Hydration of a Terminal Alkyne (Autosaved) PDF

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l~$paration of Ethyl Acetate and Butyl Acetate by Simple Distillation and Analysis of Fractions by Gas...

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Hydration of a Terminal Alkyne. Preparation of 3-Methyl-3-Hydroxy-2-Butanone Riyushi Mahadik 4/10/2014 Introduction The objective of this lab is to prepare 3-methyl-3-hydroxy-2-butanone by hydration of 2-methyl3-butyn-2-ol. The preparation process includes the usage of techniques such as heating under reflux, steam distillation, rotary evaporation, vacuum filtration, and recrystallization. Moreover, the final product will also be analyzed by IR spectroscopy, semicarbazone derivativatization, and melting point determination. When a Lewis base interacts with electrophilic reagents such as Lewis acids, an Electrophilic addition reaction produces both alkenes and alkynes. The reagents E-Nu are added to the carboncarbon multiple bonds in the process alkynes undergo an electrophilic addition reaction which is termed as hydration of the carbon-carbon triple bond. In the presence of mercuric ion and sulfuric acid, the alkyne is transformed into a ketone and the mercuric ion is converted into a cyclic mercurinium ion which facilitates the attack of water to the more substituted carbon. As shown in Figure 1, the hydration of alkynes results leaving mercuric ion forming vinyl alcohol which is very unstable and quickly tautomerizes to give the ketone.

Figure 1 The hydration of alkynes requires both sulfuric acid (H2SO4) and mercuric oxide (HgO). The H+ and Hg2+ acts like catalyst so the addition of Hg2+ forms much more stable and also low in energy mercurinium intermediate. If only H2SO4 was present, then very unstable 1 ° vinylic carbocation would form that is too high in energy. Thus, the water molecule (nucleophile) would not attack an unstable carbocation that is produced in absence of Hg2+and ketone formation would not occur. As shown in Figure 2. , hydration of an alkyne is regioselective, that is the reaction prefers one site of a functional group over other sites that could undergo the same reaction. This reaction follows the Markovnikov rule to form the final product. Based on Markovnikov Rule, a proton is added to the least substituted carbon (more hydrogen) atom to form a stable carbocation intermediate.

Figure 2 In order to perform the hydration of alkynes, different techniques are needed. For example, in this lab many techniques such as heating under reflux, steam distillation, extraction of organic solution from aqueous solution, and recrystallization are used to produce alkynes. The importance of heating mixture under reflux is to heat reaction at its boiling point for long period of time. Thus, creating a very high temperature allows the mixture to proceed for Markovnikov addition over anti-Markovnikov addition to form the ketone. The purpose of steam distillation is to separate the organic product from excess mercuric oxide. The volatile product would be codistilled with water vapor. In order to separate excess CH2Cl2 from the product, rotatory evaporation was performed. In rotary evaporator, solvents are evaporated under reduced pressure without bumping. The solution is under vacuum, lowering the pressure above the liquid decreases its boiling point which results in faster evaporation. The hot water bath heats the liquid bringing it closer to the boiling point. In rotatory evaporation methylene chloride was removed from the product. The surface area for evaporation for of the solution increases because of centrifugal and frictional forces produced. The rate of evaporation in a rotatory evaporator can be controlled by adjusting the vacuum, the temperature of the water bath, and the rate of rotation of the flask. A ketone or aldehyde functional group solutions are converted to semicarbazones that are highly crystalline solids. In this lab, semicarbazone derivatization was performed to verify the product of hydration. Semicarbazide undergoes a condensation reaction with ketones and aldehydes. The mechanism of semicarbazone formation is shown in Figure 3, so only the beta-nitrogen atom which is the second atom adjacent to the carbonyl carbon of semicarbazide undergoes addition to the aldehyde or ketone and it is the most nucleophilic. The electron pairs on the alpha nitrogen atoms that is the first atom adjacent to the carbonyl carbon are delocalized through resonance. Therefore, they are less available for bonding to other atoms, which makes the nitrogen atom less nucleophilic.

