Preparation and Purification of an Alkyl Halide

Preparation and Purification of an Alkyl Halide Bryan B. Arguilles Department of Food Science and Nutrition, University of the Philippines, Diliman, Quezon City Date Performed: February 26, 2015 Date Submitted: March 10, 2015 Abstract The study demonstrated the processes involved in the synthesis of an alkyl halide, particularly tert-butyl chloride, from tert-butyl alcohol, a tertiary alcohol with the aid of cold concentrated HCl, a hydrohalide. The process is known to be governed by unimolecular nucleophilic substitution (SN1) reaction mechanism. 20 mL of cold concentrated HCl was added to 10 mL of tert-butyl alcohol in a separatory funnel to yield the aforementioned alkyl halide. Simple distillation was performed afterwards to purify the product collected. The experiment was able to provide 6.048 g final product or 62.47% of the calculated theoretical 9.68 g tert-butyl chloride gain. Results from this study showed that the synthesis of tert-butyl chloride may be deemed successful. On the other hand, the 37.53% loss may have been caused by some side reactions, errors committed by the experimenters such as inaccuracy in the proportion of reagents, and inevitable environmental factors. Nevertheless, it has provided an accurate background regarding the basic concepts of the S N1 reaction mechanism and the synthesis of an alkyl halide itself. I. Introduction Organohalides refer to those compounds containing one or more halogen atoms. Good examples of these are the alkyl halides. As the name suggests, halogen atoms in an alkyl halide are bonded to an alkyl group with saturated, sp3-hybridized carbon atom. These halogen atoms are considered substituent to the hydrocarbon parent chain. Because of the difference in the electronegativity of carbon and halogen atoms, alkyl halides are classified as polar compounds. The carbon atoms bear a slightly positive charge while the halogens carry a slightly negative charge. This implies that alkyl halides act as electrophiles during polar reactions. [1] Alkyl halides may be classified according to the number of alkyl groups attached to the carbon atom bonded to the halogen. A primary (1°) carbon atom bonded to only one other alkyl group, will yield a primary alkyl halide while secondary (2°) carbon atom attached to two other alkyl groups will produce a secondary alkyl halide and a tertiary (3°) carbon atom bonded to three other alkyl groups will give a tertiary alkyl halide. [2]

Currently, there exists several ways of synthesizing alkyl halides. Examples are by means of hydrohalo-genation, free radical halogenation and allylic bromination. These reactions can be used to produce alkyl halides from alkanes, alkenes, alkynes and other halides. [2]

Another way is through the use of alcohol and hydrogen halides – halogens bonded to a hydrogen atom (H-X). The said reaction is governed by SN1 or unimolecular nucleophilic substitution reaction me-chanism. We know that halogens are more electro-negative than its hydrocarbon parent chain. Because of this, the halogens will donate electrons to the electrophilic carbocation intermediate. In this mechanism, the stability hierarchy is an important factor and thus should be taken into account. The rate of reaction is influenced by the stability of the carbocation formed. The greater the stability of the intermediate formed, the faster the reaction rate. [1] In this experiment, a tertiary carbocation inter-mediate was formed from the reaction of tert-butyl alcohol, a tertiary alcohol and cold concentrated HCl, a

hydohalide (Figure 1). The chloride ion will dissociate from HCl and bond with this electrophilic carbo-cation producing tertbutyl chloride, an alkyl halide, which is also the major product. This reaction also produced water as a byproduct. Because of this, further purification is needed to increase the yield of tert-butyl chloride. Simple distillation, a purifying process that separates components of a mixture or compound due to differences in boiling point, was performed to produce to final product.

Figure 1. Reaction between tert-butyl alcohol and cold hydrochloric acid The experiment’s goal is to demonstrate the synthesis of haloalkane by mixing an alcohol and a hydrohalide and to purify the product using simple distillation.

separatory funnel because water has a greater density than tert-butyl chloride. This layer was discarded afterwards. The organic layer was then transferred to a dry Erlenmeyer flask containing a small amount of solid NaHCO3 to remove traces of excess acid. After swirling, it was decanted to another dry flask. A small amount of anhydrous CaCl 2 was added to the crude tert-butyl chloride to remove the traces of water and unreacted alcohol. It was then transferred to a dry 25 mL round bottom flask for the distillation process. The distillation set-up (Figure 2) composed of two adapters, where the sample and receiving flask was attached, linked to both ends of a condenser, where a pair of rubber tubes was attached to a bucket of water with a pump, was assembled and prepared prior to distillation.

