Grignard Reagent Lab
| Reactions of Grignard Reagents with Carbonyls | | | Tuesday 1:30 | 2/28/2012 |
|
Introduction This experiment explores the reactivity pattern for the addition of Grignard reagents to three different carbonyl groups: a ketone, an ester, and a carbonate. Grignard reagents are organometallic compounds that have a carbon-metal bond, such as carbon-magnesium. Grignard reagents are formed from the reaction of an alkyl, cycloalkyl, or aryl halide and magnesium metal in dry ether. The reaction is shown below in figure 1.
Figure 1: Formation of a Grignard reagent
The Carbon bonded to the metal is both a strong nucleophile and base. The Carbon with the carbanion character can participate in nucleophile reactions …show more content…
such as nucleophile substitution or carbonyl addition. The experiment completed is an example of a carbonyl addition using a Grignard reagent. An imperative aspect of the Grignard reagent is that is must be performed under dry or aprotic conditions. The carbanion is an extremely strong base and can abstract protons from water; this will allow less carbanions to undergo the reaction. The mechanism for the reaction in the experiment is shown below:
Firgure 2: Reaction mechanism of Diethyl Carbonate with Phenylmagnesium Bromide to form Triphenylmethanol
Observations: * Benzophenone white powder * Dry Ether Clear Liquid * Grignard (Phenylmagnesium Bromide) apple juice color * Addition of Benzophenone and dry Ether to Grignard color change: red to brown to orange * Placing test tube in ice bath and adding HCl solid started to form, used stirring rod to mix solution; color change: creamy yellow color * Addition of dry ether to test tube had two layers, top layer was clear but still a bit cloudy, the bottom layer was clear * Addition of anhydrous Sodium Sulfate anhydrous Sodium Sulfate was small, white rocks, looked like salt on a pretzel; clumped up when poured into test tube, kept adding until did not clump up anymore * Decanted dry ether layer to test tube and placed in hot water bath bubbled around the rim and boiling chip * Test tube in ice bath and addition of 2 mL Petroleum Ether precipitate started to form * Addition of hot 2 mL 2-Propanol to test tube and placing in hot bath color of solution was a deep orange-yellow color * Table 2: GC Settings
Appearance of final productwhite, flakey crystals GC Settings
Injector: 330.0
(SPL1)
Column: 300.0
Detector: 330.0
Linear Velocity: 23.0 cm/s Split: 400
Table 1 Amount used in Experiment
Substance Amount
Benzophenone Reaction
Benzophenone: 0.364 grams
Dry Ether: 3.0 mL (total)
HCl: 5.0 mL (total)
Table 3: GC 1 Standards
Saturated Sodium GC 1 Standards
Peak # Ret.
Time Corrected Ret. Time 1 0.720 0 2 1.088 0.368
Chloride: 9.0 mL
Petroleum Ether: 3 mL
Diethyl Carbonate: 0.250mL
Methyl Benzoate: 0.250mL
Table 4: GC 2 …show more content…
Standards
Phenylmagnesium
GC 2 Standards
Peak # Ret. Time Corrected Ret. Time 1 0.929 0 2 1.088 0.170
Bromide: 3.0 mL
2-Propanol: 2.0 mL
Discussion: The mechanism that formed Trimethylphenol is the carbonyl addition mechanism, and that mechanism is seen in figure 2. The carbonyl carbon on the diethyl carbonate is weakly electrophilic and is attacked by the carbanionic carbon of the Grignard reagent, phenylmagnesium bromide. The attack causes the carbonyl oxygen to have a -1 charge as a sp3 bonded oxygen. The lone pair on the oxygen then formed a double bond between oxygen and carbon, making the once sp3 oxygen now sp2. The single bonded oxygen breaks away from the compound leaving methylbenzoate.
The carbonyl carbon on the methylbenzoate is still weakly electrophilic and is once again attacked by the carbanionic carbon of the Grignard reagent. The reaction is the same however this time leaving a benzophenone instead of methylbenzoate. The carbonyl carbon on the benzophenone is still weakly electrophilic and is once again attacked by the carbanionic carbon of the Grignard reagent. Once the oxygen has a -1 charge, Hydrochloric acid is then added to protonate the Oxygen. This results in the thriphenylmethanol as the final product. The average melting point of this experiment, when Benzophenone was the starting reagent, was 161.1-162.0 C. The literature melting point of triphenylmethanol is 160-163.0 C, it is evident from the melting point that there are almost no impurities. This shows how well the Grignard can extract a compound. -1 -1 -1 The IR spectra provided strong evidence that supported the formation of triphenylmethanol. The most significant peak is the O-H absorption at 3472 cm , this corresponds to the –OH in the final product. The other absorption is the C-O at 1010 cm , and the aromatic ring absorptions from 1600-1450, mainly 1445 and 1489 cm with each band having variable intensities. The percent yield of thriphenylmethanol with benzophenone as the starting product was 51.2%. In this experiment, the percent yield is relatively low in comparison to other experiments. Grignard reagents react very quickly with water, since the Grignard reagent was phenylmagnesium bromide, abstracting a proton would have formed HBr and benzene. The proton would have neutralized the Grignard. The benzene would have been removed in the separation process throughout the experiment and would not have been taken into account for the final yield. The way to avoid the formation of benzene is to make absolutely sure that all the glass wear is dry. The purity of the final product contained almost no impurities; this can be seen when looking at the GC results.
