BRANDEIS UNIVERSITY

CHEMISTRY 29A

PRELAB WRITEUP AND EXPERIMENTAL REPORT SHEET

NAME Richard Fan______T.A.'S NAME David Gay ______

DATE 7/13/06 _____EXPERIMENT# __1___TITLE _NMR Spectroscopy / Nitration of Methyl Benzoate

PURPOSE _To obtain methyl-3-nitrobenzoate from a reaction of methyl benzoate with HNO3 / H2SO4______

PRELIMINARY WRITEUP:
Materials: reaction tubes
Pipettes, beaker
0.2mL, 0.6mL H2SO4
0.3g methyl benzoate
0.2mL HNO3
0.2mL methanol
ice
Hirsch funnel, flash
Stirring rod / % yield = (experimental/theoretical) × 100%
Expected weightx = # of moles used × MWx
Methyl benzoate
MW 136.16
BP 199.6ºC
MP -12ºC
Methyl-3-nitrobenzoate
MW 181.15
MP 78ºC / HNO3 (nitric acid)
MW 63.0
MP -42ºC
BP83ºC
H2SO4 (sulfuric acid)
MW 98.18
MP 10ºC
BP337ºC
EXPERIMENTAL PROCEDURES*
(Include in preliminary writeup.)
-Add 0.6mL H2SO4 to reaction tube.
-Weigh 0.30g methyl benzoate, add to tube, keep on ice.
-Prepare mix of 0.2mL H2SO4 and 0.2mL HNO3 on ice.
-Add dropwise to reaction tube, on ice.
-Let mixture warm in room temperature water for 20-25 minutes. (Will become yellow viscous liquid)
-Weigh 2.5g of ice, pour mixture onto ice.
-Use vacuum filtration to isolate crude product.
-Wash with a few drops of ice cold methanol.
-Get crude weight, melting point.
-Recrystallize in minimal methanol.
-Vacuum filtration to isolate final product.
-Wash again with a few drops of water, and a few drops of ice cold methanol.
-Get final weight, melting point, and infrared spectroscopy. / MODIFICATIONS AND OBSERVATIONS*
(Obtained from the lab experience)
For this experiment, only 0.2966g of methyl benzoate could be effectively weighed out due to the size of the drops added. After all of the initial reactants had been added to the reaction tube, the mixture had two layers which were evenly mixed together with a stirring rod before the tube was placed in a water bath. The mixture was removed from the room temperature water bath after carrying out the reaction for 25 minutes. The reaction tube had no noticeable increase in temperature upon removal from the water bath. When this mixture was poured into a small beaker of ice, a yellowish clumped precipitate formed. This was then poured onto a filtered Hirsch funnel for vacuum filtration. The beaker was washed with a small amount of cold methanol to ensure most of the precipitate had been transferred. The crude product on the Hirsch funnel appeared slightly yellow, but turned white as the last of the beaker contents (with methanol) was transferred, and as it was washed again with a few drops of methanol. The weight of the crude product was 0.3764g, and the melting point was 64º-73ºC. After dissolving in minimal methanol over heat and recrystallizing, the final product weight was 0.3547, and the melting point was 76º-78ºC. There was no noticeable change in color of the product after recrystallization, as the crude product was already very white in appearance. See attached for IR spectroscopy.

RESULTS, DISCUSSION AND CONCLUSIONS*

Before the reaction takes place in this experiment, two separate reagent mixtures are prepared: sulfuric acid / methyl benzoate, and sulfuric acid / nitric acid. This is done to create the necessary reactive intermediates. As the sulfuric acid and methyl benzoate mixture is prepared and set aside, a hydrogen ion bonds to the double bonded oxygen to create a protonated intermediate. The sulfuric acid and nitric acid mixture creates the nitronium ion that then bonds to the protonated methyl benzoate intermediate at a meta position to then form the final product. Since a total of 0.2966g, or about 2.2mmol, of methyl benzoate was used for the reaction (0.2966g / 136.16g = 0.00218mol), about 2.2mmol (0.3946g) of methyl-3-nitrobenzoate is expected as the product (0.00218mol × 181.15g). After the first vacuum filtration, the crude product weighed 0.3764, about a 95.4% yield. This crude product also appeared very white in color, suggesting that it had relatively few impurities. Melting point analysis of this crude product did indicate the presence of impurities however, due to the wide and depressed melting point (64º-73ºC) as compared to the expected (78ºC). After recrystallizing in methanol and drying, the final product weighed 0.3547g, which was an expected but reasonable decrease from the crude weight. This final product was a 89.9% yield of the expected amount. This decrease in weight and percentage yield can probably be attributed to the loss of impurities and some product during the transfer, dissolving, and final filtration processes of the product. The melting point of the final product was observed to be 76º-78º, which is a much smaller range than that of the crude product and is much closer to the expected melting point. This showed that the final product was indeed more purified than the crude product, even though the appearance (crystal color) of the final product was about the same as that of the crude product. Infrared spectroscopy analysis of the final product shows the bonds of the product as well as the bonds of the solvent methylene chloride (see labeled on attached IR spec). Accompanying peaks may be those of remaining impurities in the final product. There was already a relatively high yield for this experiment, but it may be increased further by careful transfer of the mixtures in the experiment, as well as careful addition of methanol. While methanol acts to remove impurities, using large amounts to wash the product (especially if the methanol is not ice cold) may dissolve some of the product and carry it away in solution.

ANSWERS TO END-OF-CHAPTER QUESTIONS*

1.

Methyl benzoate is able to dissolve in sulfuric acid due to their similar polar characteristics. Sulfuric acid is very polar and has highly mobile protons, which help it as a solvent. Esters are normally not very polar, but methyl benzoate has a very high boiling point (199.6ºC) suggesting that it is very polar as well. Together, methyl benzoate is able to dissolve in sulfuric acid and produce the intermediate ions that can be used in the nitration step of the experiment reaction.

2. / The dinitro structure may have the second nitro group on the number 5 position opposite to the first nitro group. This is due to the locations of positive charge on the resonance forms of the aromatic ring. As the methyl benzoate becomes protonated, electrons shift and change the locations of double bonds, leaving resonance forms that have positions of positive charge. The nitro group, having a positive charge, will only bond to the meta positions on the ring, which are 3 and 5.

3. Unreacted methyl benzoate is removed from the product during the first vacuum filtration and again during recrystallization and the second vacuum filtration. As the mixture was poured onto ice, the product became a solid (melting point of 78ºC) but the unreacted methyl benzoate remained in solution (melting point of -12ºC). The product is then removed from the solution by filtration. If any methyl benzoate remained with the product, it was removed again in solution in methanol as the purified product recrystallized and separated with filtration.

4. When examining the 1H NMR spectrum of methyl-3-nitrobenzoate, there are five main peaks. The peak with 0 shift is most likely the solvent. The next lowest shift is around 3.9ppm and is the hydrogens attached to the ester group. The three peaks with the highest shifts (from lowest to highest shift) are the hydrogen in the para position, the ortho position opposite the nitro group, and finally the ortho position between the ester and nitro groups.

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