Introduction
In this two week project, an experiment was designed and tested. The experiment was performed to tested how a variable affects the E/Z ratio products of a Wittig reaction.
Theory
In a Wittig reaction, C=O is converted into C=C. The organophosphorus reagent or the phosphorus ylide is the nucleophile made of a positively charge phosphorus atom with three phenyl groups bonded to a negatively charge carbon atom. To form a ylide, a triphenylphosphine attacks a primary or secondary alkyl halide to form phosphonium salt which is deprotonated. To obtain the desired product in a Wittig reaction, the carbonyl compound is treated with a ylide which forms a four-member ring called oxaphosphetane. The second step is to make a more stable bond, to achieve this an elimination occurs to form Ph3P=O (triphenylphosphine oxide) and as a result, forms the alkene. The alkene can come as a trans (E) or cis (Z) isomer depending on the stability of Wittig reagents. Stable Wittig reagents are more likely to give a E isomer; meanwhile, if it is unstable that it will give a Z isomer. This type of reaction can be consider green chemistry because it reduces waste. The mechanism behind this Wittig reaction can be found at Figure 1a.
The purpose of this experiment was see how a variable affects the end result product. In this project, the
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choice for the carbonyl compound was O-tolu aldehyde and was design to see the role sterics has. The hypothesis formulated stated that the product would more likely be a Z isomer product because of the reagent, Acetonyltriphenolphosphonium Chloride has a carbonyl which allows the structure to have more resonance thus more stability. The more stable Wittig reagent is more likely to generate an E isomer. With a trans isomer the substituent groups are at opposite sides which reduces the steric hindrance clash. Given the following information, the expected product should be a trans (Z) product. Result Reagent MM (g/mol) mmol Equiv Ideal Amount Acutal Amount O-tolualdehyde 120.15 2 1 0.231mL 0.230mL Acetonyltriphenolphosphonium Chloride 354.8 2.2 1.1 0.781g 0.770 g Sodium Hydroxide 39.99 6 3 0.239 g 0.252g Discussion Due to difficulty in obtaining a clear NMR, the TA provided an NMR. The main focus was the E and Z ratio because that was the only thing that could really be analysis with this particular NMR. To identify which peak is which, the first thing to do is to look for doublets in the alkenes region. One peak that stands out is one located at 6.3 with an integral value of 1.00, which represents the E isomer. This is the E isomer because the length between the peaks is larger which is corresponding to the E isomer. The second doublet peak is around this range too and is it is located at 5.85 with an integral value of 0.025. Since the E isomer has been identified already, this is the Z isomer. To obtain the ration both integral values need to be added and divided by the integral value of each isomer. The results give an E:Z ratio of 98:2. In this reaction, the ylide--a nucleophile made of two charges attacks the carbonyl. This forms two bonds, a C-C bond and a O-P bond which generates a oxaphosphetane, a four-member ring. Through some arrow pushing, another bond between the O-P bond forms as well as a C=C bond. The Wittig reagents play a large role in the outcome of the product because the stability of these reagents will largely determine the type of (E/Z) isomers that will be produced. In conducting this experiment, no severe problems were encountered. However, an issue faced came when the extraction was done to remove the sodium sulfate because it left only nearly no solvent. This could have been avoided by decanting the solvent out and evaporating the rest. Nevertheless, it all worked out at the end but this could be an improvement implemented if this experiment was to be repeated. Conclusion In this experiment, the purpose was to conduct a Wittig reaction and analysis how the stability of the Wittig reagents contributes to the stereochemistry of the end product.
It is settled that a stable Wittig reagent will give a more an E isomer; meanwhile, a less stable Wittig reagent will give a Z isomer. By conducting this experiment, I learned how to read the NMR data to determine the E:Z ratio. Through analyzing the NMR, it is clear that the hypothesis is supported. To improve this experiment for the future, next time instead of extracting the solvent students can just decant the solvent to avoid the issue that was encounter during
lab. Figure 1a (Mechanism)
Then the reaction tube was capped but not tightly. The tube then was placed in a sand bath reflux to heat it until a brown color was formed. Then the tube was taken out of the sand bath and allowed to cool to room temperature. Then the tube was shaken until a formation of a white solid at the bottom of the tube. After formation of the white solid, diphenyl ether (2 mL) was added to the solution and heated until the white solid was completely dissolved in the solution. After heating, the tube was cooled to room temperature. Then toluene (2 mL) was added to the solution. The tube was then placed in an ice bath. Then the solution was filtered via vacuum filtration, and there was a formation of a white solid. Then the product was dried and weighed. The Final product was hexaphenylbenzene (0.094 g, 0.176 mmol,
The purpose of this lab was to perform an electro-philic aromatic substitution and determine the identity of the major product. TLC was used to detect unre-acted starting material or isomeric products present in the reaction mixture.