II. Methodology 20 mL of cold concentrated HCl was mixed with 10 mL of tert-butyl alcohol in a dry 50 mL separatory funnel and the mixture was gently swirled. It is important to note not to shake the mixture to avoid any unwanted side reactions. Since the separatory funnel used in the experiment doesn’t have a cover, there is no need to relieve the internal pressure because it is already in equilibrium with the external pressure. The mixture was left undisturbed for 20 minutes to give way for layer formation. 3-5 mL of 6 M NaCl solution was added to the mixture to facilitate the separation of layers. After the distinct separation of the two layers, a few milliliters of water were dropped to the mixture to determine which the aqueous layer is. As what the rule “like dissolves like” implies, the layer in which the water dissolves is the aqueous layer. Theoretically, the aqueous layer will form at the bottom of the

Figure 2. Simple Distillation Set-up The round bottom flask containing the sample was heated using a hot plate. A cork was used to secure the thermometer in the adapter such that the bulb is placed near the opening of the adapter leading to the condenser. This is to monitor the temperature at which the sample boils. Since the product is highly volatile, the receiving flask was kept chilled using an ice bath. The fraction that boiled at 49-52 ˚C was collected after discarding the first few milliliters. The

volume of the final product was then measured dropwise using a Pasteur pipette and recorded.

III. Results and Discussion The experiment’s aim is to produce an alkyl halide from tert-butyl alcohol and cold concentrated HCl. (1) (CH3)3COH + HCl  (CH3)3CCl + H2O The reaction is governed by unimolecular nucleophilic substitution (SN1) reaction mechanism. It involves the substitution of the alcohol’s hydroxide functional group with a nucleophile which, in this case, is the halogen chlorine that ionized from the cold HCl solution to yield the final alkyl halide product. Unimolecular denotes that the reaction’s rate depends solely on the substrate which is the tert-butyl alcohol and not the nucleophile. Tertiary alcohols works best for this type of reaction since a stable carbocation is formed. [1] The mechanism of reaction is illustrated by the following figures.

Figure 1. Protonation of hydroxyl group by the acid

Figure 3. Reaction of chloride with tertiary carbocation forming the alkyl halide product The initial step is the protonation of the alcohol by the hydronium ion from the hydrochloric acid. The reaction is a Bronsted acid-base reaction with the alcohol acting as the base or proton acceptor. This protonation yielded a good leaving group which is the stable halide ion. [2] The oxonium ion, which is the result of the protonation, releases water molecule. The oxygen takes with it an electron from the carbon where it was previously attached. This leads to the formation stable tertiary carbocation. The final step is the introduction of nucleophile chlorine into the carbocation intermediate forming the desired alkyl halide – tert-butyl chloride. This is where the substitution occurred.

Figure 2. Dissociation of water yielding a tertiary carbocation

The experiment required the use of a cold concentrated hydrochloric acid to prevent immediate evaporation of the product because of the relatively high temperature. Also, at high temperature, some 2-methylpropene can be formed as a byproduct through the reaction mechanism called acid catalyzed

elimination (E1). The process is similar to the first two steps in the synthesis of tertbutyl chloride through substitution. On the third step, however, instead of bond formation between carbocation intermediate and halide, the carbocation acts as an acid, deprotonates and gives the H+ to the water molecule producing a H3O+ ion. The hydronium ion then binds to the carbocation, forming the 2methylpropene byproduct. These eliminations are called dehydrohalogenation because the hydrogen halide has been removed from the alkyl halide. Substitution and elimination reactions often compete with each other. [2] The step by step process is clearly demonstrated by Figure 6.

since the salt reduces the amount of water in contact with the tert-butyl chloride, the possibility of occurrence of solvolysis is lowered. [3] Once the organic layer was collected, a small amount of solid NaHCO3 was added to the solution. The sodium carbonate removes traces of HCl that may initiate the E1 reaction mentioned earlier.[4] The solid form was used instead of the aqueous one in order to reduce the amount of water around the organic compound and prevent hydrolysis of the alkyl halide back to the original alcohol.[5] Prior to distillation, the organic compound was treated with anhydrous CaCl2. This is also in line with minimizing the amount of water in contact with the organic layer. CaCl2 is preferred because Calcium (II) ion forms complexes with many oxygen containing organic compounds. CaCl2 also removes untreated alcohol from the tertbutyl chloride. [6] The addition of boiling chips to the organic solution before distillation process is to prevent overheating and decrease chances of bumping. Boiling chips are small pieces of porous materials that are able to give off a stream of air bubbles when heated. [7] The rationale behind the purification by distillation is the differences in boiling points of the components of compound to be separated. [8] The boiling point of water is 100oC, tert-butyl alcohol is 82oC and tert-butyl chloride is 51oC.

