The Main Focus of Experiment 4
The main focus of Experiment 4 was the electrophilic addition of 2, 3-dibromo-3-phenylpropanoic acid to alkenes. It was deduced from the testing procedures that cold chloroform is utilized because of its capacity to dissolve cinamic acid. However, bromine compounds are dangerous molecules that require extreme caution while handling. The process for making 2, 3-dibromo-3-phenylpropanoic acid was mostly based on the neuclephilic addition property, which states that electrophiles rapidly take electrons that are given to them by extremely electron-rich alkenes. Through the experiment, it was concluded that the desired product was successfully obtained by using Cinamic acid and Bromine as the starting materials.
Introduction
An electrophilic addition to alkenes is a chemical reaction that involves breaking of the pie bonds in a chemical compound preceded by formation of a new sigma bond (Afonso, 2017, p.27). An electrophile which is an electron deficient molecule or atom becomes the driving force for the reaction to take place since it readily accepts the pair of the donated electrons. Alkenes as such undergo addition reaction in two-steps. The first step involves breaking of the pie bond of the alkenes and the pair of the electrons donated to the electrophile where they are readily accepted leading to the formation of a carbocation which is an intermediate (Atwood, Davies, & Osa, 1985, P.45). The second step involves the carbocation combining with a second reactant usually a nucleophile that is electron rich to form second covalent bond (Coelho and Sousa 2016,p.34). The aims of the experiment 4 is to use the knowledge of electrophilic addition reaction of alkenes to prepare 2, 3-dibromo-3-phenylpropanoic acid, learn how to treat and handle hazardous materials, separation using the Buchner apparatus, and learn about recrystalization as a purification technique and determine reactant’s melting point.
Method
Apparatus and Reactants
- Bromine in chloroform solution
- Dilute Thiosulphate solution in case of bromine spillage
- Test tube and bung
- Test tube holder (tray)
- Cinnamic acid
- Chloroform
- 50ml conical flask
- Retort stand and clamp
- Steam bath
- Water bath
- Buchner funnel, flask and filter paper
- Pasteur pipette
- 3x150ml conical flask
- Hotplate
Experimental Procedure
1.0 ml of aliquot of bromine was run in chloroform solution in test tube in the fume hood. The test tube was corked with a rubber bung and placed in the test tube rack and stored safely for subsequent use. 1.0 grams of cinnamic acid was dissolved in 5.0 ml chloroform in a 50 ml flask through warming the mixture gently over s team bath. When the solid completely dissolved, the flask was continuously swirled in a beaker of ice until some small crystals begun to separate. Bromine solution from the test tube was immediately added to the conical flask in a single portion.
After 10 minutes, the bromide product precipitated. The solid precipitate was collected by suction filtration using the Buchner Funnel and the flask properly and safely clamped. The solid product was washed with 2ml of cold chloroform squirted over the solid with a Pasteur Pipette. The crude product was recrystallized from a minimum quantity of aqueous alcohol that was dissolved in warm alcohol then spotted in warm water. The purified product was collected in a cleaned Buchner Funnel and flask. The product was allowed to dry in air for a few minutes then transferred into a preweighed sample container. The product mass was weighed and recorded.
Results and Discussion
The principle the experiment adopts for successful preparation of 2, 3-dibromo-3-phenylpropanoic acid is based on the electrophilic addition reactions (Indian Chemical Society, 1928, p.56). Alkenes in the chemical reaction function as the nuclephiles. Since they are highly rich in electrons, they attack the electrophiles in a two step process to result into the desired product.
The pie bond of the alkenes breaks and is transferred to the bromine molecule which then dissociates to form a free radical which is used in the second step to further react with the intermediate. Imperative to note is that the end product is stereospecific and 2 and 3 bromines are in trans-position.
During the experiment, melting point started at 1390 C and ended at 1350 C. 0.98 g of 2, 3-dibromo-3-phenylpropanoic acid was prepared. Bromine is a hazardous chemical thus was handled with excessive care during the experiment. In case of any spillage on the skin, it is recommended that the area be washed properly with water followed with dilute sodium thiosulphate in a fume chamber. Cold chloroform was used in the experiment because it dissolves Cinamic acid at room temperature. Cinamic acid is colorless while bromine has intense red color. The addition reaction is quite faster and the red color that mixture assumed immediately disappeared. The reaction proceeded and a precipitate began to form.
The traces of bromine were removed by dissolving in sufficient amount of cyclohexene. 1, 2-dibromocyclohexane is as well soluble in chloroform, thus, did not interfere with the isolation of the major product. Consequently, suction filtration was used to isolate the desired product. The dichloromethane is a volatile product thus was immediately dried in air and respective melting points aforementioned above recorded (Lehman, 2008, p.89). Considering that Cinamic acid is highly soluble in cold chloroform, it was necessary that the obtained product be washed further for purposes of obtaining a pure compound. Recrystallization nonetheless, ensured that the final product had no traces of the cinamic acid. Similarly, the melting points that were measured and recorded created an opportunity for determination of the addition mode of bromine to the pie double bonds. As such, the melting points relatively confirmed that the obtained erythro configuration emanated from anti addition.
Nevertheless, analysis of the IR spectra shows certain generalizations of the final product. For instance, the lower band intensities are due to high volatility property of cyclohaxene which makes it difficult to obtain complete and clear spectrum (Lehman, 2008, p.120). It is for that reason that the carbon-carbon double bond band of absorption 1438 cm-1 is not visible. On the other hand, the aliphatic carbon hydrogen bond is strongly visible at 2930 cm-1 and at 3020 cm-1 the apportion band is for alkenes hydrogen bond (Patai, & Rappoport, 1990,p.199).
Conclusion
The experiment demonstrates electrophilic addition of alkenes where students are were to obtain major products that are streospecific. The experiment is appropriate for exemplifying the various mechanisms involved in the addition reactions. The aims of the experiment were achieved since the compound 2, 3-dibromo-3-phenylpropanoic acid was successfully prepare. In regards to handling hazardous materials, it was noted that bromine as well as pyridine are harmful chemicals that must be treated with a lot of care. The reactant’s melting point was obtained as had been objected and the experimental procedures helped in understanding how to use the Buchner apparatus and crystallization as separation methods.
References
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Coelho, P. and Sousa, C., 2016. 4.1. 1. Electrophilic Addition to Alkenes 4.1. 1.1. Bromination of Cinnamic Acid. Comprehensive Organic Chemistry Experiments for the Laboratory Classroom, p.223.
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