SYNTHESIS OF AN ALKYL HALIDE F. F. PABLO1 AND A. C. TARRAYO2 1
DEPARTMENT OF CHEMICAL ENGINEERING, COLLEGE OF ENGINEERING UNIVERSITY OF THE PHILIPPINES, DILIMAN, QUEZON CITY, PHILIPPINES 2 DIVISION OF NATURAL SCIENCES AND MATHEMATICS UNIVERSITY OF THE PHILIPPINES VISAYAS – TACLOBAN COLLEGE, TACLOBAN, PHILIPPINES DATE PERFORMED: JUNE 25, 2015
ABSTRACT This experiment demonstrates a unimolecular nucleophilic substitution (SN1) reaction through the synthesis of tert- butyl chloride (C4H9Cl),), an alkyl halide. The reagents for synthesis were tert- butyl alcohol (C4H9OH) and concentrated hydrochloric acid (HCl). Cold concentrated HCl was used to prevent the production of an alkene through a unimolecular elimination reaction. Excess HCl was added to push the reaction forward while saturated sodium chloride was utilized to squeeze out the organic molecules and pull water molecules into the aqueous layer. The addition of solid sodium bicarbonate ensured that unreacted acid were neutralized and avoided water which could cause counter productivity if aqueous solution was used likewise. Anhydrous calcium chloride was added to remove unreacted alcohol though complexation. Using a simple distillation set- up, the crude product was purified at temperature range between 49 oC - 52 oC and with the addition of boiling chips which promoted even boiling, facilitated bubble formation and prevented superheating. With 9.536% yield, this experiment gives appreciation on the small scale production of an alkyl halide. Conducting other techniques such as halogenation and hydrohalogenation in producing alkyl halides are recommended for future experiments.
INTRODUCTION Alkyl halides, denoted by the general formula R–X, are organic compounds with a halogen attached to at least one end of an sp3 hybridized carbon atom. These organic compounds are important in the laboratory and in the industry because they are used as initial substances in the synthesis of a number of organic compounds. Alkyl halides serve as solvents for relatively nonpolar compounds.1 Insecticides, coolants and monomers in polymer production are also alkyl halides.2 CFC or chlorofluorocarbon, which is used as a refrigerant and a cleaning solvent is also an alkyl halide even though it's a major cause for ozone depletion.3 As one can see, synthesis of alkyl halides is important due to its variety of uses. Thus, the method of producing alkyl halides must be considered for reaction optimization. The most common way of synthesizing alkyl halides is by reacting an alcohol with hydrogen halides. This creates a reaction called "nucleophilic
substitution reaction", where a nucleophile, an electron-rich group and also the halide ion, replaces a leaving group, the hydroxyl group, in an organic compound. The net equation for this reaction is shown in Figure 1.
Figure 1. The reaction of an alcohol with a nucleophile A polar bond exists between C and O in the alcohol. The rate of the reaction is slowed down by this strong bond since the bond makes it difficult for the halide to replace the leaving group. This reaction is the rate determining step and produces two types of nucleophilic substitution reactions, the unimolecular nucleophilic substitution (SN1) and the bimolecular nucleophilic substitution (SN2).4 Although they produce the same products, the type of carbon where the leaving group is attached is where the predominant mechanism depends.
Since the decomposition of the alcohol is the rate determining step, the rate depends only on the concentration of the alcohol. Thus, the rate of the reaction is not affected by the excess halide ions. The rate of this type of reaction can be calculated using the following equation: Rate = k[ROH]
(1)
where k is the rate constant and [ROH] is the concentration of the alcohol. In this experiment, the production of Tert-butyl chloride (C4H9Cl), a volatile colorless liquid, is done by the reaction of tert-butyl alcohol (C4H9OH), a tertiary alcohol, and hydrochloric acid. The net equation is given by Figure 2.
Figure 2. The reaction between HCl and tertbutyl alcohol Tert-butyl chloride is used in the synthesis of many other organic compounds, such as alcohol and alkoxide salts. Production of pesticides and perfume are also other uses of tert-butyl chloride.5 The effectivity of SN1 reactions as a method of synthesizing alkyl halides along with the effectivity of distillation as a purification method were tested in this experiment. The boiling point of the alkyl halide product was also experimentally determined by noting the constant temperature at which the solution boils.
