Lab Report 6 Edited - Grade: A PDF

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Lab Report 6 Edited...

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Alkenes from Alcohols and Analysis by Gas Chromatography Lead Author: Mary Yant Reviewer: Cierra Young Editor: Hannah Strickland

CH 235- G5 Experiment 6

Introduction:

The dehydration of alcohols is an E1 reaction. An E1 reaction is an elimination reaction in which bonds are being broken and formed. Such bonds include carbon-carbon double bonds, carbon-hydrogen bonds, and carbon-leaving group bonds. Carbon-Carbon double bonds are being formed, Carbon-Hydrogen and Carbon-leaving group bonds are being broken.1 Reaction rates and physical properties are important when analyzing these types of reactions because they are indicative of the products formed. The differences between major and minor products are apparent while analyzing a particular reaction. Major products are more stable, in which is due to the double bonds present being more substituted. Minor products, however, are less stable. The double bonds present in minor products are less substituted. Each of these products are produced by the same E1 reaction. Despite this commonality, major product are more stable and more desirable. The carbocation present prefers to be on a more substituted cation, and the double bond is more stable when surrounded by other bonds. The concept of substitution can be referred back to Zaitsev’s rule, that alkene stability increases with the number of attached alkyl groups.2 This experiment observed the E1 reaction of 3-methyl-3-pentanol with phosphoric acid. The reaction performed was an acid catalyzed dehydration. Acid catalyzed dehydration reactions on alcohols works well because the OH group can act as either an acid or a base. In the presence of a strong acid, like in the experiment, the OH group acts as a base and reacts to form alkyloxonium ions. These ions are highly acidic and produce sufficient leaving groups.3 During the experiment, gas-liquid chromatography was used in order to analyze the composition of the distillate collected. Gas Chromatography separates vaporous mixtures according to their boiling points. Gas chromatography is composed of two phases: the mobile phase and the stationary phase. In this experiment, helium gas acts in the mobile phase and as the carrier. For the stationary phase in this experiment, a liquid with a high boiling point was used in order to. This liquid covers the gas chromatography column. The carrier, helium gase, then passes over the column and separates out impurities from the volatile mixture, which is separated into the liquid phase and the stationary phase. Gas chromatography is useful in analyzing data due to more stable products containing higher boiling points and less stable products containing lower boiling points. When the gas chromatography is used it yields a graph displaying multiple peaks. The peaks depict the less stable, or minor, product and end with the most stable, or major, product. Gas Chromatography further allows the identification of the mixtures products, and what percentage of each product is in the mixture.

The mechanism for an E1 reaction is as follows: INSERT MECHANISM HERE (I will do this myself Monday. I need help with the program not working.) The reagents used and their physical properties are listed below for reference. Table 1. Table of Reagents Compound

Molecular Weight Boiling point (◦C) Melting Point (g/mol) (◦C)

Density (g/cm3)

Anhydrous CaCl24 110.980

1,935.00

772.000

0.824

Bromine5

79.9µ + 0.001µ

58.80

-7.200

3.120

Phosphoric Acid6

98.000

158.00

42.350

1.880

Potassium Permanganate7

158.034

—

240.000

2.700

Water8

18.020

100.00

0.000

1.000

3-methyl-2pentanol9

102.174

123.00

-23.600

0.824

3-methyl-2pentene10

84.162

69.00

—

0.698

2-ethyl-1butene11

84.200

64.00

131.000

0.689

Experimental: A simple distillation apparatus was assembled, to begin the experiment, around 8:30 am. To assemble the apparatus, a simple distillation column, a long neck round bottom flask, rubber joint connectors, and a spin bar were obtained . The apparatus was placed in a sand bath on top of a hot plate. A thermometer was then obtained and used to observe the temperature of the sand bath and maintain the desired 100-105 degrees Celsius. The liquid mixture was placed in the round bottom flask and was mixed while the apparatus was assembled. 2 mL of 2-methylcyclohexanol was collected and placed in the round bottom flask. Then, 1 mL of 85% phosphoric acid was added to the round bottom flask and the reagents were mixed thoroughly at 8:42 am. The round bottom flask was attached to the distillation apparatus. The collection apparatus included a 25mL Erlenmeyer flask that was weighed and placed in a 140mL beaker filled with ice The beaker was placed on the hot plate to collect the distillate as a result of the hot plate itself not being turned on and just used for stirring purposes. Distillate was first observed at 9:04 am when the sand was 118 degrees Celsius. The distillation process continued until 9:32 am. The collection flask was removed and then weighed. The collection flask was weighed before the experiment and again after with the product. The calculation indicated that there was 0.677 grams of product. Then, the distillate was dried with calcium chloride. Once the product had been dried with calcium chloride, gas chromatography was performed. The process was not done, but observed due to the complexity of the machine. Results: During this experiment, major and minor products were formed. This was best displayed using the gas chromatograph. Due to problems with the machinery, another source of data was used. The two peaks shown on the gas chromatograph show first the minor product and then the major product. The minor product is 3-methyl-1-cyclohexene. The major product is 2methyl-1-cyclohexene. Percent composition of species present: Mole percentage of compound in mixture12 = (area under individual peak/ total area under all the peaks) x 100 For “really” minor product: (855/33,348) x 100 = 2.56%

For minor product: (10,421/33,348) x 100 = 31.25% For major product: (22,072/33,348) x 100 = 66.19%

Percent yield: Experimental yield/Theoretical yield x 100 = Percent yield 0.677 grams/1.2 grams x 100 = 56.42%

Figure 1: Figure 1 shows the gas chromatogram for the experiment. Peak 1 is the minor product, 3-methyl-1-cyclohexene. Peak 2 is the major product, 2-methyl-1-cyclohexene. Retention time for really minor product is 0.11. Retention time for minor product is 0.14. Retention time for major product is 0.19.

