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Preparation And Purification Of An Alkyl Halide Grefaldia, Camille E.a,, Gonzales, Anne Nicole S. a aCollege
of Home Economics, University of the Philippines Diliman, Quezon City, Philippines
A B S T R A C T Alkyl halides are essential in everyday lives and serve many purposes in different institutions such as in homes, hospitals, schools, etc. They can be synthesize from various methods although preparation from alcohol is the most generally used. The purpose of the experiment was to synthesize tert -butyl -butyl chloride from tert -butyl -butyl alcohol by reacting with concentrated HCl and then purifying using simple distillation. Tert-butyl Tert-butyl alcohol and concentrated HCl were placed and mixed in a separatory funnel where two layers of liquid formed, one was the organic layer where the tert -butyl -butyl chloride was in and the other layer is the aqueous layer. The two layers can be determined using a water drop test. The layers were then separated. The organic layer was reacted with solid NaHCO 3 and then anhydrous CaCl 3 was added. The compound was then purified by simple distillation. The theoretical yield was 9.74g and the yield obtained was 0.11g giving a percent yield of 1.13%.
1. Introduction Organic chemistry is the study of structure, properties, composition, reactions and preparations of carbon-containing compounds. A lot of carboncontaining compounds are synthesized because they are useful in variety of different industries. For example, in pharmaceutical industry, organic chemists synthesize new compounds that are potential drugs use to treat diseases. Alkyl halides are abundantly found in nature and are used in variety of everyday products such as aerosol propellants, refrigerants, anaesthetics and PVCs. Preparation of alkyl halides from alcohol is an experiment where the reaction uses an alcohol and a hydrogen halide as the starting materials for preparing an alkyl halide. Hence, the objective of this experiment is to synthesize an alkyl halide from a tertiary alcohol and hydrogen halide along with the determination of the yield and purification of the sample through simple distillation. (Carey & Guiliano, 2014) 2. Materials and Methods The materials and apparatus used were: tert -butyl -butyl alcohol, concentrated HCl, solid NaHCO 3, icebath, 50-mL separatory funnel, semi-micro distillation set-up, 50-mL Erlenmeyer flask, thermometer, hot plate, oil bath, rubber tubing and aluminum foil. *Corresponding author. Mobile: 09062510901 E-mail address: justinbirdwell@yahoo.
[email protected] com
10 mL tert -butyl -butyl alcohol and 20 mL cold concentrated HCl were placed in a dry 50 mL separatory funnel and swirled gently, gently, then then from time to time relieving pressure by opening the stopcock (Fig 1, Appendix). The ratio 10:20 was to make sure that the alcohol reacts completely with HCl. The mixture were allowed to stand for 20 minutes in order to make sure that the organic compound and aqueous compound go to their respective layer. 1 drop of NaCl was added to facilitate the separation of layers. The aqueous layer was discarded (Fig 2, Appendix). The organic layer was determined by performing a water drop test to any of the separated layer and the layer in which the water dissolved in was the aqueous layer. The organic layer was then transferred into a dry flask that contained a small amount of solid NaHCO 3 and swirled gently then decanted into another dry flask. The collected filtrate was then dried with the small amount of anhydrous CaCl 2. The crude tert -butyl -butyl chloride was then decanted into a dry 25 mL round bottom flask and then distilled with the simple distillation set-up shown in the appendix figure 3. The simple distillation set-up was prepared making sure that the water flowed into the bottom of the condenser’s cooling jacket and out from the top and a thermometer bulb placed just below the side arm of the distillation head. The round-bottom flask of appropriate volume was used and big enough so the sample filled ½ or 1/3 of its volume. The flask was
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then placed in an oil bath to regulate the temperature. The sample was then placed in the flask along with 2-3 pieces of boiling chips making sure that the sample liquid was not hot. It was made sure that the water flows through the condenser and there were no loose glass joints and that they were all well-fitted. The sample was gently heated to a gentle boil until the temperature remained constant at the boiling point. The boiling temperature was recorded. The vapors and condensate passed through the side arm into the condenser, where most of the vapor condensed into liquid, and dripped from the adapter into the receiving flask. The heat was adjusted to that distillation occurred at a rate of 2 drops distillate per second. The first few milliliters of distillate were discarded and the fraction the boiled off at 49-52 degree Celsius were collected in a pre-weighed vial cooled in an ice bath. It was made sure that it was not distilled to dryness. The sample was removed from the heat source when the sample started to boil. The distillation set-up was let to cool before disassembling it. The yield was then determined and the solubility of the purified product to water was tested. (IC, 2014)
enough rates therefore a more reactive HBr is used and the reaction takes place at a higher temperature to increase the rate of reaction compare to the reaction between a tertiary alcohol and HCl. The reaction between an alcohol and an alkyl halide is a type of substitution reaction. The functional group in alcohol which is a hydroxyl group –OH is replaced or substituted by a halogen, usually Cl or Br from the alkyl halide. However, the type of reaction does not tell us the mechanism of the reaction or the step-by-step method that gives rise to the products.