This setup was allowed to stand for 20 minutes, since one of the two reactions composing the overall process is slow. Saturated sodium chloride was added to facilitate separation of layers. It squeezed out the organic molecules and pulled water molecules into the aqueous layer. A drop of water was added to determine which one was the aqueous layer. The aqueous layer was then discarded and the organic layer was transferred into a dry flask followed by the addition of solid NaHCO3. NaHCO3 was added to neutralize HCl that may be present in the solution. NaHCO3 was added as a solid because water present in an aqueous NaHCO3 solution will cause the tert-butyl chloride to form back into tert-butyl alcohol and chloride ions.1 The resulting solution was decanted into another dry flask. The filtrate was dried with a small amount anhydrous CaCl2. Adding anhydrous CaCl2 removes water as well as unreacted alcohol from the filtrate by reacting Ca2+ to form a complex with oxygen-containing compounds such as alcohols.6 Boiling chips were added to the resulting filtrate to avoid superheating the solution. Boiling chips contained pores which provided spaces where solvent vapor can form. This step also prevented bumping and loss of solution.7 Simple distillation was used to separate the tert-butyl chloride from the filtrate . A distillation setup similar to the one used in the experiment is shown in Figure 3.
METHODOLOGY Ten mL of tert-butyl alcohol and 20 mL cold concentrated HCl were mixed in a 30-mL separatory funnel followed by gentle stirring. Cold HCl was used to prevent the dehydration of the alcohol. Dehydration of the alcohol will consequently lead to the formation of isobutene (2methylpropene).1 Also, using cold HCl will lessen the possibility of evaporating the volatile product and the possibility of lowering the calculated percentage yield of the alkyl halide.
Figure 3. Simple distillation set-up8
The solution and the boiling chips was transferred into the distillation flask and the flask was then submerged in a water bath. The temperature was determined by a thermometer inserted on the topmost hole of the three-way distilling adapter. The tip of the thermometer was kept from touching the sides of the adapter and the solution to avoid erroneous temperature readings. As the vapor moves from the distillation flask to the receiving flask, water was continuously passed through the condenser. The vapor was cooled by the water flowing through the condenser by absorbing the heat from the vapor. As the vapor cools down, it becomes denser and starts to settle on the receiving flask. To hasten the condensation of the remaining vapor, the receiving flask was submerged in an ice bath. Upon getting 1 mL of distillate, the distillate was discarded to remove impurities from the distillation. Finally, the liquid that evaporated from 49oC to 52oC was collected and its volume was measured. RESULTS AND DISCUSSION In the reaction between HCl and tert-butyl alcohol, HCl was added in excess. This was done to push the equilibrium forward. According to Le Chatelier’s Principle, when stress is added to a system, the system will proceed to the direction in which the stress is minimized. The mechanism for the reaction that occurred in this experiment is SN1, since tert-butyl alcohol is a tertiary alcohol. A tertiary carbocation and a hydroxide ion was formed by the heterolytic cleavage of tert-butyl alcohol. The chloride from the hydrochloric acid attacked the tertiary carbocation, resulting to the formation of tert-butyl chloride. This reaction mechanism is shown in Figure 4.
Figure 4. Mechanism for the reaction of C4H9OH with HCl via SN1 Tert- butyl chloride undergoes solvolysis reaction when it is dissolved in a polar solvent. To prevent this reaction with water, a polar solvent, saturated NaCl was added so that the latter would solvate Na+ and Cl- ions, hence liberation of Cl from Tertbutyl chloride is prevented. A drop of water was added to determine which one was the aqueous layer. The number of moles of tert-butyl chloride formed is equal to the number of moles of tert-butyl alcohol consumed since the mole ratio of the alcohol to alkyl halide is 1:1 and tert-butyl alcohol is the limiting reactant. This relationship is shown by this equation: molC4H9Cl = molC4H9OH
(2)
Volume, density and molecular weight are needed to solve for the theoretical amount of tert-butyl alcohol. These are listed in the following table: Table 1. Some properties of tert-butyl alcohol9
Volume Density Molecular weight
10 mL 0.7858 g/mL 74.12 g/mol
The amount of tert-butyl alcohol in moles can be calculated using this equation: volume * density
molC4H9OH(theo) = molecular weight
(3)
By using the previous equation, the calculated theoretical yield of tert-butyl chloride is equal to 0.106 moles which is also equal to the amount of tert-butyl alcohol consumed. Tert-butyl chloride has a molecular weight of 92.57 g/mol. Thus, the mass of tert-butyl chloride produced theoretically is 9.812 g. The boiling points of tert-butyl chloride and tertbutyl alcohol are 49oC and 82.41oC, respectively. With this difference between the two boiling points, distillation would be considered as a purification process.
At a temperature range of 49oC to 52oC, only tertbutyl chloride is expected to vaporize since its boiling point is 50.7oC. Thus, the 4.5 mL of liquid collected is presumed to be pure tert-butyl chloride. Tert-butyl chloride has a density of 0.851 g/mL. Multiplying the density with the volume collected will give the amount of 0.9361g as yield. The equation in calculating the percentage yield of the tert-butyl chloride is: % yield =
experimental yield theoretical yield
x 100%
(4)
Using the values obtained from earlier, the percentage yield was calculated to be 9.536%. This is considerably low for the synthesis of tert-butyl chloride. This may have been caused by several factors.
