Synthesis of an Alkyl Halide

Abstract: Alkyl halides can be synthesized from alcohols through its reaction with strong acids acting as hydrogen halides, HX (X = Cl, Br, I). The mechanisms of acid-catalyzed substitution of alcohols can be classified as either SN1 or SN2, where SN stands for nucleophilic subsitution and 1 or 2 designating its molecularity. The main objective of the experiment was to synthesize tert-butyl chloride with the utilization of an SN1 mechanism and purifying it with a simple distillation set-up, wherein a mixture of different substances separate by taking advantage of the different volatility and boiling points of the individual components. Tertiary alcohols are the most conducive alcohols whenever those must be synthesized into alkyl halides. In the experiment, hydrochloric acid was used in order to react with tert-butyl chloride to produce tert-butyl alcohol through an SN1 reaction. This reation includes three steps. The first part is the rapid, reversible (through hydrolysis) protonation of the alcohol followed by a slower rate-determining step which is the loss of water, eventually producing a more stable tertiary carbocation.

To conclude the process, the carbocation is swiftly attack by the halogen ion (chloride in this experiment) in order to form the alkyl halide. Alkyl halide is insoluble in water, making it easy for it to separate from the aqueous layer. Hydrochloric acid was then mixed with tert-butyl alcohol producing water and tert-butyl chloride and was purified through distillation. Eventually, a clear and colorless solution of alkyl halide was synthesized, 0.8 milliliters in volume, with only a 7.38% percent yield. It can be concluded that the synthesis of alkyl halides with the reaction between an alcohol and a strong acid is an accurate and effective purification procedure.

Introduction

Alkyl halides are compounds which have the general formula R – X, where R is usually an alkyl group with a halogen, such as chlorine or bromine, substitutes in place of one of the hydrogens. It can also be referred to as haloalkanes or halogenoalkanes. Multiple substitutions of halogens for hydrogens can take place, and in the process, may produce certain variations wherein not only alkanes are involved but alkenes and alkynes as well. The presence of the highly electronegative halogen in alkyl halides tend to contribute to its increased polarity. Alkyl halides can be classified depending on the number of alkyl substituents attached to the carbocation. It can be primary, secondary or tertiary. Presently, the applications of the concept of alkyl halides can still be seen.

This includes the use of carbon tetrachloride for fire extinguishers and fabric cleaners and chlorofluorocarbons (CFCs) which are usually a part in the process of making air conditioners. Because of the prevalent industrial and commercial benefits which arise from the use of alkyl halides, it is crucial that techniques must be developed in order to be able to prepare, isolate and acquire alkyl halides in the purest possible form. Two processes can be used in order to prepare or synthesize alkyl halides. One of the two methods which are used is the reaction of alcohols with sulfur and phosphorous halides. During the reaction between alcohols and sulfur or phosphorous halides such as thionyl chloride (SOCl2-), phosphorous trichloride (PCl3-), phosphorous pentachloride (PCl5-) or phosphorous tribromide (PBr3), in the process, alkyl halides can be derived from alcohols. An example of that is the synthesis of ethyl chloride or ethyl bromide from the reaction of ethyl alcohol with either sulfur or phosphorous halides. However, the more commonly used among the two is the one which involves the reaction of alcohols with the halide ions in strongly acidic environments in order to synthesize alkyl halides.

Distillation
Distillation is one the commonly used methods for the purification of liquids and separating a mixture of different liquids into its individual components. It is intended for the more volatile liquid to evaporate in a solution of less volatile and/or non-volatile substances. The vapor is then cooled, therefore, condenses in that process which results in the liquid collected with the receiver in the distillation set-up. This may be executed due to the different boiling points of the individual components of the mixture. One useful example of distillation includes the fractionation of crude oil into more beneficial and useful products such as gasoline and heating oil.

The purpose of the experiment is to be able to understand as to how alcohol and alkyl halides are connected to each other with respect to the mechanisms of the reactions. The synthesis of an alkyl halide from alcohols requires the utility of tertiary alcohols which contribute to the relative strength and stability of the tertiary carbocations. Since secondary and primary carbocations are relative weak and unstable compared to tertiary carbocations, this usually results in the derivation of inadequate alkyl halides. In addition to that, the use of more complex alcohols does not result in the product of more satisfactory alkyl halides. The reason behind it is that more complex alcohols tend to react more with strong acids which can affect the quantity of alkyl halides that can be derived. When tertiary alcohols are reacted by a strong acid, a simple displacement reaction occurs. The hydroxyl group of the tertiary alcohol is replaced by the available halide anion, provided by the strongly acidic environment, in order to form alkyl halides and water. It also intends to produce a product to the purest possible potential with the use of the process of simple distillation.