3. Results and Discussion Alcohols react with hydrogen halides according to the general equation: R – OH + H – X
R – X
+ H – OH
R represents the alkyl group and X represents a halogen. (IC, 2014)
Figure 4. SN1 reaction mechanism of tert -Butyl alcohol and hydrogen chloride
A hydrogen containing halide is always acidic because it can donate a proton. The order of reactivity of the hydrogen halides correlates their acidity: HI > HBr > HCl >> HF. HI is used infrequently and HF is not a useful reagent for the preparation of alkyl fluorides. Moreover, tertiary alcohols are observed to be the most reactive and primary alcohol is the least reactive: R 3COH > R2CHOH > RCH2OH.
Tertiary alcohols such as tert -butyl chloride undergo SN1 reaction as shown in Figure 4. The mechanism for this reaction is composed mainly of three main steps or series of equations.
Tertiary alcohols readily react with HCl at room temperature and produces alkyl halides in high yield within minutes. On the other hand, secondary and primary alcohol do not readily react with HCl at fast
Step 1 involves proton transfer. Like water, alcohols have the ability to donate a proton from oxygen and also accept a proton on oxygen. A proton transfer in water gives oxonium ion/ hydronium ion, H 3O+, in alcohol it gives an alkyloxonium ion, ROH 2+. This step involves two molecules, HCl and (CH 3)3COH that undergo chemical change, thus the proton transfer is said to be bimolecular. The intermediate formed is tert -Butyloxonium ion which is not a
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reactant nor a product but rather it lies on the pathway from reactants and products. Step 2 in Figure 4 shows a reaction in which formation of a carbocation (an ion containing a positively charged carbon) occurs. The alkyloxonium ion dissociates to a molecule of water and a carbocation. Only one chemical species undergoes a chemical change, therefore this step is said to be unimolecular. The intermediate in this step, tert -Butyl cation is a relatively unstable species which is too unstable to be isolated. The reason for its instability is that the carbon in the carbocation does not follow the octet rule and only has 6 electrons in its valence shell. However, a carbocation makes the species electrophilic or electron-loving which makes it vulnerable to a nucleophilic attack, which occurs in step 3. (Carey & Guiliano, 2014) Nucleophiles are Lewis bases that react with electrophiles because they have an unshared pair of electrons ready to form bonds. The nucleophile which is Cl-. This step is bimolecular because there are two species involved in the reaction which is the carbocation, tert -butyl cation and the nucleophile, Cl-. (McMurry, 2012) Step 2 which is the formation of the carbocation from the oxonium ion is the slowest step of the three steps and is the point of highest energy in the energy diagram throughout the whole reaction so it is called the rate-determining step. The reaction between tert -Butyl alcohol and hydrogen halide is said to follow an SN1 which is proposed by Ingold. S N stands for substitution nucleophilic follows by 1 or 2, 1 means that the reaction is unimolecular because its slow determining step only involves one species. (Carey & Guiliano, 2014) In the experiment, the separatory funnel used was closed off with a stopcock. It was necessary as well as important to perform the experiment in a closed system because tert -butyl chloride is a highly volatile organic compound and would evaporate in an open system ultimately resulting in a lower percent yield and percent purity simultaneously. The percent yield calculated(Appendix) of tert -butyl chloride from this experiment is 1.13% which is extremely low. This could have been a result of
various errors. One source of error could have been the formation of side products from side reactions. For example, one side product that might have formed is di-tert -butyl ether. This is shown by the mechanism below.
