SN1 Reaction of T-butyl clhoride PDF

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Bryanna Tanase David and Mark Synthesis and Reactivity of Tert-Butyl Chloride via an SN1 Reaction Christopher Cain CHM 2210L-022 10-13-2017

Introduction Compounds in which bromine, fluorine, iodine or chlorine (the halogens) replace a hydrogen atom on an alkane are known as alkyl halides1. They can be easily turned into other atoms or functional groups because the halogen atom on the halide can leave with its bonding pair of electrons to form a halide ion and a stable cation, known as a carbocation1. Alkyl halides can take part in both substitution and elimination reactions, but this experiment focuses on substitution where the halide is replaced by another atom when it leaves the original carbon atom it was attached to1. Nucleophilic substitution reactions happen when an electron rich atom or molecule (nucleophile) reacts with a carbon atom that has a halide attached to it2. The halide is electronegative and can serve as the leaving group, departing on its own, while the nucleophile uses its lone pair of electrons to bond to the carbon atom in the halide’s place. The general reaction scheme can be seen below in Figure 1. There are two main types of nucleophilic substitution reactions: SN1 and SN2.

Figure 1: General Substitution Mechanism

An SN1 reaction stands for first order nucleophilic substitution. The first order idea comes from the fact that the rate of the reaction only depends on the concentration of the substrate (molecule with the leaving group on it), and not the concentration of the nucleophile2. Thus, the rate law for the reaction is written as rate=k[R-LG]2. The reaction is also unimolecular, which means that only one molecule is involved in the rate determining step, or slow step of the reaction2. There are two steps in an SN1 reaction. The first is the rate determining step where the

leaving group detaches itself from the carbon atom to form a carbocation intermediate, and the second step is when the nucleophile (negatively charged atom) attacks the carbocation to form a new bond and replace the leaving group2 (see Figure 2 below).

Figure 2: General SN1 Mechanism

There are several factors that affect how an SN1 reaction occurs. For instance, the stability of the carbocation in an SN1 reaction is important because it is an intermediate involved in the rate determining step. Tertiary carbocations (three things attached) are more stable than secondary (two things attached), which are more stable than primary (one thing attached) which are more stable than methyl carbocations. The reason why tertiary carbocations are more stable is because the three different atoms attached provide more surface area for the nucleophile’s negative charge to be spread around, rather that it being concentrated in one place2. Examples of the carbocations are provided below in Figure 3. Another important factor for SN1 reactions is the nature of the leaving group, because it is also involved in the rate determining step2. The stronger the leaving group, the faster the reaction2. The best leaving groups are the conjugate bases of strong acids, since strong acids dissociate completely into a cation and anion that can stand on their own3. Examples of good leaving groups include Cl- and H2O2. Polar solvents such as water and alcohols are favored for SN1 reactions because the partial negative charge on a polar molecule like water can stabilize the positive charge of the cation. In summary, SN 1 reactions occurs for systems with good leaving groups and stable carbocations in a polar solvent2.

Figure 3: Types of Carbocations

An SN2 reaction, or second order nucleophilic substitution reaction, is bimolecular meaning that the rate determining step involves interaction between two molecules, the nucleophile and the carbon atom bearing the leaving group 2. Thus, the rate law for the reaction can be written as rate=k[Nu][R-LG]2. The pathway for an SN 2 reaction occurs all in one step where the nucleophile attacks the carbon and the leaving group leaves at the same time 2. A general mechanism for the reactions is illustrated below in Figure 4. As a result of this, there is no carbocation intermediate, but a transition state is often drawn when wring these reactions to show the simultaneous bond breaking and making that occurs2.

Figure 4: General SN2 Mechanism

The factors that affect SN2 reactions are the exact opposite of SN1. For example, a primary or methyl carbon atom is preferred over a tertiary one because they make it easier to achieve the transition state that is essential to the SN2 reaction. Since the bond between the carbon and the leaving group is broken in the rate determining step, poor leaving groups such as OH are preferred for an SN2 reaction2. The reactivity of the nucleophile is considered instead due to the fact that it is involved in the rate determining step, and the more reactive it is, the better2. The most reactive nucleophiles are strong bases such as CH3O-, because their negative charge gives them a high affinity for a positive nucleus, and they will attack the carbon bearing the leaving group so quickly that the carbocation intermediate will not form3. Polar aprotic solvents such as tBu-OK and DMSO are favored for an SN2 reaction because they can enhance the

reactivity of the nucleophile2. In summary, SN2 reactions favor poor leaving groups, primary and secondary carbon atoms, and stronger nucleophiles2. For this experiment t-butyl chloride was synthesized form t-butyl alcohol via an SN1 reaction and the mechanism by which the reaction occurs is pictured below in Figure 5.

