Synthesis Of An Alkyl Halide

A. B. Dioquino, K.P. Isleta, And M.V. Monzon1Institute Of Chemistry, College Of ScienceUniversity Of The Philippines, Diliman, Quezon City, PhilippinesDATE PERFORMED: December 8, 2009

ABSTRACT

Alkyl halides are generally formed from alcohols through treatment with hydrogen halides. The purpose of this study is to illustrate the mechanisms behind the SN1 reaction. Tert-butyl chloride is synthesized from tert-butyl alcohol using cold, concentrated hydrochloric acid. The organic layer was obtained using a separatory funnel and reagents such sodium bicarbonate and calcium chloride were added. Simple distillation was conducted in order to obtain pure tert-butyl chloride. The computed theoretical yield is 11.45mL and the obtained tert- butyl chloride is 2.7mL, which gives a 23.58% percentage yield. This relatively low yield may be due an inefficient methodology, and the presence of a simultaneous elimination reaction.

INTRODUCTION Alkyl halides, or haloalkanes, are compounds that have a halogen atom bonded to a saturated sp3-hybridized carbon atom. They are are nonflammable and nonpolar compounds which give them various uses. They are comercially used as flame retardants, fire extinguishers, refrigerants, as well as solvents for dry cleaning and degreasing applications.

This type of organic compound is generally formed through the synthesis of alcohols with hydrogen halides (HCl, HBr, or HI). The order of reactivity increases as one goes down the halogen group. These compounds may be also classified according to the number of alkyl groups attached to the carbon. Primary alkyl halides have a single alkyl group attached, secondary alkyl halides have two groups and tertiary alcohols have three. A tertiary alcohol with the form R3COH, should be applied for a more effective reaction. Primary and secondary alcohols react much more slowly and require higher temperatures. In some cases, Lewis acids such as zinc chloride are required for a more effective reaction. Thionyl chloride (SOCl2) or phosphorous tribromide (PBr3) may also be used for treatment with primary and secondary alcohols.In this experiment, tert-butyl chloride is formed from the addition of tert-butyl alcohol with concentrated hydrochloric acid. The alcohol and hydrogen halide are placed in a separatory funnel which will later on facilitate the isolation of components. Purification of the tert-butyl chloride is performed through simple distillation to remove excess water from the solution. Since tert-butyl is a tertiary alcohol, it will readily produce an alkyl halide and water at room temperature.

The purpose of this experiment is to acquaint students with the mechanisms behind alkyl halide synthesis through the synthesis of tert-butyl chloride from tert-butyl alcohol. It also aims to teach laboratory techniques such as the usage of the separatory funnel and simple distillation.

METHODOLOGY Ten milliliters of tert-butyl alcohol and 20 milliliters of cold, concentrated hydrochloric acid (HCl) were measured with a graduated cylinder and placed in a separatory funnel. The mixture was gently swirled under the fume hood. To relieve internal pressure, the stop cock was opened after some time to release gas. Afterwards, the separatory funnel was left to stand in an iron ring for 20 minutes. While waiting, the simple distillation apparatus was prepared. A cork that had been previously drilled into was inserted with a laboratory thermometer. Subsequently, the apparatus was assembled accordingly.

After 20 minutes had passed, about two milliliters of six molar sodium chloride (NaCl) solution were measured in a graduated cylinder and placed in the separatory funnel. The two layers were then separated by slowly opening the stopcock. A drop of water was placed into both layers to determine the organic layer. The aqueous layer was discarded.

The organic layer was transferred to a flask containing sodium bicarbonate (NaHCO3) and swirled gently. Afterwards, the liquid was decanted into another flask. Anhydrous calcium chloride (CaCl2) was added until there was undissolved excess solid. The liquid was then decanted into a dry, 25-milliliter round bottom flask. Two boiling chips were added to the distillation flask and the crude tert-butyl chloride was distilled. The heat and rate of distillation were controlled and the boiling point was recorded. The first one milliliter of the distillate was discarded, and the remaining solution was placed in a ten-milliliter graduated cylinder which was previously cooled in an ice bath. The volume of the product was measured and afterwards, it was placed in a vial, labeled, and submitted to the instructor.

