Analysis of SN1 and SN2 Reactions PDF

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Experiment 7 Lab Report
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Analysis of SN1 and SN2 Reactions Lead Author: Thibault Editor: Ussery

Introduction  SN1 and SN2 are two classes of nucleophilic substitution.1 Nucleophiles are negatively charged molecules that form a bond with their reaction partner by donating both electrons for that bond.2 A nucleophile’s reaction partner is an electrophile. An electrophile forms a bond to the nucleophile by accepting both bonding electrons from it.2 SN2 is a bimolecular reaction meaning that its rate depends on the concentrations of two of its reactants.3 This mechanism occurs in one step - the nucleophile attacks the carbon from the backside and the leaving group departs from the opposite side. The leaving group is usually a halide.1 A transition state forms during the reaction in which the bond to the leaving group breaks and the bond to the nucleophile forms at the same time.1 SN 1 is a unimolecular reaction meaning that the rate depends solely on the concentration of one reactant.3 This mechanism occurs in two steps - the leaving group first departs which leaves a carbocation intermediate in its place, and then the carbocation is captured by the nucleophile.1 The rate of each reaction depends on a variety of factors such as the leaving group, the structure of the alkyl halide, the solvent, and the nucleophile.1 In order for a leaving group to leave quickly, it needs to be able to accept electrons. Good leaving groups for the SN1 and SN2 reactions are usually weak bases because they can hold the charge. Strong bases donate electrons which is why they do not make good leaving groups.4 The type of solvent can also affect the rate of each reaction. A polar protic solvent makes the nucleophile less nucleophilic and stabilizes the  anionic leaving group.5 Polar protic solvents favor reactions in which ions are formed such as the 6 SN1 reaction and speed it up. SN2 reactions need polar solvents in order to dissolve the nucleophile; however, polar protic solvents slow the rate by solvating the nucleophile. Therefore, polar aprotic solvents increase the rate of SN2 by binding the cation and thus freeing the nucleophile.5 Lastly, structure of the alkyl halide also has effects on the reaction rate. The amount of steric strain between the nucleophile and the alkyl halide mostly impacts the reaction rate of SN 2 mechanisms. For SN 1 mechanisms, the stability of the carbocation intermediate mostly impacts the reaction rate. In both mechanisms, whether the alkyl halide substitution pattern is methyl, primary, secondary, or tertiary is what impacts the rate of reaction the most.1 An SN2 mechanism is mostly favored by methyl and primary substituted carbons while an SN1 reaction is mostly favored by a tertiary carbocation intermediate.7 Part A of this experiment explores the structural effects on SN2 using four alkyl halides: 2-bromo-2-methylpropane, 2-bromobutane, 1-bromobutane, and 1-chlorobutane. According to the rules about reaction rates that were explored previously, it would be predicted that 1-bromobutane would be favored due to it being primary and a better leaving group than 1-chlorobutane. Part B explores the structural effects on SN1 using the same four alkyl halides as in part A. It would be predicted that 2-bromo-2-methylpropane would be favored due to it being tertiary in structure and a better leaving group. Part C explores the solvent effect on the SN1 reaction using a 1:1 mixture of methanol/water, ethanol/water, 1-propanol/water, and

acetone/water. It would be predicted that the 1:1 1-propanol/water mixture would be favored due to it having the lowest dielectric constant of 20.1 thus making it more polar than the others.8

Figure 1. The figure above shows the SN1 reaction with a complete arrow pushing mechanism.

Figure 2. The figure above shows the SN2 reaction with a complete arrow pushing mechanism.

Table 1: Table of Reagents Compound

Molecular Weight (g/mol)

Boiling Point (Celcius)

Melting Point (Celcius)

Density (g/mL)

