SN1 AND SN2 REACTIONS: Synthesis of n-butyl bromide and t-butyl chloride

Traya, Levie Grace M.

INTRODUCTION

Alkyl halides can be synthesized when alcohols react with hydrogen halides. An alkyl halide is a hydrogen-substituted alkane, and a hydrogen halide is a compound consisting of hydrogen bonded to a halogen. Alkyl halides are classified as primary, secondary and tertiary depending on the number of alkyl substituents directly attached to the carbon bearing the halogen atom. The mechanism of acid catalyzed substitution of alcohols are termed SN1 and SN2, where S stands for substitution while sub-“N” stands for nucleophilic, and the numbers 1 and 2 describes the first and second order respectively. The 1 and 2 also represents whether the reaction is unimolecular or bimolecular reaction. The secondary alcohols favor the react with hydrogen halides by SN1 and SN2 mechanism. For primary or methyl alcohol, both molecules undergo SN2 mechanism while tertiary alcohols undergo SN1 mechanism.

In SN1 reaction, the protonated alcohol, or oxonium ion losses water molecule and forms a carbocation intemidiate in the rate-determining step. The carbocation is then rapidly attacked by halide ion to form the alkyl halide. On the other hand, SN2 reaction happens when the nucleophile assists in the expulsion if water from the

oxonium ion via bimolecular transition rate. The SN2 process is expected to be especially slow and is not observed in tertiary alcohols due to the transition state being crowded as the degree of substitution decreases at the reating center, SN2 reaction becomes dominant over SN1 reaction.

The main purpose of this experiment is to prepare n-butyl bromide from n-butyl alcohol via SN2 and t-butyl chloride from t-butyl alcohol via SN1 reaction. Also this experiment aims to understand the SN1 and SN2 mechanism involved in the reaction and to determine the yielded percentage of the experiment.

METHODOLOGY

Materials for synthesis of n-butyl bromide

This experiment will make use of the following: (a) sodium bromide, (b) anhydrous calcium chloride, (c) n-butanol, (d) conc. sulfuric acid, (e) sodium hydroxide, (f) beaker, (g) thermometer, (h) boiling chips, (i) boiling flask, (j) reflux, (k) condenser, (l) rubber tubing, (m) hot plate, (n) iron stand, (o) iron ring, (p) iron, (q) iron holder, (r) wire gauze, (s) stirring rod, (t) pipette, (u) aspirator, (v) separatory funnel,

(w) glass stopper and (x) simple distillation set-up.

Materials for synthesis of t-butyl chloride

This experiment will make use of the following: (a) t-butanol, (b) conc. hydrochloric acid, (c) aqueous sodium bicarbonate, (d) anhydrous calcium chloride, (e) separatory funnel, (f) glass stopper, (g) iron stand, iron ring, (h) pipette, (i) aspirator, (j) Erlenmeyer flask, (k) boiling chips, (l) distilling apparatus, (m) dry-apparatus set-up and (n) steam bath set-up.

Procedure for synthesis of n-butyl bromide

Figure 1. Reflux set-up

Place 24.0 g of sodium bromide in a 250-mL round bottomed flask and add 25-mL of water and 17-mL of n-butanol. Cool the mixture in a ice bath and slowly add 20-mL of conc. sulfuric acid with continuous swirling in the ice bath. Add several boiling stones to the mixture. Assemble the reflux set-up in an iron stand as shown in fig 1. with the use of a round bottom flask s reaction vessel. The inverted funnel in the beaker acts as a trap to absorb the HBr gas evolved during the reaction period. Heat the mixture with the heating mantle, an oil bath, or a flame until the mixture begins to

reflux gently. Heat the mixture under reflux for 30 mins. Two layers will form during this time.