Synthesis of tert-butyl chloride from tert-butyl alcohol involved the reaction of the alcohol with a hydrogen halide. In this case, cold, concentrated hydrochloric acid was used as the hydrogen halide. Cold HCl was used to hasten the reaction, and avoid side products. Excess HCl was added to further stimulate product formation and to ensure that all the alcohol will be used in the reaction. The reactants underwent a nucleophilic substitution reaction involving unimolecular rate-determining step, also known as the SN1 reaction. Tertiary substrates are the most favored substrates in this type of reaction. In treating an alcohol with hydrogen halides, tertiary alcohols react rapidly as compared to primary and secondary alcohols. By this principle, tert-butyl alcohol, a tertiary alcohol, reacted rapidly via SN1 and E1 pathways with HCl, its acidic nucleophile. During the SN1 reaction, the tert-butyl alcohol dissociates to form a carbocation, which then reacts with the nucleophile. The mechanism is shown below.

Figure I. Mechanism for the reaction of tert- butyl alcohol with HCl via SN1At first, the OH group is protonated by HCl which is a fast reaction. Then the protonated alcohol dissociates spontaneously, which occurs in a slow, rate-determining step. This dissociation gave rise to a carbocation intermediate plus water. Afterwards, the chloride ion attacks the stable carbocation, and the water (H2O) leaves the resulting product. Water, in this part of the reaction, is called the leaving group. In tert-butyl alcohol, OH is considered as the leaving group. However, this leaving group is not at its most stable form. In order for it to be at its most stable form, the OH group is protonated. The hydrogen atom from HCl is ionized and transformed into water. The chloride ion then attaches itself to the carbocation to yield the neutral substitution product, tert-butyl chloride.Unlike in an SN2 reaction, where displacement of leaving group and the attack of the incoming nucleophile happen at the same time, an SN1 reaction occurs first with the spontaneous loss of the leaving group then the incoming nucleophile. In between, a stable carbocation intermediate is formed. This two-step mechanism is the reason why tertiary alcohols react with HCl must faster than the primary and secondary alcohols. More stable carbocation intermediates involve a faster SN1 reaction.

It can be seen in the reaction that the substrate is the only species involved in the rate- determining step. This is because the rate of an SN1 reaction depends only on the substrate concentration, and not with the nucleophile concentration (first- order process). In the synthesis via SN1 reaction, by-products can be formed as consequences of the synthesis. This is because an opposite reaction occurs with SN1 reaction. This reaction is called the unimolecular elimination reaction, or E1. E1 reactions have the same characteristics as that of SN1, but it involves loss of H+ from the intermediate carbocation following the dissociation of process. A neutral alkene is formed from this step. SN1 and E1 reactions normally occur in competition when alkyl halides are treated with a nonbasic nucleophile. In the synthesis of tert- butyl chloride, 2- methylpropene is formed as a by-product. The reaction mechanism of 2- methylpropene is shown below.

Figure II. Mechanism for the formation of 2- methylpropene as a by- product via E1

The reaction mechanism above implies that the formation of 2-methylpropene is a first- order process. This reaction proceeds because not all tert-butyl alcohol may have reacted via SN1. After the protonation, the hydroxide ion leaves the substrate and forms water. The water can act as a base and it can remove an H+ from the carbocation. A bond will result from the C-H bond. 2-methylpropene is then formed.

In the elimination reaction, there is heat required for the reaction to proceed. As the temperature increases, the rate of E1 reaction increases as well. Therefore, by adding cold HCl, the formation of a stable tertiary carbocation, through the SN1 mechanism, is maximized and favored. In addition, based on Le Chateliers Principle to help favor the forward reaction, more reactants (HCl) must be added.

As the cold HCl is mixed with tert-butyl alcohol, the gas 2-methylpropene (BP:-6.9oC) formed from the E1 reaction is released by opening the stopcock. This is done to prevent the build up of internal pressure. However, this release of gas may result in a loss that could lead to a lower yield.

