PREPARATION AND PURIFICATION OF AN ALKYL HALIDE Farah Iman F. Deogracias Institute of Chemistry, University of the Philippines, Diliman, Quezon City Performed February 26, 2015 Submitted March 10, 2015 Abstract Alkyl halides are halogen-substituted alkanes. They are found abundantly in nature and they serve many purposes such as anaesthetics, refrigerants, and pesticides, but they are most commonly used as a solvent. Alkyl halides can be produced from various methods but synthesis from alcohol is most generally used. In this experiment the researchers conducted the synthesis by reacting a tertiary alcohol with a hydrogen halide, thus producing a tertiary alkyl halide. 7.90 g tert-butyl alcohol was reacted with 20 mL concentrated hydrochloric acid in order to yield crude tert-butyl alcohol (theoretical yield=9.87 g). Solid NaHCO 3 was added to the mixture to prevent hydrolysis. Anhydrous CaCl2 was used to remove excess water. Steam distillation technique was done to purify the alkyl halide. By virtue of the differences in volatilities, the target compound (i.e. tert-butyl chloride) was isolated as it formed into steam and was condensed. The actual yield was 6.05 g, which is a 61. 30% percentage yield. Sources of error include the occurrence of side reactions, the misuse of the reagents (in form or amount), gaps in the distillation set-up, and human error. The synthesis of alkyl halide is important and relevant because it teaches the necessities in nucleophilic substitution reactions and how they proceed. I. Introduction II. Alkyl halides — systematically named as haloalkanes — are halogensubstituted alkanes (McMurry, 2010). In these compounds, the halogen is a substituent on a parent chain alkane. Most alkyl halides we encounter are colorless and odorless liquids that miscible in organic solvents and only sparingly soluble in water (Clark, n.d.). III. Alkyl halides serve many industrial purposes, such as their use as anesthetics in medicine (Watrous, 1947), reactants to produce glyme, which is a solvent used in batteries (Tang & Zhao, 2014), as pesticides (see Hollingworth, 1976), and as refrigerants, such as the chlorofluorocarbon (CFC) compound (see Rowland, 2006) which is probably the most familiar to the general population. IV. Among these, alkyl halides are also mostly used as solvents, not only in the industrial and commercial settings but also in the academic setting. Alkyl halides are a good family of compounds to learn substitution and elimination reactions from because of the nature of their substituents (i.e. halide ions) that are easily displaced (Bruice, 2014).
V. There are multiple ways to produce alkyl halides (e.g. through radical halogenation of alkanes, allylic bromination of alkenes, using Grignard reagents, etc.) but the most generally used method of preparation is to make them from alcohols (McMurry, 2010). The general mechanism is as follows, where X is a halogen (i.e. F, Cl, Br, or I): VI. VII. | | VIII. —C—OH + H—X —C—X + H—OH IX. | | X. XI. The mechanism involves the substitution of one nucleophile by another (McMurry, 2011). First, the hydroxyl group detaches from the alkane (thus leaving the alkane with a carbocation) and attacks the hydrogen group of a hydrogen halide, forming water and a halogen ion. This nucleophilic halogen ion then attacks the electrophilic alkane group (by virtue of its carbocation), forming the alkyl halide. XII. Based on current literature, the experimenters performed the synthesis of an alkyl halide from an alcohol in order to supplement their knowledge of alkyl
halide preparation. The experimenters also related the produced yield to the procedures and processes done. XIII. In this study the experimenters opted to use for the simplicity of analysis a tertiary alcohol (i.e., wherein a hydroxyl group is attached to a tertiary carbon) because this type of alcohol has a relatively high reactivity and thus reacts readily to form a tertiary alkyl halide (McMurry, 2010). XIV. After the preparation, the experimenters also performed a distillation technique in order to purify the alkyl halide. This is a necessary step when determining the yield. XV. XVI. Methodology XVII. Materials and Apparatus used: XVIII. tert-butyl alcohol separatory funnel XIX. concentrated HCl distillation set-up XX. solid NaHCO3 Erlenmeyer flask XXI. ice bath thermometer (max. 100 °C) XXII. hot plate water bath XXIII. rubber tubing aluminum foil XXIV. round bottom flask anhydrous CaCl2 XXV. XXVI. Procedure done: XXVII. First, 10 mL tert-butyl alcohol was placed in a 50 mL separatory funnel. Twenty milliliters of concentrated HCl was added and afterwards the mixture was swirled. Internal pressure was relieved from time to time by opening the stopcock (see Figure 1). XXVIII.
XXIX.
Figure 1. Holding and Venting a Separatory Funnel © King
XXX. The mixture was left undisturbed for 20 minutes. Layers
eventually formed. To determine which the organic layer is, two drops of water was added. The water dissolved in the layer that was aqueous – this layer was then discarded (see Figure 2). XXXI. XXXII.
