ORGANIC CHEMISTRY
180-222A
Name: Joey Roy Date Performed: October 4th, 2001
Student: 0031475 Lab Day: Thursday
Locker: 299 Demonstrator: Paul
Experiment 3
THE GRIGNARD REACTION
McGill University, 2001
DATA
Source for all physical properties is http://www.chemfinder.camsoft.com
Table 1: Physical properties of chemical substances used.
Compound / Molecular Formula / Boiling Point (ºC) / Melting Point (ºC) / Density(g/ml) / Solubility in H2O
g/100ml at 25ºC
Magnesium / Mg / 1107 / 650 / 1.74 / Insoluble/Reactive
Bromobenzene / C6H5Br / 155 / -30.8 / 1.495 / Insoluble
Absolute ether / C4H10O / 34.6 / -116.3 / 0.7134 / 6.9
Calcium Chloride / CaCl2 / 1600 / 782 / Desiccant
Benzophenone / C13H10O / 305.4 / 48.5 / 1.11 / Insoluble
Sulfuric acid / H2SO4 / 280 / 3 / 1.84 / Miscible/Reactive
Anhydrous sodium sulfate / H2Na2 / 884 / 2.68
Hexanes / C6H14 / 69 / -95 / 0.659
Triphenylcarbinol / C19H16O / 360 / 163
Table 2: Quantitative properties of chemical substances used
Compound / Molecular Weight (g./mol) / Moles / Mass (g) / Volume (ml)Magnesium / 24.305 / 0.03330 / 0.8094
Bromobenzene / 157.0095 / 0.034 / 3.6
Absolute ether / 74.1224 / 0.212 / 22.0
Calcium Chloride / 110.986 / NA / Liberal use
Benzophenone / 182.2214 / 0.011 / 2.0
Sulfuric acid / 98.0734 / NA / Liberal use
Anhydrous sodium sulfate / 142.03714 / NA / Liberal use
Hexanes / 86.1766 / 0.04 / 5
Triphenylcarbinol / 260.3348 / 0.0064283 / 1.6735
Example of molar calculation by mass:
0.8094g Magnesium X 1mole/24.305g = 0.03330 moles Magnesium
Example of molar calculation by volume:
3.6ml Bromobenzene X 1.495g/ml X 1mole/157.0095 = 0.034 moles Bromobenzene
RESULTS
Yield of triphenylcarbinol: 1.6735g
Theoretical yield of triphenylcarbinol: 2.86g
(All molar ratios are 1:1) The limiting reagent in the Grignard reagent formation is Magnesium. Theoretically, 0.03330 moles of the organometallic Grignard reagent are obtained. The limiting reagent for the synthesis is Benzophenone, therefore 0.011 moles of product is expected.
0.011moles triphenylcarbinol X 260.3348g/mole = 2.86g triphenylcarbinol
% yield of triphenylcarbinol: 58.4%
% yield = Actual Yield/Theoretical Yield X 100
% yield = 1.6735g/2.86g X 100 = 58.4%
Melting point of triphenylcarbinol: 154-157 ºC
Literature melting point of triphenylcarbinol: 163ºC
Formation of the Grignard Reagent
Fig.1
Formation of Triphenylcarbinol
Fig.2
Unwanted concurrent reactions
Fig.3
The Grignard reaction is perhaps the most useful reaction in organic chemistry when dealing with the synthesis of larger products from smaller ones (molecularly speaking). As long as the required aldehyde or ketone and alkyl halide are available, a Grignard reaction can be carried out to join the two.