Figure 3 The last step of the experiment is to perform melting point of the substance to verify the purity of the product of hydration. Pure solids melt over a narrow range-usually 1ºC or less whereas an impure sample will have a broadened melting point range and a overall decrease in the melting point. The reason for not performing boiling point is because it is less accurate and less precise when using a simple distillation setup with an uncalibrated thermometer. In contrast, melting point analysis only requires a small amount of the product. Along with that, melting points of many semicarbazones are reported in the chemical literature, a comparison with the experimental values can help identify or verify the product of the reaction. As shown in Figure 3, through Markovnikov addition, the 3-hydroxy-3-methyl-2-butanone will form a semicarbazone with a melting point from 162-163°C and through anti-Markovnikov addition, the 3-hydroxy-3-methylbutanal will form a semicarbazone with a melting point of 222-223°C. Recrystallization of the semicarbazone product was performed to determine the percent yield of the hydration product. And IR was performed to analyze the structure of the product. The IR graph of 2-methyl-3butyn-2-ol will not have a peak for the carbon-carbon triple bond because dipole moment between them is very small. The structure of -hydroxy-3-methyl-2-butanone has a ketone and so there will be a peak between 1705-1725 cm-1 region. The hydration product also has a hydroxyl group showing the peak between 3200-3650 cm-1. Procedure: Part I. Preparation of 3-Hydroxy-3-methyl-2-butanone In a 250 round bottom flask, 20 mL of 3M sulfuric acid was added. Then, 0.22g of HgO and swirled to dissolve.The flask was cooled to 50°C. and it was equipped with reflux condenser with water tubes allowing the water into the condenser. From the top of the condenser, 3.6 mL of 2-methyl-3-butyn-2-ol was added to the round bottom flask and swirled for few minutes. Two water tubing were obtained and were hooked with the condenser. After the white precipitate is seen, the mixture was then heated for 30 minutes, and allowed to cool to room temperature. Part II. Steam Distillation

The steam distillation apparatus was assembled with the the reaction mixture heated in reflux condensation of 250 ml bottom flask attached to claisen adapter, stillhead, west condenser (thin column), bent vacuum adapter, separatory funnel, thermometer, and thermometer adapter. To prevent vapor from escaping apparatus, the equipment were tightly clipped with keck clips. The separatory funnel with warm water was placed at the open end of claisen adapter and stillhead was placed through another open end. A thermometer was placed into stillhead with the support of thermometer adapter. The thermometer was placed slightly below the entrance of condenser to ensure the mercury bulb of thermometer is immersed thoroughly in the vapors. Then, two water tubing were obtained and were hooked with west condenser. One of the water tubing allowed the water into the condenser and the other was there to drain it. The distillate was collected upto 48 ml in a pre-weighed graduated flask. Part III. Extraction In a separatory funnel, 48 mL of distillate collected through steam distillation was transferred and 2.5 g of potassium carbonate (K2CO3) was added to dissolve. Then, about enough NaCl was added to make the solution saturated. The mixture was then sequentially extracted with three portions of 10 mL of dichloromethane (CH2Cl2). The bottom layer was the organic layer. The organic extracts were then combined. About 1-2 g of anhydrous sodium sulfate was added to organic extractions to remove any suspended aqueous solution in the organic extract. Then, the solution was decanted into a pre-weighed 50 mL round bottom flask. Finally, rotatory evaporation was performed at 4 rotations in warm water to remove methylene chloride. Part IV. Rotatory Evaporation

The flask was attached in such a way that the solution could be heated by the hot water bath. The flask was rotated at 4 rotations per second which caused the solution to spread out over the flask. Rotation helped prevent bumping. In order to avoid excessive bumping of solution, the pressure was released frequently. The solution was under reduced pressure which resulted in lowering its boiling point and thus increasing the rate of evaporation. The product was now weighed and percent yield was calculated. Part V. Derivatization 1.3 ml of final product, 3-hydroxy-3-methyl-2-butanone was added into a test tube with 1g of semicarbazide hydrochloride and 1.6 g of sodium acetate dissolved in 5 mL of water. It was shaken vigorously and the crude solids were collected by performing vacuum filtration using. A piece of filter paper was placed in the funnel to cover the holes of the funnel. The liquid impurities go down the filtration flask as the crystalline solid gets collected in the funnel. Part VI. Recrystallization of Semicarbazone About 0.5 g of crude solids were then added to 5 ml of hot 2-Propanol. The mixture was then allowed to cool to room temperature and it was placed in ice bath for recrystallization. The final crude solid was collected after performing vacuum filtration again. Then, the melting point of solid without the recystrallization was taken in a melting point apparatus. Melting Point and Spectroscopy Due to some issues with the recrystallization, the product before the recrystallization was used to determine the melting point using the Mel-temp apparatus. The sample was transferred into a melting-point capillary tube which was placed into the melting point apparatus and the heat was gradually turned higher while both solid compounds were observed through the lens. The temperature interval at which solids melted was recorded. IR Spectroscopy test was performed of 3-hydroxy-3-methyl-2-butanone. Data Acquisition/ Calculation: The percent yield will help in determining the success of this experiment. The formula for calculating the percent yield as follows,