At the end of this reflux period, remove the heating source and allow the mixture to cool. Remove the condenser and reassemble for simple distillation. Add several new boiling stones to the round-bottomed flask. Distill the mixture and collect the distillate in a receiver cooled by an ice bath. The alkyl halide co-distills with water and then separates into two phases in the receiver. Distill the mixture until the distillate appears clear. The temperature should reach 110-115℃ by that time. While the distillation is going on, remove the receiver and collect a few drops of distillate in a test tube containing some water. Check to see whether the distillate is completely soluble. If it is, no alkyl halide present and the distillation may be stopped. If the distillate produces insoluble droplets in the water, then the distillation must be continued until the distillate becomes completely water-soluble.

Transfer the distillate to a separatory funnel, add 25-mL of water to it, and shake the mixture. Drain the lower layer. Discard the aqueous layer after making certain that the correct layer has been saved. Return the alkyl halide to the funnel and add to it 15-mL of cold sulfuric acid. Swirl the mixture until it is thoroughly mixed. Stopper the funnel and shake it thoroughly. Allow several minutes for the phase to separate. Drain the lower layer from the funnel. Allow several minutes for further separation and

drain again. Wash the n-butyl bromide with 15-mL of 10%NaOH solution. Carefully separate the layer and be certain to save the organic layer. Dry the crude n-butyl bromide over 1.5 g anhydrous calcium chloride. Stopper the flask and swirl the contents until the liquid is clear.

Decant the clear liquid into a drying distilling flask. Add boiling stones and distill the crude n-butyl bromide in a dry apparatus. Collect the material that boils between 98℃ and 102℃. Weigh the product and calculate the percentage yield.

Procedure for synthesis of t-butyl chloride

In a 125-mL separatory funnel, place 22-mL of t-butyl alcohol and 50-mL of conc. hydrochloric acid. Do not stopper the funnel. Gently swirl the mixture in the separatory funnel for about 1 minute. Stopper the separatory funnel and carefully invert it with occasional venting. Shake the funnel for 2-3 minutes. Allow the mixture to stand in the separatory funnel until the two layers have completely separated.

Wash the organic layer with one 25-mL portion of water. Again, separate the layers and discard the aqueous layer. Wash the organic layer with 25-mL portion of 5% aqueous sodium bicarbonate. Gently swirl the funnel until the contents are thoroughly mixed. Stopper the funnel, and carefully invert it from time to time with venting for 1 minute. Allow the layers to separate, and drain the lower aqueous bicarbonate layer. Wash the organic layer with on 25-mL

portion of water, and again drain the lower aqueous layer.

Transfer the organic layer in a small dry erlenmayer flask. Pour it from the top of separatory funnel. Dry the crude t-butyl chloride over anhydrous calcium chloride until it is clear. Swirl the alkyl halide with the drying agent to aid the drying. Decant the clear material into a small distilling flask. Add boiling stones and distill the crude t-butyl chloride in a dry apparatus, using steam bath. Collect the pure t-butyl chloride in a receiver cooled in ice. Collect the material that boils between 49℃ and 52℃. Weigh the product and calculate the percentage yield.

DISSCUSSION

Synthesis of t-butyl chloride

In this experiment, n-butyl alcohol, a primary alkyl halide, is prepared by reacting the alkyl halide with sodium bromide and sulfuric acid. From the chemical reaction presented below, the reaction produces hydrogen halides during the reflux when sodium bromide reacted with sulfuric acid.

2 NaBr + H2SO4 → 2 HBr + Na2SO4

The produced hydrogen halide is used to convert the alkyl halide into n-butyl bromide via nucleophilic substitution. Meanwhile, the excess sulfuric acid serves as a reagent that speeds up the reaction by shifting the equilibrium and thus produces more hydrobromic acid. Sulfuric acid will

also be protonated as hydroxyl group of the alkyl halide in order to remove water from the equation rather than the hydroxide ion OH-. Sulfuric acid will also be protonated to produce water and be deactivated to become a nucleophile to avoid the n-butyl bromide from being converted back into its alcohol form because water can do a nucleophilic attack on the n-butyl bromide.