Figure 5. The elimination reaction of tertbutyl carbocation1 The presence of this reaction is proven by the evolution gas, especially in the separatory funnel. The synthesis of di-tert-butyl ether from 2 moles of tert-butyl alcohol may have been another side reaction that occurred. This reaction is called an alcohol condensation reaction. The mechanism is shown in Figure 6.
One of these factors, which also might have been the first error to occur, is the instrumental error from the glassware used, whether the glassware were damaged or downright faulty. Parallax error may have been committed when reading measurements. Also, not the entire organic layer may have been collected. Another factor is that not all the tert-butyl alcohol reacted with excess HCl. When transferring the solutions to next container, it was unavoidable to leave some of the solution behind with the previous container. Meanwhile, some of the the tert-butyl chloride may have formed back into tert-butyl alcohol and chloride ions due to the trace amounts of water in the distillation setup. Another factor is that not all the tert-butyl chloride vapor may have condensed early enough to be collected. Thus, some of the uncondensed tertbutyl vapor was not accounted for. Side reactions may have also occurred. For one, an elimination reaction might have occurred where the carbocation intermediate may have lost one hydrogen atom, forming gaseous isobutene (2methylpropene).1 This reaction is shown in Figure 5.
Figure 6. Alcohol condensation reaction11 These side reactions lowered the amount of tertbutyl alcohol available for reaction. Thus, lowered the percentage yield of tert-butyl chloride as well. In the experiment, precautions were made to avoid these side reactions as much as possible. One of
which is the use of cold HCl. Cold HCl avoided the elimination reaction of the carbocation intermediate. Another precaution was that the experiment was carried out at room temperature to avoid the formation of isobutene. CONCLUSION AND RECOMMENDATION The experiment aims to synthesize an alkyl halide from tert-butyl alcohol and hydorochloric acid. With that in mind, the experiment was successful in producing an alkyl halide even though the percent yield was only 9.536%. The method still proves to be effective since the errors are easy to detect. The experiment can still be improved in many ways. For one, take note of the equipment/materials to be used. Check if the equipment/materials need to be fixed or need to be replaced. Be cautious when taking measurements. When converting the alcohol to alkyl halide, it would be recommendable to increase the reaction time in order to make reaction more complete. Likewise, the time for condensation of tert-butyl chloride must also be increased to allow more of the vapor to condense. Maintaining an acidic system will also improve the experiment since the formation of unwanted isobutene is hastened by a basic system.1 The reaction of alcohols and halide ions is not the only way to produce alkyl halides. Halogenation and hydrohalogenation of alkanes, alkenes and alkynes can also form alkyl halides.10 Although these methods also produce alkyl halides, the alcoholhalide reaction is still the most favored since it requires less energy and money. Also, chlorine has a low selectivity and isomerism occurs when synthesizing chloroalkanes via halogenations and hydrohalogenation.10 REFERENCES [1] Solomons, T. W. G.; Fryhle, C. B. Organic Chemistry, 10th ed.: John Wiley & Sons, Inc.: USA, 2010; pp. 230-284 [2] Khramkina, M.N. Laboratory Manual of Organic Synthesis. Mir Publishers: Russia, 1980; pp.49-50, 123-149.
[3] Chemguide. Uses of Halogenoalkanes. http://www.chemguide.co.uk/organicprops/halo alkanes/uses.html [4] Mcmurry, J. Organic Chemistry, 7th ed.: Thomson Learning, Inc.: USA, 2008; pp. 333, 372381 [5] Triveni Interchem Pvt. Ltd. Chlorotrimethylethane. http://www.trivenigroupindustries.com/chlorotri methylmethane--1010209.html [6] University of the Philippines Diliman Institute of Chemistry. Organic Chemistry Laboratory Manual. 2008 [7] University of Colorado at Boulder Department of Chemistry and Biochemistry. Boiling Chips. http://orgchem.colorado.edu/Technique/Proced ures/Distillation/Boilingchips. [8] University of Missouri–St. Louis Department of Chemistry and Biochemistry. Distillation. http://www.umsl.edu/~orglab/documents/distill ation/dist.html [9] Tert-butyl alcohol; CAS No.: 75-65-0 [Online]; ScienceLab Chemicals & Laboratory Equipment http://www.sciencelab.com/msds.php?msdsId=9 923195 [10] Wade, L. G. Organic Chemistry, 8th ed.: Prentice Hall: USA; pp. 226 - 230 [11] University of Calgary. Departmartment of Chemistry. http://www.chem.ucalgary.ca/courses/350/Carey 5th/Ch15/ch15-4-4-1.html