Figure 5: T-Butyl Chloride Synthesis Reaction

This substitution mechanism is unique because it involves an alcohol, which is generally not a good leaving group. As a result, the HCl molecule acts as a Bronsted Lowry Base and donates its hydrogen to the t-butyl alcohol, breaking its bond with chlorine. Thus, the alcohol is turned into water, which is a much better leaving group, and the substitution reaction proceeds as normal, with the water leaving to form a carbocation, and the chlorine nucleophile attaching in the alcohol’s place to form t-butyl chloride. A possible side reaction for this mechanism is pictured below in Figure 6. This side reaction is characterized as an elimination reaction due to the formation of a double bond and the fact that the chlorine ion leaves with the hydrogen ion rather than bonding to the carbocation.

Figure 6: Side Reaction

Procedure The procedure for this lab experiment involved two distinct parts: synthesizing t-butyl chloride from t-butyl alcohol by a SN1 reaction, and to test the reactivity of the formed t-butyl chloride by reacting it with silver nitrate and sodium iodide. The first step in setting up the SN 1 reaction was to cool 15 mL of HCl to 5°C in an ice bath and transfer it to an unstoppered separatory funnel, and add 5 mL t-butyl alcohol once the HCl was placed in. The stopper was placed onto the funnel and the reaction mixture was swirled for 20 minutes and left on a ring stand, to allow the layers to separate. Once the layers were separate, a drop of water was added to the funnel via a dropper to check the layers and tell them apart. Upon identifying the layers, the HCl one was drawn out from the funnel into an aqueous waste beaker and set to the side. 30 mL of H2O were added to the funnel to wash the t-butyl alcohol layer, the layers were left to separate, and the excess H2O was drawn out into the aqueous waste beaker. Following this, 10 mL of 5% NaHCO 3 were added to the separatory funnel to wash the alcohol, and the funnel was vented frequently by removing the stopper due to the large amount of carbon dioxide gas produced by the reaction between t-butyl alcohol and NaHCO3. When all the pressure from the reaction subsided, the H2O layer was removed into the aqueous waste beaker, and 10 mL H2O were added to wash the product layer, which now contained t-butyl chloride. The t-butyl chloride was removed from the separatory funnel into a clean and dry 125 mL Erlenmeyer flask. The t-butyl chloride was then dried by adding 3.65 g of CaCl2 to the flask, and the dry product was transferred to a 125-mL round bottom flask for purification by distillation. The simple distillation apparatus was assembled as shown in Figure 7 below, the temperature regulator was set to 30°C so that the temperature would increase at a steady rate, and the Erlenmeyer flask was well buried in the sand bath on the heat source. Boiling chips were

added to the flask to ensure an even boil and the t-butyl chloride was distilled into a preweighed sample bottle at a range of 49-54°C. The exact boiling point of the t-butyl chloride during the distillation was recorded along with the final mass of it so that the percent yield could be calculated. The final portion of the procedure was to perform reactivity tests on the newly purified t-butyl chloride. Four test tubes were cleaned and put on a test tube rack to be used for each trial of the reaction. The first test tube contained a combination 1 mL NaI/acetone and 0.075 mL t-butyl chloride, the second test tube contained a 1 mL 1% AgNO 3/ ethanol and 0.075 mL t-butyl chloride mix, test tube 3 held 0.2 mL of 1-clhorobutane and 1 ml NaI/acetone, and test tube 4 contained 0.2 mL t-butyl chloride and 1 mL AgNO 3/ ethanol. For all test tubes, the procedure went as follows: place stopper in tube, shake to mix the contents, and record time of first appearance of a precipitate along with the color and appearance of the reaction at completion. If no reaction occurred in 5 min, the tubes were placed in an ice bath and monitored for a reaction after 5-6 minutes of being left undisturbed

Figure 7: Simple Distillation Apparatus5

Table of Chemicals Chemical

Molar Mass

Melting Point

Boiling Point

Sodium iodide

149.89

65.1

NA

t-butyl chloride

92.57

-25

51

Toxicity/Hazards

contact with skin or eyes. May cause blindness blistering, and may be toxic to mucous membranes Do not inhale, ingest, or put in contact with skin or eyes. Toxic to nervous system. Wear PPE Do not inhale, ingest, or put in contact with skin or eyes. Wear PPE.

Table 1: Physical and Chemical Properties

Results Test Tube 1 (t-BuCl and NaI)

2 (t-BuCl and AgNO3) 3 (1-Cl butane and AgNO3) 4 1-Cl butane and NaI)

Color Slightl y yellow Clear

Appearance Clear

Precipitate No

Total Reaction Time 5 mins

Cloudy

Yes, at 9:09

Clear

Clear

No

1 mins, positive result 5 mins

Slight yellow

Cloudy

Yes, at 9:15

1 mins, positive result

Boiling Point T-butyl Chloride: 38°C Percent Yield T-butyl Chloride: 6.36% Table 2: Reactivity Test Results

Calculations Weight t-butyl chloride= 0.3 g Density t-butyl alcohol=0.775 g/ml Weight t-butyl alcohol= 0.775 g/ml x 5 ml= 3.775 g Moles t-butyl alcohol= 3.775 g x

1 mol = 0.051 moles t-butyl alcohol 74.12 g/mol

I mole t-butyl alcohol produces 1 mole t-butyl chloride 0.051 mol t-butyl alcohol produces 0.051 mol t-butyl chloride Theoretical yield t-butyl chloride= 0.051 mol x