Discussion: The products observed suggest an E1 reaction took place. The major and minor products were formed by acid catalyzed dehydration, and they are indicators of the E1 reaction1. The gas chromatogram did not yield all possible products. The expected graph would have shown one small peak followed by a larger peak in which represented the minor and major products respectively. This data is not what resulted from the test of the collected product. The smaller peak was not present, the higher peak was first, and it was followed by a very small peak. It is known that the minor product would have shown up first because the minor product is less stable and therefore has a lower boiling point. This information makes it impossible to consider that the second peak present could have been the minor product. The minor product would never have followed the major product. It is likely that the minor product was present in such a low quantity that the gas chromatograph machine did not register it as present, and that the second peak was a result of an air bubble or a trace impurity. It is also plausible that there was an error with the gas chromatograph machine and the error was causing it to give faulty readings. Temperature maintenance is a part of the experiment that is always prone to error, for this group in particular. Too high of a temperature can cause the experiment to proceed too fast yielding low quality results. Too low of a temperature can cause the experiment to take much longer than necessary. A mistake that could have been made was the overheating of thermometers which could cause an explosion. Another mistake could be misuse or mishandling of phosphoric acid. Percent yield is used to calculate a reaction’s efficiency.

Percent yield shows how the actual yield compares to the theoretical yield and it gives a ratio between the two. The theoretical yield is calculated based on the limiting reactant. It determines how much product could be formed. The actual yield is how much product was formed by the reaction. This will usually be different than the theoretical yield due to limitations and errors.13

Conclusion: In this experiment, the dehydration of alcohols using acid catalyzed dehydration was performed. The E1 reaction was observed and the products were determined which included major and minor products. The gas chromatography was used to determine the amount of major and minor products in the mixture and to visualize the stability of the products. While this experiment went exceptionally well, the experiment could have been improved by a longer arm on the distillation column for the extraction of products. The short arm made catching the products. Some of the distillate could have been lost due to a short distillation arm which could have caused a lower percent yield, thus giving inaccurate results. Also, instead of simple distillation, fractional distillation could have been preformed in order to give a more pure distillate, eliminating impurities, and giving a more accurate percent yield. This would also eliminate impurities to give a more accurate gas chromatography graph.

References: 1. The E1 Reaction http://www.masterorganicchemistry.com/2012/09/19/the-e1-reaction/ (accessed Nov 1, 2016). 2. Zaitsev’s Rule http://www.masterorganicchemistry.com/tips/zaitsevs-rule/ (accessed Nov 1, 2016). 3. Alkenes by Dehydration of Alcohols http://chem.libretexts.org/@api/deki/files/2164/info (accessed Nov 1, 2016). 4. CALCIUM CHLORIDE | CaCl2 - PubChem https://pubchem.ncbi.nlm.nih.gov/compound/ calcium_dichloride (accessed Nov 1, 2016). 5. Bromine https://pubchem.ncbi.nlm.nih.gov/compound/5360770 (accessed Nov 1, 2016). 6. Phosphoric acid | H3PO4 - PubChem https://pubchem.ncbi.nlm.nih.gov/compound/ phosphoric_acid (accessed Nov 1, 2016). 7. POTASSIUM PERMANGANATE | KMnO4 - PubChem https://pubchem.ncbi.nlm.nih.gov/ compound/potassium_permanganate (accessed Nov 1, 2016). 8. water | H2O - PubChem https://pubchem.ncbi.nlm.nih.gov/compound/water (accessed Nov 1, 2016). 9. 3-METHYL-2-PENTANOL | C6H14O - PubChem https://pubchem.ncbi.nlm.nih.gov/ compound/3-methyl-2-pentanol (accessed Nov 1, 2016). 10. cis-3-Methyl-2-pentene | C6H12 - PubChem https://pubchem.ncbi.nlm.nih.gov/ compound/643935 (accessed Nov 1, 2016).

11. 2-ETHYL-1-BUTENE | C6H12 - PubChem https://pubchem.ncbi.nlm.nih.gov/compound/2ethyl-1-butene (accessed Nov 1, 2016). 12. Hill, R. K.; Barbaro, J. Experiments in organic chemistry; Contemporary Pub. Co. of Raleigh: Raleigh, NC, 2005. 13. Calculating Theoretical and Percent Yield - Boundless Open Textbook https:// www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/mass-relationshipsand-chemical-equations-3/reaction-stoichiometry-44/calculating-theoretical-and-percentyield-234-4704/ (accessed Nov 1, 2016).

Questions (From Pages T8-9): 1. The type of stationary liquid phase used, the column’s length, the column temperature, and the flow rate of the carrier gas. 2. Benzene, Toulene, o-xylene, p-xylene. 3. The cyclohexylmethanol because the one that boils first will have the first peak. 4. A) It decreases the solubility of a gas in a liquid. B) There are more opportunities for interactions between the components of the sample and the stationary liquid phase. This gives for a better separation. C) The compounds will also move through more quickly. 5. Diethylene glycol succinate (DEGS) because it has the lowest maximum temperature that would be nearest the temperatures listed. 6. It can result in the liquid phase becoming volatile, and therefore causing the absorbent to bleed off of the column interfering with the separation and making the column ineffective or useless in the future. 7. Mole Percentage: (area under individual peak/ total area under all of the peaks) x 100 For 29 mm2: (29/375) x 100 = 7.73 For 210 mm2: (210/375) x 100 = 56 For 136 mm2: (136/375) x 100 = 36.27...