2-Bromo-2methylpropane9

137.02

73.3

-16.2

1.21

2-Bromobutane10

137.02

91

-112

1.255

1-Bromobutane11

137.02

101.4 - 102.9

-112.5

1.276

1-Chlorobutane12

92.57

78.5

-123.1

0.89

Methanol13

32.04

64.7

-98

0.791

Ethanol14

46.07

78.37

-114.1

0.789

Water15

18.02

100

0

1

Propanol8

60.0952

97

-126

0.785

Acetone17

58.08

56

-95

0.791

Silver Nitrate18

169.87

440

212

4.35

Sodium Hydroxide19

39.99

1388

318

2.13

Phenolphthalein20

318.32

557.8

260

1.28

Experimental Part A of this lab experiment explored the structural effects on SN2 reactions. To begin, four clean, dry test tubes were obtained and placed in a test tube rack. Five drops of each of the following alkyl halides were then placed into the separate test tubes: 2-Bromo-2-methylpropane, 2-Bromobutane, 1-Bromobutane, and 1-Chlorobutane. Twenty drops of 15% NaI solution in acetone was then added to each of the four test tubes, and the exact time that the first drop was added to each test tube was noted. After the addition of the NaI solution, the contents of each test tube were gently shaken, and the appearance of cloudiness was watched for. The exact time of appearance of any cloudiness was noted. The appearance of a precipitate (solid) on the bottom or the sides of the test tube was watched for as this indicated that a reaction had occurred. The exact time of appearance of the precipitate was noted. If no reaction occurred after 5 minutes, the test tubes were placed in a warm water bath (50-60 degrees Celsius), and change was watched for for another 5-6 minutes. Part B of this lab experiment explored the structural effects on SN1 reactions. The contents from part A were first emptied out, and each test tube was rinsed with ethanol or acetone. Again, 5 drops of each alkyl halide from part A were added to separate test tubes. Twenty drops of a 1% AgNO3 solution in ethanol were added to the four test tubes, and the exact time that the first drop was added to each test tube was noted. After the addition of the AgNO3 solution, the contents of each test tube were gently shaken, and the appearance of cloudiness was watched for. The exact time of any appearance of cloudiness was noted. The appearance of a precipitate on the bottom and sides of the test tube was watched for, and the exact time of precipitate appearance was noted. If no reaction occurred after 5 minutes, the test tubes were placed in a warm water bath, and change was watched for for another 5-6 minutes. Part C of the experiment looked at the effect of solvent polarity on the rate of the SN1 reaction. Again, four clean, dry test tubes were placed in a test tube rack. A 1:1 methanol/water mixture was added to the first test tube, then 1:1 ethanol/water to the second, then 1:1

1-propanol/water to the third, and finally 1:1 acetone/water to the fourth test tube. Five drops of 0.5 M NaOH and 3 drops of 1% phenolphthalein were then added to each test tube. After the addition of the phenolphthalein, the solutions were light pink in color. Three drops of 2-bromo-2-methylpropane were added to each test tube, and the exact time that the first drop was added to each tube was noted. After the addition of the alkyl bromide, the contents of each test tube were gently shaken. The disappearance of the pink color was watched for, and the exact time of disappearance was noted. Results The first portion of this lab experiment focused on what effects alkyl halide substitution (primary, secondary, tertiary, etc.) and leaving group ability had on the rate of a SN2 reaction. 4 alkyl halides were provided and the procedure was run as detailed in the experimental section of this report. The time was recorded at the initial drop of each halide, first observation of cloudiness, and solid formation. The substitution of the halide and the relative leaving group ability was also recorded. Data from part A can be found in Table 2. Table 2: SN2 Reactions and Structural Effects. Compound

Substitution of Alkyl Halide

Relative Leaving Group Ability

Time of Addition of Alkyl Halide (hh:mm:ss)

Observation of Cloudiness (hh:mm:ss)

Solid Formation (hh:mm:ss)

Total Time Elapsed (hh:mm:ss)

1-Bromobutane11

Primary

Low

0:00:00

0:00:11

0:00:27

0:00:27

2-Bromobutane10

Secondary

N/A

0:00:00

0:05:57

0:07:00

0:07:00

1-Chlorobutane12

Primary

High

0:00:00

0:09:03

0:09:27

0:09:27

2-Bromo-2-methy lpropane

Tertiary

N/A

0:00:00

0:05:54

0:07:31

0:07:31

In part B of the experiment, the impact of alkyl halide substitution (primary, secondary, tertiary, etc.) and leaving group ability on the rate of an SN1 reaction was investigated. The same 4 alkyl halides were utilized as in part A and the procedure ran as detailed in the Experimental section of this report. The time was recorded at the initial drop of each halide, first observation of cloudiness, and solid formation. The substitution of the halide and the relative leaving group ability was also recorded. Data from part B can be found in Table 3.

