Synthesis of an Alkyl Halide 2003-3
An experiment on synthesizing an alkyl halide from an alcohol...
Synthesis of an Alkyl Halide Ma. Queenie Rose G. Amosco, Gim Thane G. Colorado, Elaine Joyce S. Oquendo, Ian B. Paguigan. Institute of Chemistry,University of the Philippines, Diliman, Quezon City 1101 Philippines Date Performed: April 25, 2013; Date Submitted: April 29, 2013 The purpose of this study is to synthesize an alkyl halide with the use of an alcohol and a hydrohalogen, and at the same time observe the corresponding reaction mechanisms. The reaction mechanism provides a perception of the products expected to be yielded in different reactions involving similar compounds, and also positively creates a better execution of step procedures to come up with more refined products. In this study, tertbutyl alcohol was reacted with HCl as the hydrogen halide resulting to the formation of tert-butyl chloride and water. The tert-butyl chloride was separated from aqueous layer using its solubility characteristics. The obtained alkyl halide was purified through the process of distillation. Distillation of the alkyl halide ensures the exclusion of other compounds like water and HCl. From the performed study, the computed mass of tertbutyl alcohol used was 7.858 grams with a theoretical yield of 11.69 mL of tert-butyl chloride. The amount of the product is 55.60% of the theoretical amount,which suggests more improvement and caution in executing the procedures. Introduction Alkyl halides are monohalogen derivatives of alkanes. They are classified as primary, secondary or tertiary depending on whether the halogen atom is connected to a primary, a secondary or a tertiary carbon. A primary alkyl halide has the general formula of RCH2X, where R and X represents generalized alkyl group and the halogen in the alkyl halide, respectively. The general formula for secondary alkyl halides is R2CHX and while for tertiary alkyl halides, the formula is R3CX. There are several methods developed for preparing alkyl halides; a) addition reactions of HX and X 2 with alkenes, by electrophilic addition reaction, b) reaction of an alkane with X2 and c) treating alcohols with HCl or HBr. Among these , the most generally useful method is the synthesis through the use of alcohols. Tertiary alcohols works best when this method is employed. Primary and secondary alcohols react much more slowly in when this method is applied to such alcohols. The general reaction for tertiary
Figure 1. General reaction for alkyl halide synthesis through the reaction of an alcohol and HX
Distillation is a technique that is used to purify a mixture of liquids or to obtain a boiling point of a pure liquid. Essentially, the liquid is heated to boiling and the vapors condensed above the boiling liquid. There are many types of distillation. Simple distillation is best for separating liquids where one of the components has a boiling point great than 25°C than the other. Fractional distillation should be used when the boiling point differences between the two components of the mixture is less than 25°C. In steam distillation, one of the components is water (hence, the name “steam”) and the other component is an insoluble organic compound . 1
In this experiment, the synthesis of an alkyl halide from an alcohol was performed. Specifically, the alkyl halide is tert-butyl chloride, which was synthesized from tert-butyl alcohol using HCl as the hydrogen halide. The process of purification through distillation was also applied in this experiment, wherein the tert-butyl chloride was purified by simple distillation. To calculate the theoretical yield and % yield in the experiment, the following equations were used; Theoretical yield =
% yield =
Experimental Details Ten mL of tert-butyl alcohol was mixed with 20 mL of cold concentrated HCl in a dry 30-mL separatory funnel. A cold concentrated HCl was used to react with the tertiary alcohol so that the SN1 reaction is favored. Two possible reactions can either take place during the reaction of strong acid and tertiary alcohol: an elimination or substitution reaction. Elimination reaction favors high temperature while substitution reaction occurs at low temperature. Since the desired product is alkyl halide which is only produced from substitution reaction, to prevent the synthesis from undergoing elimination reaction, the reaction should be in low temperature. Hence, a cold concentrated HCl was used. Additionally, HCl was added in excess to hasten the progress of the desired reaction. The mixture was then swirled gently, taking care not to shake the mixture. During swirling, the funnel’s stopcock was slowly opened from time to time to relieve pressure in the system. The mixture was then allowed to stand for 20 minutes undisturbed. Two separate layers, an organic and aqueous, were formed after the allotted time. The aqueous layer was then discarded while the organic layer was transferred on a dry flask containing small amount of solid NaHCO 3. NaHCO3 was used to
wash off any unreacted acid which may have mixed with the tert-butyl chloride in the organic layer. It is important to note that solid NaHCO3 was used and not an aqueous solution of the compound. Using an aqueous solution will not have any effect on the organic layer since the two liquids are immiscible. The HCO3- ions in the aqueous solution cannot react with the alcohol in the organic layer. Using solid NaHCO3 solves this problem. The mixture of organic layer and solid NaHCO 3 was swirled gently and after swirling, the mixture was decanted into another dry flask. After the decantation process, the collected filtrate was transferred into a dry, 25 mL round bottom flask. Often, CaCl2 is added to the filtrate since it removes excess water. CaCl2 forms complexes with compounds containing oxygen and nitrogen so it is a good drying agent . By drying, the crude alkyl halide becomes pure before undergoing distillation. Some boiling chips were then added into the flask and the crude tert-butyl chloride was distilled afterwards. The pure tert-butyl chloride produced from the distillation was then collected on a 10-mL graduated cylinder cooled in an ice bath. This was done to prevent loss of tert-butyl chloride, since it is a highly volatile compound. The distillate (pure tert-butyl chloride) was the fraction collected when the distillation reached a temperature range of 4952°C.
Figure 2. A simple distillation set-up.