There were only two peaks, the first being acetone which is the reference point, the second peak was the thriphenylmethanol. If any other compound was present in the solution, it would have appeared in the results. In all three starting materials, it still gave a GC result of only two peaks. In our run of all three solutions together, there was a third peak, the percent area of that peak was only 0.55%, the other group that ran the same solution of all three mixed together did not have the third peak. In comparison with all three different starting materials, the reagent that yielded the most products was diethyl carbonate, it yielded 64.4%. The reagent that yielded the least was methyl benzoate; it yielded 45.17%, which in reality is not that low for this experiment. It can be hypothesized that the diethyl carbonate will yield that most product of thriphenylmethanol. The way to test this would be to repeat the experiment to see if diethyl carbonate will once again yield that most
thriphenylmethanol.
Results:
Amounts of Final Product Collected
Starting Reagent Amount Collected
Benzophenone: 0.266 grams
Diethyl Carbonate: 0.346 grams
Methyl Benzoate: 0.235 grams
Melt Point Results from Benzophenone as starting reagent
Melt Point 1: 161.0-162.0
Melt Point 2: 162.1-163.0
Average Melt Point: 161.5-162.5
Literature Melt Point: 160-163
Melt Point Results from Diethyl Carbonate as starting reagent
Melt Point 1: 161-163.0
Melt Point 2: 162.2-163.0
Average Melt Point: 162.0-163
Literature Melt Point: 160-163
All three Solution GC Results
Peak # Ret. Time Cor. Ret. Time Cor. % Area 1 0.935 0 0 2 1.272 0.337 0.55 3 1.410 0.475 99.44 Percent Yield Table
Starting Reagent % Yield
Benzophenone 51.2
Methyl Benzoate 45.17
Diethyl Carbonate 64.4 Single Solution GC Results
Peak # Ret. Time Cor. Ret. Time Cor. % Area 1 0.940 0 0 2 1.412 0.472 54.84
Melt Point Results from Methyl Benzoate as starting reagent
Melt Point 1: 163.1-164.2
Melt Point 2: 163.2-165.0
Average Melt Point: 163.1-164.6
Literature Melt Point: 160-163.0
|
Introduction This experiment explores the reactivity pattern for the addition of Grignard reagents to three different carbonyl groups: a ketone, an ester, and a carbonate. Grignard reagents are organometallic compounds that have a carbon-metal bond, such as carbon-magnesium. Grignard reagents are formed from the reaction of an alkyl, cycloalkyl, or aryl halide and magnesium metal in dry ether. The reaction is shown below in figure 1.
Figure 1: Formation of a Grignard reagent
The Carbon bonded to the metal is both a strong nucleophile and base. The Carbon with the carbanion character can participate in nucleophile reactions …show more content…
such as nucleophile substitution or carbonyl addition. The experiment completed is an example of a carbonyl addition using a Grignard reagent. An imperative aspect of the Grignard reagent is that is must be performed under dry or aprotic conditions. The carbanion is an extremely strong base and can abstract protons from water; this will allow less carbanions to undergo the reaction. The mechanism for the reaction in the experiment is shown below:
Firgure 2: Reaction mechanism of Diethyl Carbonate with Phenylmagnesium Bromide to form Triphenylmethanol
Observations: * Benzophenone white powder * Dry Ether Clear Liquid * Grignard (Phenylmagnesium Bromide) apple juice color * Addition of Benzophenone and dry Ether to Grignard color change: red to brown to orange * Placing test tube in ice bath and adding HCl solid started to form, used stirring rod to mix solution; color change: creamy yellow color * Addition of dry ether to test tube had two layers, top layer was clear but still a bit cloudy, the bottom layer was clear * Addition of anhydrous Sodium Sulfate anhydrous Sodium Sulfate was small, white rocks, looked like salt on a pretzel; clumped up when poured into test tube, kept adding until did not clump up anymore * Decanted dry ether layer to test tube and placed in hot water bath bubbled around the rim and boiling chip * Test tube in ice bath and addition of 2 mL Petroleum Ether precipitate started to form * Addition of hot 2 mL 2-Propanol to test tube and placing in hot bath color of solution was a deep orange-yellow color * Table 2: GC Settings
Appearance of final productwhite, flakey crystals GC Settings
Injector: 330.0
(SPL1)
Column: 300.0
Detector: 330.0
Linear Velocity: 23.0 cm/s Split: 400
Table 1 Amount used in Experiment
Substance Amount
Benzophenone Reaction
Benzophenone: 0.364 grams
Dry Ether: 3.0 mL (total)
HCl: 5.0 mL (total)
Table 3: GC 1 Standards
Saturated Sodium GC 1 Standards
Peak # Ret.