Figure 4. Formation of alkene byproduct 2-methylpropeme The acid should be added in excess (HCl:t-BuOH 2:1) to ensure that all of the alcohol would be consumed. It also pushes the reaction to the product side or to proceed (Le Chatelier’s Principle). After the two layers have separated, 6M NaCl was added to the solution. The salt aids the separation of layers by breaking the intermolecular forces of attraction between the organic and aqueous parts since sodium chloride has a higher affinity for water than the organic layer. Also,

The vapor of the tert-butyl alcohol will come in contact with the thermometer thus reading the boiling point of that component. The vapor will then travel through the condenser where cold water is continuously flowing. This is where the vapor will reliquify. The water should be continuous to ensure that all of the vapor will be reliquified before reaching the receiving flask or else, the percent yield of the experiment will decrease. [8] The first few drops of the distilled product was discarded because since it boiled and evaporated at temperature lower than

that of the tert-butyl chloride, it is not the desired product. Tert-butyl chloride is volatile in nature. That is why the collecting flask is submerged in an ice bath: to prevent unwanted evaporation of components. Although this purification process is fast and relatively straightforward, it also has its disadvantages or limitations. Separation of heat sensitive compounds cannot be aided with this process since side reactions might occur during procedure. Another limitation is that compounds whose component’s boiling points are relatively close cannot be fully separated.[9] After all the steps have been followed subsequently, pure tert-butyl chloride has been isolated. The following table shows the experi-mental results. Table 1. Experimental Results Weight of tertbutyl alcohol Weight of tertbutyl chloride Theoretical yield Percent yield

7.75 g 6.05 g 9.68 g 62.47%

The experiment was able to provide 6.048 g tert- butyl chloride. Based on the data from Table 1, this lead to a quite high percent yield (62.47%) using the following equation (1)

yield=

actual yield x 100 theoretical yield

The 37.53% loss may be attributed to some errors committed by the experimenters. One example would be the distillation set up may not have been properly prepared. Some parts may have been loosely connected allowing the escape of vapors that should have been reliquified as a final product. Also, the discarded initial distillate may have exceeded the advisable amounts, leading to lower percent yield.

Occurrence of side reactions also lowers the percent yield. The SN1 and E1 reaction are both competitive reactions. 2methylpropene, a side product whose formation was discussed earlier in this paper may have been produced. Lastly, the effects of anhydrous calcium chloride are not instantaneous. For the drying to have been surely maximized, a waiting time of around twenty minutes should have been strictly followed. But due to time constraints, unfortunately, this was not the case. Water molecules in contact with tert-butyl chloride were not completely removed and thus, solvolysis may have occurred.

IV. Conclusion Having able to purify 7.2 mL or 6.95 g of tert-butyl chloride which is 62.47% of the theoretical 9.73 g, the experiment is can be considered a bit successful. However, 37.53% of the product was lost. This may be due to several factors mostly committed by the experimenters themselves. But still, the objective of the experiment is to synthesize and purify tert-butyl chloride from tert-butyl alcohol and this was properly achieved in the experiment. It has provided basic background regarding the concept of alkyl halide and purification by simple distillation. To increase the percent yield, it is recommended that the waiting time for drying reagents such as NaHCO 3 and CaCl2 to absorb water be longer. Also, the distillation set-up should be assembled properly to avoid the escape of vapors from the system.

V. References

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[1] McMurry, John. Organic Chemistry, 8th ed.; Edited by Lisa Lockwood. Cengage Learning: Belmont, CA, 2010; pp. 344-346

[6] Organic Chemistry Laboratory Manual 2008 ed. Institute of Chemistry, University of the Philippines Diliman, Quezon City, 1101.

[2] Leory G. Wade, Jr. Organic Chemistry, 8th ed,; Prentice Hall: New Jersey, USA, 2011; pp. 220. PDF file.

[7] University of Colorado. Boiling Chips. July 14, 2013. Web

[3] University of Colorado. Drying Organic Solutions. July 14, 2013. Web . [4] WiseGEEK. What Happens When Sodium Bicarbonate and Acid Meet in the Stomach?. July 14 2013. Web [5] The McGraw-Hill Companies. Hydrolysis of Alkyl Halides. July 14, 2013. Web