The mixture was allowed to stand for 20 minutes and then separated. Sodium chloride (NaCl) solution was added to make the layers more visible since it can draw out the water from the organic layer to the aqueous layer. Crude tert- butyl chloride is obtained. Since it still has impurities, solid sodium bicarbonate (NaHCO3) is added in small amounts to neutralize any excess HCl. This addition will cause effervesence of carbon dioxide (CO2). Solid NaHCO3 is used rather than the aqueous NaHCO3 to ease the separation of NaHCO3.

Drying the collected filtrate with a small amount of anhydrous calcium chloride (CaCl2) followed. This was done before distillation to remove traces of water left and unreacted alcohol which may interfere in the process.The solution was decanted and boiling chips were added before distillation. Boiling chips are small, insoluble, porous objects made of calcium carbonate or silicon carbide that can trap air and provide spaces where bubbles of solvent vapor can form. When a boiling chip is heated in a solvent, it releases tiny bubbles that ensure even boiling and prevent bumping and boiling over. It does not make the boiling faster and avoids product loss by preventing the bumping of the solution during boiling.Subsequently, simple distillation was conducted. Distillation of the solution was done to purify the tert-butyl chloride by separating the impurities through their different boiling points. Continuous flow of water on the condenser during distillation is important to properly cool and condense the hot product vapors. It affects the cooling capacity of the condenser which would affect the product recovery.Table I. Experimental Results

Weight of tert-butyl alcohol, g : 7.80Volume of tert-butyl alcohol, g : 2.7Theoretical yield, mL : 11.45% yield : 23.58%

The synthesis via SN1 and E1 resulted in a tert-butyl chloride with a volume of 2.7ml. Pure tert-butyl chloride has a colorless appearance and our result yielded a similar substance. However, impurities may still be present, such as minute traces of HCl. It would be difficult to detect these errors because they come from different sources. A possible source of these errors is the addition of the NaHCO3 and CaCl2. It is possible that not enough NaHCO3 was added to neutralize the excess HCl, or the addition of anhydrous CaCl2 was not enough to react with all the unreacted alcohol and water. These errors would lead to an impure product.

Tert-butyl chloride is a volatile substance which can cause the loss of product. The way the process was conducted may have resulted in reagent loss. During separation of layers, the separatory funnel is not covered and some tert- butyl chloride may have vaporized. The separation of the layers through the stopcock is another source of error. Some aqueous layer might have been retained with the organic layer due to improper separation. Also, the decantation and distillation of the solution may contribute to product loss. Inadequate drying of the filtrate may have lead to backward reaction and heating the distillation system too abruptly and erratically will cause a sudden increase in temperature. These possible errors may have accounted for the low yield.CONCLUSION AND RECOMMENDATION

With a percent yield of 23.58%, it can be concluded that not all of the tert-butyl alcohol reacted to form the desired product, tert-butyl chloride. As expected, some of the product formed 2-methylpropene, which was released in a gaseous form and resulted in a lower yield. However, the results show a relatively low percent yield despite precautions. After adding certain reactants to favor the forward reaction, the SN1 was not effectively maximized and the E1 reaction was favored instead.

For further improvement, the usage of a very cold concentrated hydrochloric acid is recommended. More anhydrous calcium chloride should also be added until there is undissolved CaCl2. Careful observation should also be observed in the steps that could result in side reactions.REFERENCES[1] McMurry, J. Organic Chemistry 6th edition. Brooks/Cole-Thomson Learning., USA. 2004

[2] Mackenzie, C.A. Experimental Organic Chemistry. Prentice hall, Inc., New Jersey. 1967

[3] March, Jerry. Advanced Organic Chemistry. 4th edition. John Wiley & Sons. Inc.

[4] Organic Chemistry Group. Organic Chemistry Laboratory Manual. Institute of Chemistry, College of Science, University of the Philippines, Diliman, Quezon City. 2008.APPENDIX (see attached)

RESULTS AND DISCUSSION

: 7.80: 2.70: 11.45: 23.58%