I. Objectives :
1. To produce tert-butyl chloride from tert-butyl alcohol
2. To understand the SN1 and SN2 mechanism involved in the reaction
3. To determine the yield of percentage of t-butyl chloride
An alkyl halide is a derivative of alkanes. Alkanes are hydrocarbons with a functional group C-C. The hydrogen atom is then replaced by a halogen (F, Cl, Br, I). Therefore, alkyl halides are compounds that have a halogen atom bonded to a saturated, sp3 hybridized carbon atom. These could be classified according to the number of alkyl groups attached to the carbon that is bonded to the halogen atom. A methyl halide does not contain any alkyl group, a primary halide has one alkyl group, a secondary halide has two alkyl groups, and a tertiary halide has three. Synthesis of alkyl halides can be performed from a variety of starting materials and specific mechanisms: from alkenes by addition, from alkanes by substitution, and from alcohols via nucleophilic substitution. The reaction of alcohols with hydrogen halides, like HCl, HBr, and HI, would result to their corresponding alkyl halides and water. The formation of alkyl halides has different mechanisms, depending on the alcohol used for the synthesis. Tertiary alcohols react with hydrogen halides faster compared to the secondary and primary alcohols. Tertiary alcohols could react with hydrogen halides rapidly at room temperature, while the reaction of primary alcohols with hydrogen halides takes a longer time and should be at a high temperature. Tertiary alcohols can be converted to their corresponding alkyl chlorides by the addition of concentrated hydrochloric acid to the alcohol. In this experiment, concentrated HCl is added to tert-butyl alcohol to produce tert-butyl chloride via SN1 reaction.
R3COH > R2CHOH > RCH2OH > CH3OH
Tertiary alcohols react readily with HX alone to form alkyl halide, while secondary and primary require catalyse in the halo hydrogenation reaction. Alkyl halide can be prepared from alcohol by reacting them with a hydrogen halide, HX (X=Cl,Br, or I). 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 number “1” and “2” is described as first order and second order respectively. The “1” or “2” is also represent the reaction is unimolecular or bimolecular reaction. The secondary alcohols are more favor to react with hydrogen halides by both SN1 and SN2 mechanisms. For primary or methyl alcohol, both molecules undergo SN1 mechanism while tertiary alcohol undergoes SN2 mechanism. In the reaction of a tertiary alcohol and a hydrogen halide, the initial steps are the protonation of the alcohol oxygen and then the formation of the carbocation. The hydrogen ion of the hydrogen halide would first bond with the hydroxide ion, OH-, of the alcohol forming water. The water formed could easily leave the alcohol, and this would result to the formation of a carbocation. The halide ion would then react with the carbocation, forming the alkyl halide. Based on the reactivity of hydrogen halides towards alcohols, HCl could readily react with tertiary alcohols. In this experiment, the reaction of tert-butyl alcohol with concentrated HCl at room temperature is analyzed.
A. Synthesis of Tert-Butyl Chloride
The major reactants needed in this experiment are the tert-butyl alcohol and the concentrated hydrochloric acid, HCl. First, 5mL tert-butyl alcohol and 15mL concentrated HCl were placed in a 125-mL separatory funnel. The mixture was swirled gently without shaking, and was relieved of internal pressure by slowly opening the stopcock. The separatory funnel was then placed on the ring stand allowing the two layers to separate. The lower aqueous layer was drained and placed into a 250mL Erlenmeyer flask. Then, 40mL of saturated
sodium bicarbonate solution was added into the organic layer remaining in the funnel. The previous steps was repeated. The organic layer was then transferred into a dry 50mL flask with a small amount of anhydrous calcium chloride. The liquid was decanted into a preweighed 50mL beaker and the crude product was weighed.
B. Qualitative Chemical tests for Reactivity
Four small test tubes was labeled properly. Into two test tubes, 2-5 drops of the synthesized tert-butyl chloride was added. Into the two another test tubes, 2-5 drops of chlorobenzene was added. To one test tube of each compound, 1mL of potassium iodide solution was added and in the other two test tubes 1mL of silver nitrate solution was added. The stopper was placed in the test tube and the content was shaked vigorously. The time it takes to form any precipitation was noted. The differences in the results was also noted.
IV. Data and Discussion
Data 1.1. A. Synthesis of Tert-Butyl Chloride
Hydrochloric acidt-butanolt-butyl chloride
Molecular FormulaHCl C4H10O C4H10Cl
Condensed FormulaHCl (CH3)3COH (CH3)3CCl
Molecular Weight 36.458 g/mol74.12 g/mol 92.562 g/mol g 17.7 g 3.904 g 0.64 g
Volume 15 mL 5 mL % yield 13.23 %
Data 1.1. B. Sample Computation
1. Weight of the vial container=13.65g
Weight of the crude product + vial =14.29g
Weight of the crude product = 0.64g
2. Theoretical Yield =4.8391g
3. Percentage Error=86.78%
gHCl = (Density HCl)(volume HCl)gt-butanol = (Density t-butanol)(volume t-butanol)
= 1.18 g/mL HCl * 15mL = (0.7809 g/mL)(5mL)
= 17.7 g = 3.904 g
# of mol of (CH3)3COH = 3.875g / 74.12 g.mol = 0.0526 mol
(CH3)3COH(aq) + HCl(aq) (CH3)3CCl(aq) + H2O(l)
1 mol of (CH3)3COH produces 1 mol of (CH3)3CCl
0.00523 mol of (CH3)3COH produces 0.0523 mol of (CH3)3CCl
Theoretical wt. of (CH3)3CCl = 0.0526 mol * 92.562 g.mol-1 = 4.8687 g
Percentage Yield = Experimental Value X 100
= 0.64 g X 100
= 13.14 %
Percentage Error = theoretical value – experimental value X 100
= 14.8391 g – 0.64g X 100
= 86.78 %
Qualitative Chemical tests for Reactivity
Reagent | Reactivity | Time
(seconds)| t-butyl chloride + Nal| No ppt. formed, clear solution| ————| Benzyl chloride + Nal| Bubbles formed| ————| t-butyl chloride + AgNO3| White ppt. formed| Right after AgNo3 was added = 0.8 s | Benzyl chloride + AgNO3| No ppt. formed| ———–|
In this experiment, 5ml of tert-butyl alcohol was reacted to Hydrogen chloride (HCl). By nucleophilic substitution, the chloride was substituted to the alcohol to obtain the reaction below:
It should be noted that excess amount of HCl was added to keep the reaction from going too fast and to avoid the formation of side reactions such as isopropene. In addition, concentrated HCl was used because concentrated HCl is very volatile, thus, it prevents the possible escape of HCl vapor during its reaction with tert-butyl alcohol. The reaction yielded the product tert-butyl chloride. Data 1.1 shows the details of this reaction.