Percent yield t-butyl chloride =

92.57 g /mol = 4.715g t-butyl chloride 1mol

actual yield x 100= theoretical yield

0.3 g 4.715 g

x 100=6.36%

Discussion During the distillation of the synthesized t-butyl chloride, the distillation apparatus leaked due to an improper seal on the condenser and other pieces of the device. This resulted in a loss of some of the t-butyl chloride, which is why only 0.075 mL t-butyl chloride were added to test tubes 1 and 2 as opposed to 0.1 mL. The smaller amount of t-butyl chloride also gave a lower percent yield than desired. It could also be possible that the reaction did not fully complete, resulting in s lower yield of t-butyl chloride. The reactions observed in test tubes 1-4 went exactly as expected. As noted in Table 2, test tube 1 containing the t-butyl chloride and sodium iodide/acetone mixture did not form a precipitate. Instead it was clear and had a slightly yellow color to the presence of iodide. The idea that a precipitate did not form makes sense because t-butyl chloride is a tertiary alkyl halide

and thus cannot react with iodide by an SN2 mechanism. In addition, acetone is a nonpolar solvent, and tertiary alkyl halides prefer polar solvents instead because they help them generate a stable carbocation intermediate. In test tube 2, where t-butyl chloride was reacted with AgNO3 and ethanol, a precipitate was formed immediately because ethanol is a polar solvent that allows for the formation of a stable carbocation, and the silver cation pairs up with the detached chloride anion to form a molecule of insoluble AgCl2, which indicates an SN1 reaction. Test tubes 3 and 4 contained 1-clhorobutane which is a primary alkyl halide. Based on the conditions for substitution reactions discussed, it would be expected that a primary alkyl halide would participate in an SN2 mechanism with a nonpolar solvent. These expectations are consistent with the observations in test tubes 3 and 4 as noted in Table 2. Test tube 3 contained 1-clhorobutane and the AgNO3/ethanol mixture did not form a precipitate, and instead was clear throughout. This is consistent with the information provided because ethanol is a polar solvent and primary alkyl halides prefer nonpolar solvents instead because they allow for the nucleophile to be more reactive. In addition, ethanol is normally used in SN1 reactions, and primary alkyl halides cannot participate in SN1 mechanisms because they generate unstable carbocations. However, in test tube 4, a precipitate did form because 1-clhorobutane was mixed with NaI/acetone. Acetone is a nonpolar solvent which dissolves NaI, and as discussed primary alkyl halides prefer nonpolar solvents because they increase the reactivity of the nucleophile. Because 1-clhorobutane is a primary alkyl halide, it reacted with the iodide ions in NaI by an SN2 mechanism to produce a precipitate of NaCl. The solution in the test tube was cloudy due to the precipitate and yellow from the iodide. Conclusion

The objectives for this experiment were to synthesize t-butyl chloride from t-butyl alcohol by an SN1 reaction and to test the reactivity of the synthesized t-butyl chloride through tests with AgNO3 and NaI. The experiment accomplished what it set out to do because t-butyl chloride was successfully synthesized and the reactivity tests went as expected. The results of the reactivity tests as shown in Table 2 reveal that the factors affecting SN 1 and SN2 reactions are key indicators as to whether or not a reaction will occur. In test tube 1 (t-butyl chloride and AgNO3/ethanol), a reaction occurred because ethanol is a polar solvent that helps separate the chloride ion from the t-butyl chloride and t-butyl chloride is a tertiary alkyl halide which forms a stable carbocation. Also, the silver ion can react with chlorine to create insoluble silver chloride and promote an SN1 reaction. In a similar way, a precipitate formed in test tube 4, which contained 1-clhorobutane and NaI/acetone because 1-clhorobutane is a primary alkyl halide which can react with iodide ions by an SN2 mechanism to produce NaCl, and the nonpolar solvent acetone allows for increased reactivity of the nucleophile, which increase the SN2 reaction rate. The techniques in this lab can be applied to the food industry in that SN1 reactions are responsible for the way nitrosamines, chemicals found in cured meats and many other foods, can act as toxins4. Nitrosamines are formed when amines that occur in food react with the preservative sodium nitrite, a preservative added to meat to prevent the growth of bacteria which causes food poisoning4. In he presence of heat, nitrosamines are converted to diazonium ions, a good leaving group, which can react with biological nucleophiles (DNA or enzyme) in the cell. If the substitution reaction occurs at a critical site in a cell, it can disrupt the function, leading to cell death or cancer4.

References [1] Weldegirma, S. Experimental Organic Chemistry Laboratory Manual, 7th ed.; Procopy Inc: Tampa, Florida

[2] Hunt, I. Ch 8: Nucleophilic Substitution http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch08/ch8-11.html (accessed Oct 15, 2017). [3] University of Illinois. Nucleophile Reactivity [4] Ch. 7: Alkyl Halides and Nucleophilic Substitution. [5] Plymouth State University. Distillation Apparatus http://jupiter.plymouth.edu/~wwf/distillation.htm (accessed Oct 15, 2017)....