Experimental Details

In the experiment, the use of a reaction between a strong acid and an alcohol was used in order to synthesize an alkyl halide. Two parts were executed in the experiment. The first one involves the derivation of tert-butyl chloride from tert-butyl alcohol with the use of HCl. The second part involves the purification of the synthesized tert-butyl chloride. All necessary materials and lab apparatus were prepared. In a dry 30-milliliter separatory funnel, 10 milliliters of tert-butyl alcohol and 20 milliliters of cold concentrated hydrochloric acid (HCl) were added. The mixture was swirled gently, making sure that the mixture is not agitated too much. While the mixture was being swirled, the stopcock was opened from time to time in order to relieve the internal pressure which builds up inside the separatory funnel. After the two compounds are sufficiently combined, the mixture was set aside for about 20 minutes, making sure that it would not be disturbed. After the required time has elapsed, the layers were separated which was facilitated by the use of 3-5 milliliters of sodium chloride (NaCl) solution. Once that step has been executed, the lower layer of the mixture was drained and the aqueous later was discarded.

Figure I. Separatory Funnel Set-up

The organic layer within the mixture was transferred into a dry flask which contains a minute amount of sodium bicarbonate (NaHCO3). The flask, which contains the organic layer with NaHCO3, was swirled gently eventually decanted into another dry flask. The filtrate which was collected was dried by the addition of a small amount of anhydrous calcium chloride (CaCl2). The mixture was decanted into a dry 25-milliliter round bottom flask. A few pieces of boiling chips were added to the decanted mixture, making sure the mixture was cool before those were added. The crude tert-butyl chloride was being prepared for distillation.

Figure II. Separation of Layers

Once the tert-butyl chloride is synthesized, a simple distillation set-up was employed in order to push through with the purification process. The students were cautious in making sure that the water flowed into the bottom of the condenser’s cooling jacket and out from the top. Also, a thermometer bulb was placed just below the side arm of the distillation head. A round bottom flask was utilized and it was filled to about ½ of its volume. An ice bath was used in order to regulate the temperature of the distillation set-up. Once the sample was placed inside the flask, the water supply was turned on, making sure that the water flow is continuous within the condenser and the ground glass joints fitting well. The sample in the flask was then gently headed to a soft boil. The temperature reading on the thermometer was observed intently. At some point, the temperature will remain constant which signifies the sample has reached the boiling point which was recorded by the students. The vapor and condensate which passed through the side arm and into the condenser, where most of the vapor will condense into liquid due to the cool water which flows into the cooling jacket.

Most of the vapor that has condensed into liquid eventually drips into the adapter leading to the receiving flask. The flame was also adjusted in a way to minimize the amount of distillate, to 2 drops per second at most, in order to regulate the distillation process. The first 1 milliliter or 10 drops of distillate was discarded since those are considered as impurities within the sample. The fraction that is distilled when it reaches a constant temperature is the one which was collected, taking into consideration that it must not be distilled until the sample becomes dry. Eventually, when the sample started to boil, the heat source was then removed and the entire set-up was allowed to cool before dismantling the set-up. The pure tert-butyl chloride was collected in a 10-milliliter graduated cylinder was cooled in an ice bath in order to minimize its evaporation since it is highly volatile. The fraction that was distilled at around the 49-52°C was the one which was collected. It was finally submitted to the instructor after transferring it to a labeled vial.