In the diagram above, the t-butyl alcohol acts as a nucleophile which attacks the proton from the hydronium ion in the solution. According to Bronsted-Lowry Theory, the t-butyl alcohol is considered as a base in this reaction. This is because it accepts a proton from the hydronium ion and hence t-butyloxonium ion is formed. In order to become a stable molecule, the bond between the carbon and oxygen of the t-butyloxonium ion breaks heterolytically. The breaking of bond between carbon and oxygen leads to the formation of a carbocation and a leaving group of water.
As shown in the diagram 5, the carbocation is formed and it is acts as eletrophile which is the species lack of electron. Due to the lacking of electron, another nucleophile, chloride ion, Cl-, tends to attack the carbocation and hence to achieve a stable molecule. The carbocation acts as a Lewis acid which accepts electron from the chloride ion, Cl- to form t-butyl chloride. The formation of t-butyl chloride is synthesized via SN1
mechanism is shown.
The addition of concentrated hydrochloric acid into the t-butyl chloride causes the formation of cloudy solution is formed when stirring. The reaction between t-butyl alcohol and hydrogen chloride is a simple reaction which can take place in the room temperature. Two layers are formed after transferring the mixture into a separatory funnel.
Aqueous sodium bicarbonate solution is added into the organic to neutralize the acidic medium that caused by concentrated hydrochloric acid added. Since sodium bicarbonate is an alkaline solution. The neutralization process between sodium carbonate and hydrochloric acid could be shown in the following chemical equation.
NaHCO3 (aq) + HCl (aq CO2 (g)) –> NaCl (aq) + H2O (l) + CO2 (g)
The sodium chloride salt, water, and gaseous carbon dioxide are formed in the neutralization process. The two layers are formed second time due to the formation of water in the neutralization. The sodium bicarbonate is highly soluble in aqueous layer which is being discarded together with aqueous layer.
CaCl (s) + H2O (l) –> CaCl.nH2O (s)
calcium chloride drying agent
The presence of tertiary alkyl halides can be tested by using silver nitrate test. Some of the product formed in the experiment is added with silver nitrate solution. The observation we obtained is a white precipitate is formed after addition of silver nitrate solution. This is because the t-butyl chloride containing tertiary alkyl group which reacts rapidly via SN1 mechanism with the silver nitrate to form a precipitate of silver chloride.
As shown in the diagram above, a highly polar solvent (ethanol) is used to dissolve the butyl chloride. The chloride will ionize to the butyl cation and chloride ion. The butyl cation will react with the alcohol solvent to form the butyl ethyl ether via formation of C-O bond. The HCl is formed in this reaction too. In this case both products are soluble; however, if silver ion is present in the solution, insoluble AgCl will form and a precipitate will be visible. Primary halides do not react in this test, and secondary reacts only slowly with heating.
1. Write the complete step-wise mechanism for the reaction of t-butyl alcohol with concentrated hydrochloric acid.
2. What is the purporse of sodium bicarbonate solution ? dehydrate calcium chloride?
Sodium bicarbonate was added to act as neutralizer of the acid in the medium. Dehydrate calcium chloride was added to remove the water droplets. It acts as a solidifying agent.
3. What is the limiting reagent ?
The limiting reagent is tert-butyl alcohol.
4. Give two observations for this experiment.
On the addition of AgNO3 solution, the tert-butyl chloride resulted as white solution with white precipitate occurred directly after adding AgNO3. While chlorobenzene it has a clear solution with bubbles at the bottom. On the other hand, addition of potassium iodide solution on tert-butyl chloride was observed as colorless and no precipitate form while in the chlorobenzene, a colorless s solution with a bubble formation at the bottom was observed.
5. Explain the results of the calculated percentage yield and percentage error.
It can be seen in Data 1.1B that an actual yield of 0.64 g of tert-butyl chloride was obtained; and compared to the theoretical yield of 4.8391 g, the actual yield was considerably distant. From this data, the actual percent yield is 13.23%; in addition, it can be identified that a deviation of the actual yield from the theoretical gave 86.78% error, which suggests a substantial product yield less than the expected product yield.
In this experiment, the synthesis of an alkyl halide from an alcohol was carried out, using HCl as the hydrogen halide. Through nucleophilic substitution, the reaction between tert-butyl alcohol and HCl gave off tert-butyl chloride and water.From the synthesis process done, 0.64 g of tert-butyl chloride was obtained, suggesting an actual percent yield of 13.23%. It can therefore be concluded that the reaction between HCl and tert-butyl alcohol can give considerable results to produce tert-butyl chloride.
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