Percent Yield = [(Actual Yield) / (Theoretical Yield)] x 100 Compound Density Molar Mass 2-methyl-3-butyn-2-ol (1) 0.8608 g/mL 84.118g/mol 3-hydroxy-3-methyl-20.971 g/mL 102.13 g/mol butanone (3) semicarbazide hydrochloride 111.53 g/mol Semicarbozone (8) 155.19 g/mol Reference from: http://www.chemsynthesis.com/base/chemical-structure-14072.html Theoretical Yield: 3.6 ml 2-methyl-3-butyn-2-ol (0.8608 g/ml) (84.118g/mol) = 0.0368 mols of 2-methyl-3-butyn2-ol 0.0368 mols of 2-methyl-3-butyn-2-ol (1 mol of 2-methyl-3-butyn-2-ol/ 1 mol of 3-hydroxy-3methyl-2-butanone) (102.13 g/mol 3-hydroxy-3-methyl-2-butanone) = 3.76 g 3-hydroxy-3methyl-2-butanone

Actual Yield: Mass of empty flask = 29.76 g Mass of the flask + product = 31.25 g Mass of the product (3-hydroxy-3-methyl-2-butanone) = (Mass of the flask + product) – (Mass of empty flask) = 31.25 g – 29.76 g = 1.49 g 3-hydroxy-3methyl-2-butanone Percent Yield: [(1.49g) / (3.76g)] x 100 = 39.6% Semicarbazone derivative: Semicarbazide + 3-hydroxy-3-methyl-2-butanone = semicarbazone (8) Theoretical Yield: (1g semicarbazide)( 111.53 g/mol)(1 mol carbazone/1mol carbazide) = 0.0089 mol semicarbazone (1.3 mL 3-hydroxy-3-methyl-2-butanone)( 0.971 g/mL)( 102.13 g/mol)(1 mol carbazone/1mol 3hydroxy)= 0.0123 mol semicarbazone Limiting reactant is semicarbazide (0.0089 mol semicarbazone) (155.19 g/mol) = 1.39 g semicarbazone

Actual Yield of semicarbazone: 0.01 g Percent Yield: [(0.01 g) / (1.39 g)] x 100 = 0.72 %. Melting Point The melting point was obtained to confirm the purity of the collected crystal and if the Markovnikov rule was carried out by hydration reaction. Substance Semicarbazone

True melting Point (°C) 162 – 163 °C

Experimental Melting Point (°C) 155 – 160 °C

Infrared Spectroscopy The infrared spectroscopy was performed to determine the presence of functional groups in the final product. Frequency 3454.17 cm-1 2978.39 cm-1 1708.42 cm-1

Intensity Broad, Short Short Strong, Sharp

Functional Group(s) O-H (Alcohol) Hybridized sp3 C-H bond C=O (of Ketone group)

Conclusion: The purpose of the lab is to prepare 3-methyl-3-hydroxy-2-butanone by hydration of 2-methyl-3butyn-2-ol. The preparation process includes the usage of techniques such as heating under reflux, steam distillation, rotary evaporation, vacuum filtration, and recrystallization. Moreover, the final product will also be analyzed by IR spectroscopy, semicarbazone derivativatization, and melting point determination. All these methods were successful because the final product was achieved at the end. Due to IR peaks, it was verified that the product resulted after hydration was at the end was 3-methyl-3-hydroxy-2-butanone (3) showed three peaks: a short, broad O-H peak at 3454.17cm-1, a short sp3 hybridized C-H bond peak at 2978.39 cm-1, and a strong, sharp carbonyl peak of the ketone at 1708.42 cm-1. Even the melting point determination further identified the isolated pure compound semicarbazone by melting at 155 – 160 °C. The produce was derived from 3-hydroxy-3-methyl2-butanone. The melting point range was close to the original melting point range. This confirms that our derivatization of semicarbazone was successful through reaction based on Markonikov rule. The other major identification was that if the reaction gone under anti-Markonikov rule then, the melting point range for the semicarbazone would have been close to 222-223°C. Similarly, percent yield specifically examined the amount of product produced and how efficiently. The percent yield was calculated twice; first percent yield was calculated after steam distillation and rotatory evaporation were performed, and second percent yield was calculated after derivatization of semicarbazone. The percent yield of 3-hydroxy-3-methyl-2-butanone was 39.6%. The reason why the entire formed product was not recovered could be that some of the

ketone could have been lost in the transformation as the experiment procedure involved several steps or could have evaporated due to its high volatility. The percent yield of semicarbazone was only 0.72%. Such low percent yield suggests that we might have made errors while performing derivatization process. Also while performing vacuum filtration after derivatization, we didn’t obtain any of the crystalline product. Therefore, derivatization was performed twice followed by vacuum filtration and this might have reduced the quantity of our product. Despite having low percent yields of the final products, overall our experiment was successful because our data for the IR and melting point range were mostly accurate. Therefore, hydration of 2-methyl-3-butyn-2-ol to obtain 3-hydroxy-3-methyl-2-butanone was successfully performed and the purity of our final product was verified by IR, derivatization, and melting point analysis. Reference...