For the purification of the crude tertbutyl chloride, a simple distillation set-up as shown in Figure 2 was prepared. The cooling 2
water must enter at the bottom part of the condenser and goes out at the top part. To prevent risks of explosion, there must be a continuous flow of water in the condenser . A thermometer was then placed just below the side arm of the three-way distilling adapter to accurately monitor the temperature of the vapor and not the air inside. An appropriately-sized distillation flask was chosen, big enough so that the sample fills 1/2-2/3 its volume. The flask should not be full since the surface area for rapid evaporation will become small, resulting to slow distillation process. A water bath was used as a source of heat. After choosing the appropriate distillation flask, the sample was placed inside it. Two to three boiling chips were added into the flask. Boiling chips are small, porous stones made up of calcium carbonate. These chips have pores inside which provide spaces for bubbles of solvent to form. These bubbles ensures boiling and prevents bumping and loss of solution . The water supply was turned on and the water flow was checked through the condenser. All glass joints were also checked, so that the joints fit well. After the distillation set-up was checked properly, the sample was slowly heated to a gentle boil. The heat source temperature was adjusted so that the distillation occurs at a rate of 2 drops distillate per second. The first 1 mL distillate that was collected in the receiving flask was discarded and the fraction that distilled at a temperature range of 49-52 deg Celsius was collected. Before dismantling the whole set-up, it was allowed to cool first.
Figure 3. Reaction of tert-butyl and HCl to produce tert butyl chloride
This synthesis follows the SN1 nucleophilic substitution reaction. According to McMurry (2008), nucleophilic substitution involves the switching of one nucleophile by another. It has two types of reactions: S N1 and SN2, respectively. SN1 stands for substitution, nucleophilic, unimolecular reaction. It requires the use of weak nucleophile, and RX that will form stable carbocation. Three steps are involved in the synthesis of tert-butyl chloride. These are shown below.
Figure 4. In the first step, tert-butyl alcohol accepts proton from hydronium ion, forming tertbutyloxonium and water. The alcohol acts as the nucleophile hence becoming protonated
Figure 5. The carbon-oxygen bond in terbutyloxonium ion breaks heterotically, forming a carbocation and water. This is the rate determining step.
Results and Discussion Alkyl halides are compounds that can be synthesized using alcohols and hydrogen halides. In this study, researchers used tertbutyl alcohol, a tertiary alcohol, and hydrochloric acid to produce tert-butyl chloride. Structures and balanced equation are shown below.
Figure 6. The last step involves the reaction of carbocation and chloride atom, forming tert-butyl chloride.
This mechanism explains why hydrochloric acid, a relatively weaker acid than hydrogen iodide or hydrogen bromide, can only react readily with tertiary alcohols as was done in the experiment. The stable tertiary 3
carbocation allows for the attachment of the Cl ions that do dissociate from the HCl in solution. The rate of formation of tert-butyl chloride is dependent on the amount of alcohol used since the synthesis follows SN1 reaction, a first order rate reaction. Since the amount of alcohol is less than the amount of HCl and follows a stoichiometric ratio of 1:1 as seen from the balanced chemical equation, the researchers inferred that tert-butyl alcohol is the limiting reactant and will determine the rate of formation of tert-butyl chloride. To determine the theoretical yield, the amount in grams of tert-butyl alcohol was obtained using its density, molecular weight and the amount used in the study (refer to appendix for calculations). The calculated value for grams of alcohol was 7.858. Using this value, theoretical yield was found out to be 11.69 mL, which is equal to about 9.8 grams of tert-butyl chloride. Given the actual yield in the experiment, the percent yield was then calculated, giving a value of 55.60%, indicating that the amount of tert-butyl chloride that could be obtained was not maximized in the experiment. Possible sources of error include those encountered during the distillation process. The first of which is that the adapter and condenser were not properly attached to each other, hence leakage occurred, which contributed to the decrease in the actual yield of the product. The completeness of distillation process can also be a source of error. It is possible that not all of the sample compound was converted to tert-butyl chloride, resulting to the lower product yield. It is also possible that the separation of layers was not done efficiently, thus leaving some of the organic layer with the aqueous layer disposed. This might have reduced the amount of crude tert-butyl chloride distilled, again resulting to the lower product yield. To reduce occurrence of errors, always make sure to prepare distillation set-up properly and perform the steps in the procedure which were made to ensure the minimalization of errors. Such was the use of an ice bath during distillation,, since tert-butyl chloride, the desired product, is highly volatile. Another technique performed to reduce errors was the use of sodium bicarbonate, which removes excess HCl, which
is also highly volatile, from the organic layer so that it will not interfere in the distillation process such that it will produce a false positive error. Conclusion This study sought to synthesize an alkyl halide, specifically tert-butyl chloride, from the reaction of tert-butyl alcohol with HCl. Since a polar protic solvent is best for an SN1 reaction as it strongly solvates ions with the hydrogen bonding, and a tertiary halide mixture at low temperature favors SN1 reaction, the researchers conclude that this synthesis made use of the first type of nucleophilic substitution reaction, where upon the addition of hydrohalogen HCl to tert-butyl alcohol an endothermic intermediate reaction occurred where a stable carbocation set in energy was formed, with which the electronegative Cl ion made a bond. The reaction followed first order rate, and zero order for the nucleophile, which supports the dependability of the amount of product produced from the alkyl halide tertbutyl alcohol and gives a rate law of R =k[halide]. The formation of the carbocation is the slow step thus it is the rate determining step of the reaction. Researchers were only able to recover 55.60% of theoretical amount of tert-butyl chloride, which may suggest some degree of errors including the occurrence of leaks in an improperly sealed distillation set-up, or failure to quantitatively transfer the reagents from one glassware to another. On the other hand, to yield more accurate results the study requires extra caution upon performing every step. Such examples are observing the right and proper amounts of reagents needed, to accurately and quantitatively separate the aqueous layer from the organic layer, and also to properly prepare the distillation set-up ensuring no leaks can occur.
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