Since a Grignard reagent is very reactive, it cannot be stored for long periods of time and must therefore be synthesized before shortly before it must be used. Also, since it is such a good nucleophile, the Grignard reagent can attack water and produce an unwanted byproduct (see Fig.3a). For this reason, the reaction flask setup has a drying tube full of calcium chloride to absorb any moisture that may enter the reaction apparatus. To begin the reaction, elemental magnesium and bromobenzene are combined in a small amount of ether. The ether is a good solvent to use in this case because it is not hydrogen-active and will not react with the Grignard reagent being formed. Only 6ml are used at first, this is done to keep the reactants in close contact so that the reaction will get vigorous relatively quickly (this is important). The magnesium is crushed lightly to expose a reacting surface free of oils and MgO. Once bubbling starts, the reaction (Fig.1) has begun and once it gets quite vigorous, 10ml of ether is added to keep the reaction under control. This reaction is exothermic and as the reaction progresses, the ether is heated past its boiling point, the condenser mounted above the reaction flask serves to reflux the ether instead of letting it all boil away. Once the reaction nears completion, a steam bath is used to keep the ether refluxing (steam is used instead of a flame because ether is VERY flammable). This is done to make sure the largest amount of product is obtained, the heat added provides the activation energy that the slowing reaction can no longer provide. It also keeps the reaction going at a speedy rate. It is important that the reaction proceed at a quick pace because, as with any Grignard reagent formation, some of the alkyl halide will react with the newly formed Grignard and produce an unwanted by-product (Fig.3b). The faster bromobenzene can be used up by reacting with magnesium, the faster its concentration goes down and the less biphenyl can be produced. Once the reaction is complete, the mixture should be dark brown. If the mixture is cloudy, as it was in this case, water was present during the reaction and a white crystal was formed (again, Fig.3a). This impurity will be eliminated later on during the isolation of the final product.
Now that the Grignard has been formed, it can be reacted with benzophenone to form triphenylcarbinol (Fig.2). Since this reaction is exothermic, the phenyl magnesium bromide is cooled to help keep the reaction manageable. The benzophenone is dissolved in ether in a separatory funnel and slowly dripped into the Grignard mixture. Again, the ether is prevented from boiling off using the water condenser (and an ice bath if needed). As the product salt is formed, it precipitates out of the ether as a crystal since it is insoluble in non-polar solvents. To ensure the maximum amount of product is formed the flask is refluxed for thirty minutes using a steam bath. The product salt is clear, any colours are due to impurities and/or unused Grignard reagent. The mixture is poured into a flask containing cold sulfuric acid to protonate the product salt into its alcohol form by hydrolysis. This alcohol then dissolves in the organic phase, leaving magnesium sulfate in the aqueous phase. To ensure maximum yield, any product left in the previous flask is extracted into the current flask using some ether and acid. Acid is added until all the solid is dissolved, the layers are separated in a separatory funnel and the lower aqueous layer is discarded. The ethereal layer is washed with acid (to complete any hydrolysis) and a saturated salt solution. The salt solution creates a sharp contrast between the two phases and pulls most aqueous/polar substances out of the organic phase. Once the aqueous layer has been removed, the ether layer is passed through a filter cone filled with anhydrous sodium sulfate. This removes any solid impurities and water present in the solution. The filtrate is then rotary evaporated to remove most of the ether. Once the triphenylcarbinol crystals start to appear, 5ml hexanes is added to dissolve out everything but the product. Since the major by-product is biphenyl and biphenyl is soluble in hexanes while triphenylcarbinol is not, the product will crystallize and the impurities and ether will remain in the hexanes solution. Since it is the only solid, the crystal product is collected using vacuum filtration.
The sample can be deemed quite pure since its melting range is narrow and in the vicinity of the literature melting point of triphenylcarbinol. The discrepancy between the two points can be attributed to instrument error and varying conditions in the lab. The yield is a respectable 58.4%, the loss can be explained by a few factors. Firstly, if too much Grignard reagent has reacted with water and/or bromobenzene, there is not enough of it available to be in excessive amounts when reacting with benzophenone. In this case, the molar amount of product would equal that of the phenyl magnesium bromide instead of that of benzophenone. It is also possible that some benzophenone remained on the walls of the separatory funnel when it was drained and was therefore not available to react. A third possible factor is that some of the product salt was not removed from the reaction flask and could therefore not be converted to triphenylcarbinol by the sulfuric acid. Finally, it is certain that a small amount of product remained trapped in the filter paper along with some of the ether when it was filtered/dried. In microscale experiments, these factors feasibly explain the sub 100% yield.