The synthesis of this alkyl halide is SN2 reaction. The hydrogen bromide reacts with the alkyl halide attacking it from the opposite side, thereby inverting stereochemical configuration, of the leaving group. The mechanism is shown in the figure 2 as below.

Figure 2

Sodium bicarbonate is added to serve as neutralizer to the acidic solution. The drying agent anhydrous calcium chloride is added in order to absorb water droplets in and to purify the organic layer. To ensure that all water droplets are eliminated, an excess anhydrous calcium chloride should be added because if there is any water droplets that remains in the materials collected, it would interfere with the analysis. After the drying agent is filtered out, the boiling stones are added to prevent over boiling during distillation. The

distillation is done to purify the n-butyl bromide in the temperature range of 98℃ and 102℃.

Synthesis of n-butyl chloride

In this experiment, t-butyl alcohol is converted to t-butyl chloride. In order to synthesize t-butyl chloride from t-butyl alcohol, hydrochloric acid is used to react with t-butyl alcohol. During the reaction, t-butyl alcohol will undergo SN1 reaction.

The SN1 reaction is where the rate of formation of t-butyl chloride is dependent only on the concentration of the alcohol and is independent to the amount of acid used. A strong concentrated HCl is added to t-butyl alcohol to provide an acidic medium and to protonate the electron rich hydroxyl group allowing it to leave as a molecule of H2O.

The t-butyl alcohol will act as a nucleophile which will attack the proton from the hyrdonium ion in the solution. Considering Bronsted-Lowry theory, t-butyl alcohol is considered as a base in this reaction. It is because t-butyl alcohol accepts proton from the hydronium ion and hence forming t-butyl oxonium ion. In order to synthesize the molecule, the bond between carbon and oxygen of the molecule must be broken. This breaking of the bond will result to the formation of a carbocation and H2O. This carbocation will act as an electrophile. Due to the lack of electron, a nucleophile, Cl-, will attack the carbocation and therefore stabilizing the molecule. The carbocation will accept the

electron given by Cl- forming t-butyl chloride.

In adding HCl to t-butyl chloride a simple reaction will be observed. A visible 2 layers will be form of which (with the aid of a sepatatory funnel) the two layers will be the t-butyl chloride (upper) and the aqueouas solution (lower). After draining, anhydrous calcium chloride will be added and will act as a drying agent where it will remove any amount of water from the organic solution. It also helps in decreasing the solubility of the solution to aqueous solvents. The steam bath will be performed to purify the crude product.

In addition, to neutralize the acidic medium that was caused by HCl, a sodium bicarbonate will be added. This can be shown as:

NaCHO3 + HCl ---- > NaCl + H2O + CO2

A salt sodium chloride, water and carbon dioxide will be formed during the neutralization process. Due to the neutralization, water will be produce giving a visible, yet, another two layers, as being highly soluble; sodium chloride will be discarded together with the aqueous layer. To isolate and reduce the solubility of the organic layer from water, distilled water will be used to wash the organic layer. To produce the crude product of the t-butyl alcohol, a drying agent, anhydrous calcium chloride is added.

REFERENCES

Books:

Bruice, Paula Y. Organic Chemistry. 4th ed. Pearson Prentice Hall. 2004

Boyd, R. N. & Morrison, R.T. Organic Chemistry. 6th Ed. Prentice Hall. 1992

Carey, Francis A. Organic Chemistry. 4th ed. McGraw-Hill. 2000

Chang, Reymond. Chemistry. 10th ed. McGraw-Hill. 2010

McMury, John.Organic. Chemistry.8th ed. Brooks/Cole, Cengage Learning. 2012

Web:

(2013, 02). Nucleophilic Substitution: Synthesis of N-Butyl Bromide and T-Pentyl Chloride. StudyMode.com. Retrieved 18 April 2015 from http://www.studymode.com/essays/Nucleophilic-Substitution-Synthesis-Of-n-Butyl-Bromide-1460558.html