Figure III. Simple Distillation Set-up

Results and Discussion

The experiment allowed the students to synthesize tert-butyl chloride, an alkyl halide which was formed from the addition of a strong acid to an alcohol. The properties of the synthesized tert-butyl chloride are as follows: |Properties | |Mass |0.692 g | |Molecular Weight |92.45 g/mol | |Density |0.865 g/mL | |Mmol |7.485 mmol | |Volume |0.8 mL | |Color |Colorless | |Solubility
|Insoluble | |Boiling Point |47.0°C |

After the execution of the experiment, the theoretical yield for the synthesized tert-butyl chloride was computed. The value of the percent yield of the derived alkyl halide was ___________. Synthesizing alkyl halides for this experiment involves the reaction through the direct contact of the tertiary alcohol with the strong acid, with strength and stability provided by the halogenic characteristic of the acid. The product formed from this method of synthesis of alkyl halides is of increasing stability with the decreasing atomic numbers, namely, RF < RCl < RBr < RI. The synthesis of alkyl halides with the utility of tertiary alcohols is the most appropriate for this since tertiary alcohols react readily with other reagents. The use of primary and secondary alcohols is not recommended for this case since they react less readily, therefore, producing a longer reaction time which is not conducive for the time allotted for the experiment. The equation for the synthesis is given below:

This kind of reaction is called a nucleophilic aliphatic substitution. It is one of the reactions which contributes to the field of organic chemistry. It involves the reaction between a nucleophile, which acts as an electron pair donor, and an electrophile, an electron pair acceptor. For an sp3-hybridized hybridized electrophile, a leaving group is required in order for the reaction to proceed. One example that will highlight this reaction is the synthesis of an alkyl halide from an alcohol. Nucleophilic substitution is a reaction which is common for aliphatic compounds in which the leaving group is attach to a carbon which is sp3 – hybridized. The mechanism for this synthetic transformation depends mostly on the structure of the alkyl group which generates the leaving group. Two pathways for the mechanism which may apply for the substitution reactions include SN1 and SN2 reactions. SN1 reactions involve polar protic solvents, such as alcohols, while SN2 reactions involve polar aprotic solvents such as acetone. In the experiment, an SN1 reaction proceeded since it utilized tert-butyl alcohol, a tertiary alcohol.

A faster reaction would take place since it only involves one step and because both the nucleophile and substrate are already involved in the rate determining step. This produces both a cation and an anion during the reaction. These contributes to the relative stability of the charges on the ions formed during solvation. Molecules of compounds that have extremely stable cations undergo SN1 mechanisms. Usually, only compounds which produce tertiary carbocations undergo SN1 rather than SN2 reactions. For this case, it would be easily seen as to why the reaction was considered as SN1. The stability of carbocations of tertiary alkyl halides is accounted for by the original molecules which exhibit steric hindrance on the rear lobe of the bonding orbital, inhibiting SN2 reactions from occurring. The aforementioned happens the other way around for SN1 reactions. With the contact between the proton (H+) derived from the acid and the tertiart alcohol within an elevated temperature it produces a reaction wherein the OH group in the alcohol is acknowledged as the leaving group. The OH group, along with the H+, combine together to form water and the carbocation intermediate.

The carbocation is attacked by the free chlorine anions within the aqueous solution which results to the formation of tert-butyl chloride and water.

The procedures for the experiment were followed strictly. But along the way, there were limits which have contributed to some failures before achieving the desired product. One error done was the addition of the higher bound amount of NaCl solution have resulted in some white precipitate to perform within the separatory funnel. This may well have limited the amount of organic layer that could have formed and separated which can account for the lower percent yield of tert-butyl chloride. This may also have affected the purity of the alkyl halide which added to the difficulties experienced by the experimenters since they had to distill it not only once. But still, some steps were executed in order to help alleviate the situation.

One of those is the addition of excess of the concentrated HCl in order to, at least, guarantee the chances of the reaction pushing through. It also drives the reaction in a way to fully optimize the formation of an alkyl halide. With the addition of HCl, it also increases the amount of H+ which prevents the formation of other tert-butyl side products which act as impurities and may affect the percent yield of the experiment. Also, the presence of large number of H+ prevents the formation of tert-butyl side products. In addition to those, the use of cold HCl instead of warm HCl since cold HCl prevents the dehydration of tert-butyl alcohol to isobutylene, (CH3)2C=CH2, which is a result of increased temperature.

Another step taken in order to ensure the success of the experiment was during the distillation part wherein solid sodium bicarbonate (NaHCO3) was used in order to eliminate excess H+ in the final aqueous product. Moreover, it would be much easier to decant neutralized H+ compared to solid Na2CO3. Solid NaHCO3’s other purpose was to neutralize HCl sufficiently since excess HCl was added in the first part in order to optimize the success of the reaction.