Figure 5. Di-tert -butyl ether mechanism Another side product that might have formed is 2methylpropene as shown in the mechanism below:
Figure 6. 2-methylpropene mechanism Again, the presence of these side products would have competed with the target product and resulted in a lower yield of tert -butyl chloride. In order to limit or prevent the formation of these possible side products, it was made sure that the temperature remained at a high temperature to make sure that these side products would not form and only tert butyl chloride would be collected. Moreover, the concentrated HCl used was cold and this was to prevent the formation of 2-methylpropapane. Also, HCl being cold was to prevent volatilization of organic compound and to avoid the sudden release of heat which is really important because boiling chips would have avoided this however, boiling chips were not used. Excess, concentrated HCl was used in order for the alcohol to completely react with it and become thoroughly solvated, This would have been another source of error if HCl was not cold enough to have prevented these from occurring. Another reagent used was NaHCO 3 which is commonly used in experiments to neutralize acid spills. It was made clear that solid NaHCO 3 were to be used instead of an aqueous sodium bicarbonate as to prevent the addition of water in the sample
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solution by hydrolysis. Furthermore, anhydrous CaCl2 was used to remove excess water. This was a necessary step to prevent inaccurate results as the presence of water would have resulted in a higher yield than what was actually expected and this would cause a miscalculation in actual yield and theoretical yield. Another method used to prevent inaccuracies was in the simple distillation set-up itself. It was made sure that the water flowed continuously all throughout the distillation part of the experiment. This was important because it ensured that the condenser was kept warm throughout and made sure that the all the vapour condensed into the liquid and collected in the receiving flask. Otherwise, less condensation meant less yield of tert -butyl chloride. (Bruice, 2014) Some common possible errors were human errors in reading of the temperature on the thermometer, contamination of the reagents and the reagents might not have been up to standards and had low quality. These could all have resulted in discrepancies in the results and the reason why the yield is 0.11g and percent yield, 1.13% is very low especially if compared to the calculated theoretical yield, 9.74g. 4. Conclusion and Recommendations The objective of the experiment of synthesizing tert -butyl chloride was met because we were able to collect a small amount of the target product. However, the yield obtained in this experiment was extremely low. Hence, the experiment was deemed inaccurate in obtaining a higher yield because of all the possible limitations and sources of error during this experiment. Therefore, a more careful and thorough experiment should be done. Moreover, other alternative methods in purifying alkyl halide could be performed. For example, instead of using simple distillation, fractional distillation could be use which is essentially similar to simple distillation except that there is a fractioning column (Figure 6, Appendix) placed between the boiling flask which is filled with glass or plastic beads improving the separation of liquids being distilled. Moreover, an alternative in preparing an alkyl halide was to react a primary or secondary alcohol to thionyl chloride,
hydrogen bromide, hydrogen iodide, phosphorus(III) chloride, or phosphorus (IV). Any of these alternatives might result in a better yield.
References [1] Carey, Francis A., Giuliano, Robert, M., 2014. Organic Chemistry 9 th Edition. McGraw-Hill, New York [2] Bruice. P. 2014. Organic Chemistry 7 th Edition. Pearson, New York [3] Clark, Jim. 2003(modified in 2015). Replacing the –OH group in Alcohols by a Halogen. http://www.chemguide.co.uk/organicprops/alcoh ols/halogen.html. Retrieved on September 21, 2017. Web. [4] Institute of Chemistry(IC), 2014 edition. Organic Chemistry Laboratory Manuel. University of the Philippines. Diliman [5] McMurry, John E. 2012. Organic Chemistry, Chapter 11: Reaction of Alkyl Halides: Nucleophilic Substitutions and Eliminations. Cengage Learning
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Appendices
Figure 1. Holding and Venting a Separatory Funnel
Figure 2. Draining the Lower Layer
Figure 3. Simple Distillation Set-up
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Calculations mass of tert -butyl alcohol(g) = density x volume = 0.78g/mL x 10 mL = 7.8g mmol of tert -butyl alcohol(mmol) = mass / molar mass = (7.8g/1000)/74.12mg/mmol = 105. 23mmol mass of tert -butyl chloride(g) = weighed vial after experiment – pre-weighed vial = 33.67g -33.56g = 0.11g mmol of tert -butyl chloride(mmol) = mass/molar mass = (0.11g/1000)/92.57mg/mmol = 1.2mmol theoretical yield(g) = [mol tert -butyl alcohol x (1 mmol tert-butyl Cl/ 1 mol tert -butyl alcohol)] x molar mass = 0.10523 mol x (1/1) x 92.57g/mol = 9.47g percent yield(%) = (actual yield/ theoretical yield) x 100% = (0.11g/9.74g) x 100% = 1.13%