...teraction of the HOMO of the diene and the LUMO of the dienophile. This reaction was done at relatively low temperatures as the dry ether has a boiling point of 34.6 °C. At low temperature the endo preference predominates unless there is extreme steric hindrance, which in this case there is not. The endo product forms almost exclusively because of the activation barrier for endo being much lower than for exo. This means that the endo form is formed faster. When reactions proceed via the endo for the reaction is under kinetic control. Under kinetic control the adduct is more sterically congested, thus thermodynamically less stable. The endo form has a lower activation energy, however, the EXO form has a more stable product. As this is a symmetrical Diels-Alder reaction there is not two possible isomers of the product.
Input variables In this experiment there are two main factors that can affect the rate of the reaction. These key factors can change the rate of the reaction by either increasing it or decreasing it. These were considered and controlled so that they did not disrupt the success of the experiment. Temperature-
In a separate beaker, acetone (0.587 mL, 8 mmol) and benzaldehyde (1.63 mL, 16 mmol) were charged with a stir bar and stirred on a magnetic stirrer. The beaker mixture was slowly added to the Erlenmeyer flask and stirred at room temperature for 30 minutes. Every 10 minutes, a small amount of the reaction mixture was spotted on a TLC plate, with an eluent mixture of ethyl acetate (2 mL) and hexanes (8 mL), to monitor the decrease in benzaldehyde via a UV light. When the reaction was complete, it was chilled in an ice bath until the product precipitated, which was then vacuum filtrated. The filter cake was washed with ice-cold 95% ethanol (2 x 10 mL) and 4% acetic acid in 95% ethanol (10 mL). The solid was fluffed and vacuum filtrated for about 15 minutes. The 0.688 g (2.9 mmol, 36.8%, 111.3-112.8 °C) product was analyzed via FTIR and 1H NMR spectroscopies, and the melting point was obtained via
The results of this experiment are shown in the compiled student data in Table 1 below.
We thank the University of Oklahoma and the chemistry faculty for providing the space, instructions, and equipment for the development of this report and experiment.
The overall objective of this experiment was to perform a Wittig reaction from creating an ylide and mixing it with a carbonyl (C=O) compound, cinnamaldehyde. The completion of the reaction was confirmed ultimately from the initial TLC analysis. Since TLC separates the components of the spotted material, as long as the retention factor values were different for cinnamaldehyde, the starting reagent, and the product(s), it was evident that some of the reaction had gone to completion. However, as seen in Figure 3, there was some blurred area between the product spots. This indicated that there still existed some impurities, most likely the starting reagent, which was affecting the movement of the compounds through the solvent, petroleum
== Refer to Chemistry Lab # 2 – Investigating Changes. No changes have been made in this experiment. Results = ==
Going into details of the article, I realized that the necessary information needed to evaluate the experimental procedures were not included. However, when conducting an experiment, the independent and dependent variable are to be studied before giving a final conclusion.
The input variables are the ones that I can change in order to affect the experiment and the outcome variables are the ones I will measure to see how the input variable has affected it. Input Variables --------------- Amount of calcium carbonate Amount of hydrochloric acid Surface area of calcium carbonate Concentration of hydrochloric acid Temperature of hydrochloric acid Introduction of a catalyst Outcome variables ----------------- Amount of calcium chloride released Amount of water released Amount of carbon dioxide released Change in weight
The purpose of the experiment is to identify and understand reactions under kinetic and thermodynamic control. A reaction under kinetic and thermodynamic control can form two different types of products. A reaction under kinetic control is known to be irreversible and the product is formed quickly. A reaction under thermodynamic control is known to require rigorous conditions. It is also reversible. The final product is more stable than the product made by kinetic control. The chart below shows the two types of reaction coordinates:
Chemical kinetics is a branch of chemistry that involves reaction rates and the steps that follow in. It tells you how fast a reaction can happen and the steps it takes to make complete the reaction (2). An application of chemical kinetics in everyday life is the mechanics of popcorn. The rate it pops depends on how much water is in a kernel. The more water it has the quicker the steam heats up and causes a reaction- the popping of the kernel (3). Catalysts, temperature, and concentration can cause variations in kinetics (4).
In this lab, it was determined how the rate of an enzyme-catalyzed reaction is affected by physical factors such as enzyme concentration, temperature, and substrate concentration affect. The question of what factors influence enzyme activity can be answered by the results of peroxidase activity and its relation to temperature and whether or not hydroxylamine causes a reaction change with enzyme activity. An enzyme is a protein produced by a living organism that serves as a biological catalyst. A catalyst is a substance that speeds up the rate of a chemical reaction and does so by lowering the activation energy of a reaction. With that energy reactants are brought together so that products can be formed.
The authors has given very well stated facts and information to attract the readers so that they can easily get the main ideas from each chapter. The authors also provided the examples and experiments as a evidence to support their information. 5. What are the weaknesses of the