Tert-butyl chloride, once synthesized, should not be exposed to a lot of water because it may hydrolyze and turn it back to alcohol. Also, another thing that would prevent the hydrolysis of tert-butyl chloride would be drying it with calcium chloride (CaCl2) before distillation. If it did not undergo drying, distillation would instill the halide with moisture which will hydrolyze it. Even before distillation, the product was already dried with the use of anhydrous CaCl2 in order to minimize the amount of water and untreated alcohol which may affect the results of the distillation of tert-butyl chloride.

Another integral part to the success of the distillation process is the continuous flow of water within the condenser. The condensation of the liquid sample within the condenser process requires an adequate or, if possible, highly cool water flow within the condenser. If the water flow is insufficient or discontinuous, this may result in the low efficiency of distillation of the liquid sample due to the decreased cooling capability within the condenser.

Returning to the synthesis of tert-butyl chloride, the free hydride (H-) which remains within the aqueous solution may also react with the carbocation intermediate before it reacts with the chloride anion in the solution. The formation of tert-butane as a product may be a possible side reaction for the synthesis of tert-butyl chloride with use of strong acids in treating tertiary alcohols. The synthesis of tert-butyl chloride involves the reaction of a tert-butyl carbocation intermediate. It may undergo the process of elminiation of a beta hydrogen in order to produce 2-methypropene as a side product. Whenever alcohols are treated with strong acids, the OH group changes as it becomes more protonated, therefore, potentially changing it to a better leaving group, as water, which can be substituted by the entering halide.

A small amount of its protonated form may lose water which may then form the free carbonium ion which readily stabilizes by forming an alkene minus a proton at the second carbon. This is usually a minor product formed because primary carbocations which are free in the solution are unstable. The loss of water along the concurrent attack would be more favored. Although, this may be the case, it can be detached easily with distillation because alkenes have lower boiling points compared to alkyl halides. Removal of the side products, which act as impurities during distillation, can therefore be done easily. The inclusion of the side reactions’ products with the main product, tert-butyl chloride, may result in the inaccuracies reflected on several physical properties when compared to the purified sample such as the boiling point. In order to ensure more accurate results in succeeding trials with its synthesis, some ideas must be taken into account.

For the experiment to be more accurate in succeeding trials, some recommendations and points should be taken into consideration. The synthesis of the pure tert-butyl chloride was very low, producing only 7.38%. A possible reason behind the low yield could be separating the different laters within the separatory funnel. Separating the aqueous layer, which was used for the experiment, from the t-butyl layer leaves a lot of room for t-butyl to be accidently discharged from the solution alongside the aqueous later. In addition to that, if the anhydrous CaCl2 added is insufficient, it may waterlog the sample and increase the measured mass of the products. This can lead to the inaccurate assumption that more product was synthesized.

Conclusion

Because of the resulting yield of 7.38%, it can be concluded that side reactions involved were not minimized. Also, some steps along the way may have affected the percent yield such as the addition of more NaCl than the least possible amount to be added, the less amount of organic layer that has been derived from the separatory funnel and the amount of distillate that have been thrown out due to the impurities combined from the crude sample. In addition to that, the simple distillation set-up may not surely optimize the amount of tert-butyl chloride that can be acquired. One of the experimenter believes that some mechanisms and steps can still be taken in order to improve its yield. One of the main weaknesses of the way the experiment was handled was the time consumed and the speed at which the experimenters performed the experiment. A fast and efficient execution of the experiment can reduce the risk of letting side reactions occur during the length of the experiment. Another recommendation is to wash the receiving flasks with cold alcohol since water would not become an agent of a side reaction.

By the end of the experiment, despite the many failures that the experimenters have faced in synthesizing the alkyl halide from alcohol, they managed to bring it into fruition. The objectives of the experiment were satisfied: tert-butyl chloride was successfully synthesized from tert-butyl alcohol with the use of HCl which acts as the hydrogen halide. Purification techniques were mastered due to the numerous failures which were encountered along the way. Some notes on the procedures, such as the amount of reagent that must be added, can be changed in a way in order to optimize the amount of yielded distillate. Despite the failures faced along the way, the experiment was deemed to be a success.

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