Time Corrected Ret. Time 1 0.720 0 2 1.088 0.368
Chloride: 9.0 mL
Petroleum Ether: 3 mL
Diethyl Carbonate: 0.250mL
Methyl Benzoate: 0.250mL
Table 4: GC 2 …show more content…
Standards
Phenylmagnesium
GC 2 Standards
Peak # Ret. Time Corrected Ret. Time 1 0.929 0 2 1.088 0.170
Bromide: 3.0 mL
2-Propanol: 2.0 mL
Discussion: The mechanism that formed Trimethylphenol is the carbonyl addition mechanism, and that mechanism is seen in figure 2. The carbonyl carbon on the diethyl carbonate is weakly electrophilic and is attacked by the carbanionic carbon of the Grignard reagent, phenylmagnesium bromide. The attack causes the carbonyl oxygen to have a -1 charge as a sp3 bonded oxygen. The lone pair on the oxygen then formed a double bond between oxygen and carbon, making the once sp3 oxygen now sp2. The single bonded oxygen breaks away from the compound leaving methylbenzoate.
The carbonyl carbon on the methylbenzoate is still weakly electrophilic and is once again attacked by the carbanionic carbon of the Grignard reagent. The reaction is the same however this time leaving a benzophenone instead of methylbenzoate. The carbonyl carbon on the benzophenone is still weakly electrophilic and is once again attacked by the carbanionic carbon of the Grignard reagent. Once the oxygen has a -1 charge, Hydrochloric acid is then added to protonate the Oxygen. This results in the thriphenylmethanol as the final product. The average melting point of this experiment, when Benzophenone was the starting reagent, was 161.1-162.0 C. The literature melting point of triphenylmethanol is 160-163.0 C, it is evident from the melting point that there are almost no impurities. This shows how well the Grignard can extract a compound. -1 -1 -1 The IR spectra provided strong evidence that supported the formation of triphenylmethanol. The most significant peak is the O-H absorption at 3472 cm , this corresponds to the –OH in the final product. The other absorption is the C-O at 1010 cm , and the aromatic ring absorptions from 1600-1450, mainly 1445 and 1489 cm with each band having variable intensities. The percent yield of thriphenylmethanol with benzophenone as the starting product was 51.2%. In this experiment, the percent yield is relatively low in comparison to other experiments. Grignard reagents react very quickly with water, since the Grignard reagent was phenylmagnesium bromide, abstracting a proton would have formed HBr and benzene. The proton would have neutralized the Grignard. The benzene would have been removed in the separation process throughout the experiment and would not have been taken into account for the final yield. The way to avoid the formation of benzene is to make absolutely sure that all the glass wear is dry. The purity of the final product contained almost no impurities; this can be seen when looking at the GC results.
There were only two peaks, the first being acetone which is the reference point, the second peak was the thriphenylmethanol. If any other compound was present in the solution, it would have appeared in the results. In all three starting materials, it still gave a GC result of only two peaks. In our run of all three solutions together, there was a third peak, the percent area of that peak was only 0.55%, the other group that ran the same solution of all three mixed together did not have the third peak. In comparison with all three different starting materials, the reagent that yielded the most products was diethyl carbonate, it yielded 64.4%. The reagent that yielded the least was methyl benzoate; it yielded 45.17%, which in reality is not that low for this experiment. It can be hypothesized that the diethyl carbonate will yield that most product of thriphenylmethanol. The way to test this would be to repeat the experiment to see if diethyl carbonate will once again yield that most
thriphenylmethanol.
Results:
Amounts of Final Product Collected
Starting Reagent Amount Collected
Benzophenone: 0.266 grams
Diethyl Carbonate: 0.346 grams
Methyl Benzoate: 0.235 grams
Melt Point Results from Benzophenone as starting reagent
Melt Point 1: 161.0-162.0
Melt Point 2: 162.1-163.0
Average Melt Point: 161.5-162.5
Literature Melt Point: 160-163
Melt Point Results from Diethyl Carbonate as starting reagent
Melt Point 1: 161-163.0
Melt Point 2: 162.2-163.0
Average Melt Point: 162.0-163
Literature Melt Point: 160-163
All three Solution GC Results
Peak # Ret. Time Cor. Ret. Time Cor. % Area 1 0.935 0 0 2 1.272 0.337 0.55 3 1.410 0.475 99.44 Percent Yield Table
Starting Reagent % Yield
Benzophenone 51.2
Methyl Benzoate 45.17
Diethyl Carbonate 64.4 Single Solution GC Results
Peak # Ret. Time Cor. Ret. Time Cor. % Area 1 0.940 0 0 2 1.412 0.472 54.84
Melt Point Results from Methyl Benzoate as starting reagent
Melt Point 1: 163.1-164.2
Melt Point 2: 163.2-165.0
Average Melt Point: 163.1-164.6
Literature Melt Point: 160-163.0