Table 3: Part B Data Compound

Substitution of Alkyl Halide

Relative Leaving Group Ability

Time of Addition of Alkyl Halide (hh:mm:ss)

Observation of Cloudiness (hh:mm:ss)

Solid Formation (hh:mm:ss)

Total Time Elapsed (hh:mm:ss)

1-Bromobutane11

Primary

Low

0:00:00

0:00:05

0:00:32

0:00:32

2-Bromobutane10

Secondary

N/A

0:00:00

0:00:15

0:00:17

0:00:17

1-Chlorobutane12

Primary

High

0:00:00

0:09:27

0:10:15

0:10:15

2-Bromo 2-methylpropane9

Tertiary

N/A

0:00:00

0:00:05

0:00:10

0:00:10

The final portion of the experiment assessed the influence of solvent polarity on the rate of a SN1 reaction. Approximately four solvent mixtures were obtained each containing a 1:1 mixture of a certain alcohol and water. Added to each mixture was 2-bromo-2-methylpropane and a phenolphthalein indicator. The procedure then ran as detailed in the experimental portion of this lab report. The time of the addition of the alkyl bromide was recorded as well as the time of disappearance of color. Data from part C can be found in Table 4. Table 4: Part C Data Compound

Dielectric Constant of each solvent in mixture

Time of Addition of Alkyl Halide (hh:mm:ss)

Disappearance of color (hh:mm:ss)

Acetone16/water15

21.01

0:00:00

0:01:14

Methanol13/water15

32.6

0:00:00

0:05:27

Ethanol14/water15

24.6

0:00:00

0:02:27

Propanol8/water15

20.1

0:00:00

0:01:46

Discussion The first portion of this experiment the rate of observed changes during the SN2 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 the four reactions. The general reaction order of a SN2 reaction was found in part A of this experiment. The nature of substitution regarding a SN2 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 2 suggests that as the number of substitutions rose, so did the total reaction time. This is indicative of an “overcrowded” reaction due to multiple methyl groups and nucleophilic elements being exchanged. The halides that were less substituted were observed to have much quicker reaction rates due to the ease of ability of nucleophilic substitution at the intermediate carbocation. The second portion investigated alkyl halide substitution on a SN1 reaction. As seen in Table 3, 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 SN2 reaction of a tertiary alkyl halide. This is due to the carbocation intermediate formed in the SN 1 reaction. An SN1 reaction took place in multiple pieces unlike the concerted steps found in an SN 2 reaction. The departure of the leaving group from the alkyl halide formed the carbocation in the earliest steps of the reaction. The next steps of the reaction ran much more quickly due to the stability of charge formed by the more substituted carbocation. The final portion of the experiment explored the impact of polarity of solvents on the rate of a SN1 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 most 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. This data could be a product of the evaporation of alcohol outside of the experiment, causing the solvents that are less dense to evaporate more quickly. While the data in Table 4 proves to be the opposite of what was expected, this portion of the experiment is most at risk for common error.

Conclusion This experiment demonstrated the impact of alkyl halide substitution on two nucleophilic substitution reaction mechanisms (SN1 and SN2). It can be determined that halide substitution can have a wide range of effects on the reaction mechanisms. Reaction rate can be affected by multiple stimuli including molecular structure, polarity, temperature, and the presence of a catalyst. In part A of this experiment, alkyl halide substitution was investigated as it pertains to

the impact on the reaction rate of SN2 reactions. It was determined that the less substituted alkyl halides had a much quicker reaction due to a lack of steric hindrance between the nucleophile's ability to bond to the intermediate carbocations. Part B explored the effect of alkyl halide substitution on the SN1 reaction mechanism and found that the more substituted halide had a much faster reaction. This is in part because of the charge of the carbocation and it being more stable as substitution increases. Furthermore, in Part C it was determined that the rate of the SN1 mechanism increases as the solvent becomes more polar. This was displayed between the dielectric constant of the solvent mixture versus the reaction rate. Overall, the SN2 reaction favored less substituted alkyl halides that lacked steric hindrance, while the SN1 reaction favored more substituted alkyl halides that provided more carbocation stability. Additionally, the rate of SN1 reaction mechanisms sped up as the dielectric constant and polarity of the solvents also increased. To improve this experiment, a stronger consideration for the evaporation of alcohol outside the realm of the experiment should be taken. As seen in Table 4, the data collected assumed that a solvent being more polar would decrease the reaction rate of an SN1 reaction. This would be incorrect and could be explained by the less dense solvent mixtures evaporating out of the solution much quicker. A better focus on the consistency of each solvent mixture should also be taken to obtain more stable data. Holding each part of the experiment at a specific temperature may also ensure a more accurate set of data.

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