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CH 236: Structural and Solvent Effects on the SN1 and SN2

Introduction Nucleophilic substitution is a class of chemical reactions in which nucleophiles replace a functional group within another molecule, an electrophile. It is a useful and well studied organic reaction. SN1 and SN2 are extremes and both shown in Figures 1 and 2. SN2 occurs in two steps SN1 reactions or unimolecular nucleophilic substitution occurs in one. In SN2 reactions the nucleophile attacks the from the backside cardon and the leaving group departs from the other side. While this is happening a transition state is formed when the bond to the leaving group breaks simultaneously as the formation of the nucleophile. The SN1 reaction occurs in one step. The leaving group departs first which generates a carbocation intermediate.

Figure 1 & 2: SN1 and SN2 mechanisms SN2 reactions are converted in one step and both nucleophile and substrate are involved in the rate determining step. The bigger barrier in the SN2 reaction is steric and in which the reaction will only proceed if the empty orbital is accessible. The rate of these reactions go from fastest to slowest. Each extreme occurs under a specific set of conditions and the rate of reactions depend on the structure of the alkyl halide, the leaving group, the nucleophile, and the solvent. Figure 3: Table of Reagents Compound

Molecular Weight (g/mol)

Melting Point (°C)

Boiling Point (°C)

Density (g/cm^3)

Dielectric Constant

2-bromo-2methylpropane

137.02

-16.20

73.30

1.271

10.10

2-bromobutane

137.02

-112.00

91.20

1.267

8.6

1-bromobutane

137.02

-112.40

101.30

1.272

7.16

1-chlorobutane

92.55

-123.10

778.50

0.874

9.60

Sodium Iodine

149.89

660.00

1304.00

3.670

7.28

Silver Nitrate

169.87

212.00

440.00

4.350

___

Methanol

32.04

-97.60

64.70

0.753

33.0

Ethanol

46.07

-114.10

78.20

0.789

25.3

Propanol

60.10

-126.10

97.20

0.786

20.1

Acetone

58.08

-94

56

0.773

20.7

Sodium Hydroxide

39.99

318

2530

2.130

57.5

Phenolphthalein

318.32

262

79

1.386

___

Experimental This experiment was broken up into three separate parts. Structural Effects on SN2 To begin part A we were to clean and dry four test tubes and place them in a test tube rack and label them. Next to pipette 5 drops of each of the following: 2-bromo-2- methylpropane, 2-bromobutane, 1-bromobutane, and 1-chlorobutane into separate test tubes. After separating the tubes 5 mL of 15% of sodium iodide in acetone was also pipetted into the four test tubes but making the exact time the first drop hit. After the 20 drops were added into the test tube one was to shake the tube gently and wait for the cloudiness in appearance to occur and or a precipitate to form along the edges of the tube. The exact time that the reaction occurred via the appearance of the precipitate was also recorded. If neither showed we were to place the test tube tinto a warm water bath not reachich above 60° C. Structural Effects on SN1 Part B was extremely similarly to part A but instead of using the water to clean and rinse out four test tubes we used acetone. We also used 5mL of 1% silver nitrate instead of the 15% sodium iodide. Once again the test tubes were observed for the appearance of cloudiness and precipitate formation. The time of the first drop was recorded as well as the first spotting of a precipitate and cloudiness. If no reaction occurred after the allotted 5 minutes the test tube was placed into a warm water bather not exceeding 60°C. Solvent Effects on the SN1 Reaction To conclude the experiment we began with once again four clean dry test tubes in a rack which we added a one to one ratio mixture of methanol and water, ethanol and water, propanol and water, and acetone and water. We labeled them and then noted the exact time at which we placed the first drop each 1% phenolphthalein and the 5 drops of sodium hydroxide. 5 drops of 2-bromo-2methylpropane were also added to each test tube for the disappearance of the pink color. Each test tube was shaken until the pink color disappeared.

