ground glass joints should be lightly greased.
Leave room under the flask to fit an ice-water bath later.
2. Obtain 6.7mL of bromobenzene and 2.5g of magnesium. Grind the magnesium in a mortar and pestle. Add the Mg to the flask along with 20mL of anhydrous ether. Remove the addition funnel and, using a pipet, add approximately 2mL of the bromobenzene below the surface of the ether to obtain a high concentration of the bromobenzene close to the surface of the magnesium. Replace the addition funnel and warm the solution with warm water or the palm of your hand. Formation of the Grignard reagent is evidenced by bubbling and a cloudy appearance to the reaction mixture.
NOTE: DO NOT ADD THE BROMOBENZENE ALL AT ONCE. NO SECOND CHANCES!!!!!!
Record masses to 0.1g.
Record the volume of bromobenzene to 0.1mL.
Formation of the Grignard reagent takes place at the surface of the Mg. What is the purpose of grinding the Mg?
If after 5-10 minutes of warming and swirling the flask, you have difficulty obtaining a vigorous reaction, inform your instructor or TA.
Why should the bromobenzene be added dropwise, a little at a time?
3. When the reaction mixture is vigorously refluxing, turn on the water to the condenser. Mix the remaining bromobenzene and 20-25mL of anhydrous ether and add the solution to the addition funnel. Add the bromo-benzene solution to the reaction mixture dropwise at a rate to maintain a steady reflux. Swirl the flask occasionally.
How does the reaction mixture reflux without the application of heat?
4. When all the bromobenzene has been added, heat the reaction mixture as needed to maintain a gentle reflux for 20 minutes. Turn on the heating mantle and set it to a very low voltage.
To do this, turn it up to 30-40 then
back it down to about 10 or to the point where the lamp just stays lit.
When the reaction no longer boils very much on its own, even after swirling, carefully warm the flask by lowering it, not right into the mantle but above it just to the point where the ether boils gently. Remember that ether boils at 35° and if you lower the flask too close to the heat, the ether may boil violently and evaporate. Some Mg will remain.
While the reaction mixture is refluxing, obtain the reagents for the next step.
5. Mix 4mL of methyl benzoate with 12mL of anhydrous ether and add the solution to the addition funnel. After the reflux period, cool the reaction flask briefly in an ice-water bath. Add the ester solution dropwise with swirling of the flask to insure mixing. Add the ester at a rate that maintains a gentle reflux. If necessary, cool the flask in the ice-water bath to control the reaction. When the addition is complete, reflux the reaction mixture for 30 minutes.
Record the volume of methyl benzoate to 0.1mL.
You should record some physical changes at this point.
If the reaction mixture is left until the next lab period, the round bottom flask should be stoppered with a well-greased glass stopper and placed in a beaker so that it will not tip over.
6. Add about 15g of ice and 25mL of 10% sulfuric acid to a 125mL Erlenmeyer flask. Pour the reaction mixture over the ice/acid mixture. This is a messy step. Use a funnel. It is helpful when pouring the reaction mixture, which contains solid, to swirl and suspend as much solid as possible so that the solid is transferred to the Erlenmeyer during the pouring. Eventually all the solid will react and dissolve so that finally there will be two liquid layers (ether and aqueous) and NO solid in the Erlenmeyer or RB flasks (except a little Mg). Rinse the reaction flask with a small amount of 10% sulfuric acid followed by a few mL of ordinary ether (not anhydrous) and add these rinses to the Erlenmeyer flask. Write your name clearly on the Erlenmeyer flask and set it into the
hood, uncorked, but set next to a stopper or cork that fits the Erlenmeyer. We will stopper it later, after the reaction has clearly ceased.
Why is ice used?
What reaction is occurring at this step?
Why is only “ordinary” ether
called for in this step?
Any solid left behind in the RB
flask represents lost product
and will result in a decreased
yield.
Week 2
7. Transfer the contents of the flask to a separatory funnel, decanting or using a pipet so that unreacted magnesium is left behind. Separate the layers. Wash the ether layer with 10% sulfuric acid, then with a saturated sodium chloride solution. Unreacted Mg should be placed in the “solids” container in the hood.
What is the purpose of the saturated sodium chloride solution?
8. Dry the ether layer over magnesium sulfate. Filter the solution through a cotton plug to remove the drying agent. Put the cotton and the recovered drying agent in the “solids” container. Add 15mL of high-boiling petroleum ether and evaporate the solution slowly on a steam bath in the hood until crystallization of triphenylmethanol begins.
What is the difference between ether and petroleum ether?
The sample MUST be dry. Be patient. If the liquid appears cloudy, it is NOT DRY.
9. Cool in an ice-water bath to complete the crystallization. Collect the product by suction filtration and dry and weigh the solid. Pour the filtrate into a waste container in the hood. Calculate the %yield. Obtain the melting point and the infrared spectrum of the product.
Record masses to 0.1g.
Remember to record the melting point as a range.
Obtain the IR spectrum from a KBr pellet.
prosedures:
1. All glassware, including a 10mL and a 25 or 50mL graduated cylinder and a glass stirring rod, should be clean and should have been dried overnight in the drying oven. Remove any plastic or rubber parts from the apparatus before placing it in the oven. Set up a reflux apparatus as follows: attach a Claisen adapter to the neck of a 100mL round bottom flask. To the curved neck of the adapter attach a reflux condenser and to the straight neck attach an addition funnel. Fit a CaCl2 drying tube to the end of the condenser.
Why must the apparatus be rigorously dry?
All ground glass joints should be lightly greased.
Leave room under the flask to fit an ice-water bath later.
2. Obtain 6.7mL of bromobenzene and 2.5g of magnesium. Grind the magnesium in a mortar and pestle. Add the Mg to the flask along with 20mL of anhydrous ether. Remove the addition funnel and, using a pipet, add approximately 2mL of the bromobenzene below the surface of the ether to obtain a high concentration of the bromobenzene close to the surface of the magnesium. Replace the addition funnel and warm the solution with warm water or the palm of your hand. Formation of the Grignard reagent is evidenced by bubbling and a cloudy appearance to the reaction mixture.
NOTE: DO NOT ADD THE BROMOBENZENE ALL AT ONCE. NO SECOND CHANCES!!!!!!
Record masses to 0.1g.
Record the volume of bromobenzene to 0.1mL.
Formation of the Grignard reagent takes place at the surface of the Mg. What is the purpose of grinding the Mg?
If after 5-10 minutes of warming and swirling the flask, you have difficulty obtaining a vigorous reaction, inform your instructor or TA.
Why should the bromobenzene be added dropwise, a little at a time?
3. When the reaction mixture is vigorously refluxing, turn on the water to the condenser. Mix the remaining bromobenzene and 20-25mL of anhydrous ether and add the solution to the addition funnel. Add the bromo-benzene solution to the reaction mixture dropwise at a rate to maintain a steady reflux. Swirl the flask occasionally.
How does the reaction mixture reflux without the application of heat?
Explanation / Answer
Q1.
Why must the apparatus be rigorously dry? --> Because moisture (H2O) may interfere with the original reaction. Also, the Mg will react with water to form Mg(OH)2 which is a storng base, we do not want that!
Q2.
Why should the bromobenzene be added dropwise, a little at a time?
To ensure reaction and no excess reagent is used... This way, we will just have a drop of excess material. Also for safety/hazard rules, it could react vigorously!
Q3.
How does the reaction mixture reflux without the application of heat?
It is due to the heat of the same reaction + the condenser
Q4.
Why is ice used?
We use ice in order to decrease the water's temperature
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