XXXIII. Figure 2. Layer Separation and Draining © El-Fellah
XXXIV.The organic layer was then transferred into a dry flask containing a small amount of solid NaHCO3 and was gently swirled. The mixture was decanted into another dry flask. XXXV. The collected filtrate was dried with a small amount of anhydrous CaCl2. This was added until clumps were formed. This was done in order to remove the H 2O produced in the reaction. The crude tertbutyl chloride was then decanted into a dry 25 mL round-bottom flask. XXXVI. A simple distillation setup was prepared (as shown in Figure 3). The experimenters made sure that the water flowed from the bottom of the condenser’s cooling jacket and out from the top. A thermometer was placed in the 3-way distilling adapter, its bulb placed just below the side arm of the distillation head. XXXVII. A round bottom flask was chosen, big enough for the sample to fit in two-thirds of its volume. A water bath was used to regulate the temperature.
XXXVIII.
XLVIII.
LI.
color
LIV. solubil ity LVII. boiling point
XLIX. t ert-butyl alcohol LII. c olorless LV. m iscible LVIII. 8 3°C
L. tertbutyl chloride LIII.
colorles s LVI. slightly miscible LIX. 51°C
LX. Figure 4. Properties of tert-butyl alcohol and tert-butyl chloride
LXI. The balanced equation for the reaction is the following:
LXII. LXIII. XXXIX. Figure 3. Simple Distillation Apparatus © Nimitz
XL. The crude tert-butyl chloride previously decanted inside a flask was then installed to the distillation set-up. The water pump was turned on. XLI. The sample in the flask was heated to a gentle boil. The temperature reading on the thermometer rose rapidly and when the reading remained constant the boiling point was recorded. The recorded boiling point was 80°C. XLII. The vapors and condensate passed through the side arm and into the condenser (where most of the vapor condensed into liquid). The experimenters made sure that when the mixture started boiling, the heat source was adjusted accordingly. The condensate then eventually poured into the receiving flask. The experimenters watched for drops disregarding the first five drops. XLIII. The experimenters determined the collected fraction by using a Pasteur pipette. The total number of drops was then converted into milliliters, garnering a 7.2 mL yield of tert-butyl chloride. XLIV. The wastes were disposed into the appropriate waste containers: the aqueous solutions diluted and poured into the sink, and the organic compounds disposed in appropriate organic waste jars. XLV. XLVI. Results and Discussion XLVII. To complement the results, the following table (Figure 4) shows a summary of the properties of tert-butyl alcohol and tert-butyl chloride:
(CH3)3COH + HCl (CH3)3CCl + H2O
LXIV. LXV. This implies that 1 mmol of (CH3)3COH is equivalent to 1 mmol of (CH3)3CCl. In the experiment, ten milliliters of tert-butyl alcohol was prepared. This is equivalent to 7.90 g which is equal to 106.58 mmol, as shown in the following equation: LXVI.
10 mL ×
0.79 g =7.90 g mL
LXVII.
mol 1000 mmol 7.9 g × × =106.58 mmol 74.12 g mol LXVIII. LXIX. The yield of the tert-butyl chloride was 7.20 mL. This is equivalent to 6.05g, and thus 65.36 mmol produced, as calculated in the following equation: LXX.
7.20 mL ×
0.84 g =6.05 g mL
LXXI. LXXII.
6.05 g ×
mol 1000 mmol × =65.36 mmol 92.57 g mol
LXXIII. LXXIV. A summary of the reaction scheme is shown below (Figure 5), along with the experimental results: LXXV. LXXVI. (CH3)3COH —1)HCl; 2)NaHCO3—> (CH3)3CCl LXXVII. LXXVIII. LXXIX.tert- LXXX. ter butyl t-butyl alcohol chloride
LXXXI.Mass LXXXIV. m olecular weight LXXXVII. d ensity XC. mmol XCIII.
LXXXII. 7.90g LXXXV. 74.12g/mo l LXXXVIII. 0.79g/mL XCI. 106. 58 mmol
LXXXIII. 6.05g LXXXVI. 92.57g/m ol LXXXIX. 0.84g/mL XCII. 65. 36 mmol
various sources of error. One source of error could be due to the formation of side products (from side reactions). One of the possible side reactions of the synthesis is di-tert-butyl ether, as shown by the mechanism below: CIII.
Figure 5. Experimental Results
XCIV. The theoretical yield of tertbutyl chloride, when limited by the reagent tert-butyl alcohol, is 9.87g, as shown in the following equation: XCV.
106.58 mmol×
mol 92.57 g × =9.87 g 1000 mmol mol
CIV.