Results Compound

Time of addition

Time of cloudiness

Time of solid formation

Total time elapsed

2-bromo-2methylpropane

8:20:00

8:21:12

8:36:54

16:54

2-bromobutane

8:20:25

8:38:00

8:39:22

18:57

1-bromobutane

8:20:55

8:23:20

8:34:50

13:55

1-chlorobutane

8:21:25

8:30:34

8:37:48

16:23

Table 1: Part A The results show the time of addition of the first drops into the test tubes, the appearance of cloudiness and the time of precipitate formation. The final column is the total time elapsed from the first drop of 15% of sodium iodide and the solid precipitate formation. Compound

Time of addition

Time of cloudiness

Time of formation

Total time elapsed

2-bromo-2methylpropane

8:20:42

8:21:00

8:25:11

6 min

2-bromobutane

8:21:05

8:21:15

8:27:51

6 min

1-bromobutane

8:21:55

8:26:19

8:32:04

12 min

1-chlorobutane

8:40:17

8:42:56

8:47:21

8 min

Table 2: Part B The results show time of addition of the first drop of 1% silver nitrate into the four test tubes. The time of cloudiness appearance and the time of the start of the reaction by precipitate formation. The final column is the total time elapsed. Test tube four including 1-chlorobutane was placed in a warm water bath to speed up the rate of the reaction. Solvent Mixture

Time of addition

Time of Disappearance

Total TIme Elapsed

1:1 methanol/water

0:00

02:27

2:27

1:1 ethanol/ water

0:00

01:14

1:14

1:1 propanol/ water

0:00

01:46

1:46

1:1 acetone/ water

0:00

03:07

3:07

Table 3: Part C Four solvent mixtures were obtained containing a one to one mixture of alcohols and water which was then added to 2-bromo-2-methylpropane and phenolphthalein. The time of the addition of the alkyl bromide was recorded as well as the time of disappearance of color. The format is different because the in-lab data was inconclusive and we had to use COVID lab data.

Discussion Part 1 of this experiment the rate of observed changes during the SN2 reaction directly correlated to the amount of alkyl halide substitution present. While each reaction rate for part A differed considerably, the end results were similar between all three reactions The nature of substitution regarding a SN2 reaction favored less substituted alkyl halides as it eliminated any excess steric hindrance between the nucleophile and its ability to bond to the intermediate cation. This is common among bimolecular reactions. The data from Table 1 suggests that as the number of substitutions rose, so did the total reaction time. This is displayed as a crowded reaction due to multiple methyl groups and nucleophilic elements being exchanged. The halides that were less substituted had much quicker reaction rates due to the quickness nucleophilic substitution at the intermediate carbocation. Part B investigated alkyl halide substitution on a SN1 reaction. As seen in Table 2, the reaction of 2-Bromo-2-methylpropane took only a matter of seconds to reach solid formation. This data differs tremendously from the time recorded regarding the SN2 reaction of a tertiary alkyl halide. This is due to the carbocation intermediate formed in the SN1 reaction. An SN1 reaction took place in multiple pieces unlike the concerted steps found in an SN2 reaction. The departure of the leaving group from the alkyl halide formed the carbocation in the earliest steps of the reaction. The final portion of the experiment explored the impact of polarity of solvents on the rate of a SN1 reaction. In general it was predicted that the solvent with the highest dielectric constant would react much more quickly. This would be because the solvent was more polar, which in a substitution reaction is expected to have a faster reaction rate. It should be noted that unusual data was formed in this procedure so we utilized the COVID data for finalization. While the data in Table 3 proves to be the opposite of what was expected, this portion of the experiment is most at risk for common error.

Conclusion In conclusion, SN1 and SN2 were experimented in various ways to determine reaction rates. The experiment was separated into three parts with Part A comparing alkyl halides reaction rates with sodium iodide and acetone. Part B compared reaction rates of alkyl halides with silver nitrate in ethanol while Part C tested the effects on the SN1 reaction. Part A concluded that SN1 the alkyl halide, 2-bromo-2-methylpropane formation of cloudiness had the shortest recorded time. The 2- bromobutane reacted the fastest when recording the time of the precipitate. In part B of the experiment, 1-chlorobutane is favored by SN1 in terms of cloudiness while 2-bromobutane was the fastest. And 2-bromo-2-methylpropane was the fastest to form a precipitate. proved otherwise and had the shortest time recorded for cloudiness. However, the experiment did prove that 2-bromo-2-methylpropane reacted the fastest out of the other alkyl halides. The results for art C the 1:1 methanol in water should have resulted in the fastest reaction but it resulted in being acetone instead. Lastly, this experiment could have been improved by repeating the trials for more accurate results.

References 1

Casselman, B. A Tale of Two Reactions. The University of Alabama at Birmingham.

Casselman, B. A Tale of Two Reactions: Structural andSolvent Effects on the SN1 and SN2 Reactions background and procedure. The University of

2

Alabama at Birmingham. 3

Ashenhurst, J. Substitution Reactions. Master Organic Chemistry Blog.

https://www.masterorganicchemistry.com/2012/08/08/comparing-the-sn1-and-sn2-reactions/...