Figure 7. Di-tert-butyl ether Mechanism
CV. Another possible reaction is 2methylpropene, as shown in the illustration below: CVI.
XCVI. XCVII. The percent yield is therefore 61.30%, as calculated below: XCVIII.
6.05 g (obtained) ×100=61.30 9.87 g (theoretical)
XCIX. The mechanism involved in the reaction between tert-butyl alcohol and HCl is a unimolecular nucleophilic substitution (SN1), where the reaction occurs in a step-wise manner. This means that the nucleophilic hydroxyl group of the tert-butyl alcohol will attack the hydrogen atom of the hydrogen halide. This will give rise to a tert-butyl oxonium compound. The electron-poor oxygen atom will then be attacked by the carbon attached to it. This will give rise to 2methylpropane and water. The carbocation of the 2-methylpropane will be attacked by electrophilic Cl ion and will thus form tert-butyl chloride. This is illustrated below:
C. CI.
Figure 6. Tert-butyl chloride Mechanism
CII. The yield was only 61.30%. It is possible that this number was caused by
CVII.
Figure 8. 2-methylpropene Mechanism
CVIII. To limit the formation of these side-products however, the experimenters made sure that the temperature was regulated to high temperatures to ensure that only the target compound will be achieved. Also, cold HCl was used to prevent the formation of 2-methylpropene. CIX. In line with this, the HCl used was cold so as to prevent volatilization and to avoid the sudden release of heat (this is important because in the experiment boiling chips—used to prevent sudden heating—were not used). The HCl used was also concentrated so that all of the alcohol sample will be solvated. Another reagent used was NaHCO 3. It was important to use the solid and not the aqueous form so as to neutralize the excess HCl in a moderated manner. Also, it is to prevent the introduction of water (i.e. hydrolysis) in the reaction. These measures were done in order to reduce the error. CX. Another source of error could be from the distillation process (before and after) and set-up. In the experiment,
anhydrous CaCl2 was incorporated to remove the excess H2O. This is a necessary step before proceeding to distillation because if the excess water was not removed, there would be a huge discrepancy between the theoretical and actual yields. CXI. In terms of the set-up, one crucial part was keeping the flow of water continuous in the condenser. This is important because the water in the condenser will prevent it from getting warm. The removal of the heat will aid the condensation process. This is why the experimenters made sure that the water flowed continuously. CXII. In line with this, a possible source of error is the experimenter. She may not have been able to read the temperature correctly, there must have been an error in the amount of reagents used, etc. Some experimenter variables such as fatigue and distraction may have also caused inaccurate findings. CXIII. Due to the various combinations of these sources of error, that probably occurred during the experiment, the actual yield was not close to the theoretical yield, but good enough to exhibit the synthesis of tert-butyl chloride. CXIV. CXV. Conclusion CXVI. The formation of an alkyl halide is important to perform so that one is able to understand the mechanisms and reactions that occur in each step of the synthesis. The experiment also serves as a good starting point when learning about nucleophilic substitution reactions. In the synthesis of an alkyl halide the importance of following the procedures carefully and also in choosing the correct form (e.g. solid or aqueous) and amount of reagents can also be observed. Knowing the proper procedures and techniques in the synthesis will serve as an aid and a guide in future experiments. CXVII. CXVIII. References CXIX. Bruice, P. 2014. Organic Chemistry 7th ed. Pearson, New York.
CXX. Clark, J. n.d. Properties of Alkyl Halides. Retrieved March 7, 2015 from http://chemwiki.ucdavis. edu/Organic_Chemistry/Alkyl_Halides/P roperties_of_Alkyl_Halides. Licensed under CC BY-NC-SA 3.0 US. CXXI. El-Fellah, M. 2012. Laboratory Course in Organic Chemistry. Organic Chemistry Lab I 5332. Accessed March 6, 2015. http://www.elfellah.com/Organic Chemistry Lab. I 5332.htm. CXXII. Hollingworth, RM. 1976. Chemistry, biological activity, and uses of formamidine pesticides. Environmental Health Perspectives. 14, 57-69. CXXIII. King, C. n.d. Preparation of TButyl Chloride. Accessed March 6, 2015. http://christopherking .name/Organic%20I %20Labs/Preparation%20of%20t-butyl chloride.htm. CXXIV. McMurry, JE. 2011. Fundamentals of Organic Chemistry 7th ed. Brooks-Cole, New York. CXXV. McMurry, JE. 2010. Organic Chemistry 8th ed. Brooks-Cole, New York. CXXVI. Nimitz, JS. 1991. Experiments in Organic Chemistry. Prentice Hall: New Jersey. CXXVII. Rowland, FS. 2006. Stratospheric ozone depletion. Philosophical Transactions of the Royal Society B: Biological Sciences. 361(1469), 769-790. CXXVIII. Tang, S & Zhao, H. 2014. Glymes as versatile solvents for chemical reactions and processes: From the laboratory to industry. RSC Advances. 4(22), 11251-87. CXXIX. Watrous, RM. 1947 Health hazards of the pharmaceutical industry. British Journal of Industrial Medicine. 4 (2), 111-125. CXXX. CXXXI. Appendices CXXXII. Attached is the data sheet for this experiment.
