Experiment 7 Lab Report Edited PDF

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Relative Reactivity of Alkyl Halides in Nucleophilic Substitution Reactions Lead Author: Cierra Young Reviewer: Hannah Strickland Editor: Mary Yant CH 235- G Experiment 7Introduction: The most common reaction in which alkyl halides undergo is known as a nucleophilic substitution. During such reactio...

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Relative Reactivity of Alkyl Halides in Nucleophilic Substitution Reactions Lead Author: Cierra Young Reviewer: Hannah Strickland Editor: Mary Yant CH 235- G5 Experiment 7

Introduction: The most common reaction in which alkyl halides undergo is known as a nucleophilic substitution. During such reaction, the nucleophile breaks the bond between a halide leaving group and an R group. This displacement produces a substitution product in which the nucleophile is bound to the R group. This reaction can occur through two different mechanisms, SN2 and SN1. The SN2 mechanism consist of a one-step biomolecular displacement process in which the formation and dissociation of bonds occur concurrently1. As the leaving group in a reaction departs and dissociates from its bond with an R-group, it is assisted by a nucleophile as it enters. The nucleophile then begins to form a bond with the R-group, attaching itself to the reactive center, opposite of the leaving group. This is known as a “backside attack”2. Nucleophiles, during SN2 reactions, are hindered from attaching to any other location on an R-group due to their steric hindrance2. This barrier allows for the nucleophile’s “attack” and bond formation to be easier for methyl, primary, and secondary alkyl halides than tertiary alkyl halides. Aside from SN2 reactions mechanism, they favor polar aprotic solvents whose anions are weakly solvated and free to partake in nucleophilic substitution reactions2. SN2 reactions often favor sufficient partially negative leaving groups and nucleophiles, as well as produce a second order reaction. The SN1 mechanism consist of a two-step unimolecular displacement process in which the formation and dissociation of bonds occur separately at different rates2. During this particular reaction the bond between the carbon (R-group) and the halogen dissociates and the halide ion leaving group departs at a slow rate. This “rate-determining step” proceeds to produce a carbocation intermediate1. As the nucleophile enters the reaction, it quickly grasps the

intermediate forming a bond between the two. The carbocation produced during the ratedeterming step of the reaction is a high energy intermediate. As a result of its transition state’s similarity to carbocation in energy, it can easily form bonds with secondary and tertiary alkyl halides than with primary alkyl halides2. Tertiary and secondary carbocations are more stable and contain a lower activation energy than primary and methyl carbocations. Like SN2 reactions, SN1 reactions favor partially negative leaving groups. They prefer polar protic solvents in which the anions are highly solvated and less free to partake in the nucleophilic substitution reactions2. SN1 reactions often favor weak nucleophiles and produce first order reactions as well. The mechanisms for both SN2 and SN1 reactions as well as a nucleophilic substitution are shown below.

Figure 1: Figure 1 shows the mechanism for a nucleophilic substitution

Figure 2: Figure 2 shows the mechanism for a SN2 reaction, bimolecular displacement

Figure 3: Figure 3 shows the mechanism for SN1 reaction, unimolecular displacement During this experiment a series of test were performed in order to determine the effects that specific alkyl groups, leaving groups, and solvents have on both SN2 and SN1 reactions. The particular organic reagents as well as their physical properties are seen below in Table 1.

Table 1. Organic Reagents Name

Molar Mass

Melting Point

Boiling Point

Density

1-bromobutane3

137.02 g/mol

-112.5 °C 101.4 – 102.9 °C 1.27 g/cm3

2-bromobutane4

137.02 g/mol

-112.6 °C 91.0 °C

1.26 g/cm3

1-chlorobutane5

92.57 g/mol

-123.1 °C 78.0 °C

0.89 g/cm3

2-bromo-2-methylpropane6

137.02 g/mol

-16.2 °C

73.3 °C

1.22 g/cm3

Acetone7

58.08 g/mol

-95.0 °C

56.0 °C

0.773 g/cm3

Ethanol8

46.07 g/mol

-114.0 °C 78.37 °C

0.789 g/cm3

Sodium iodide9

149.89 g/mol

661.0 °C

1,304.0 °C

3.67 g/cm3

Sodium hydroxide10

39.99 g/mol

318.0 °C

1,388.0 °C

2.13 g/cm3

Phenolphthalein11

318.33 g/mol

260.0 °C

557.8 °C

1.28 g/cm3

Water12

18.02 g/mol

0.0 °C

100.0 °C

1.00 g/cm3

Propanol13

60.10 g/mol

-126.0 °C 97.0 °C

0.786 g/cm3

Methanol14

32.04 g/mol

-97.6 °C

64.7 °C

0.753 g/cm3

Silver nitrate15

169.87 g/mol

212.0 °C

444.0 °C

4.35 g/cm3

Experimental: This experiment was separated into three separate sections. Part A. Structural Effects on the SN2 Reaction To begin this section of the experiment, four clean test tubes, four clean pipets, and a test tube rack was obtained. The four test tubes were placed in the test tube rack and labeled. The first test tube was labeled 2-bromo-2-methylpropane, the second test tube was labeled 2bromobutane, the third was labeled 1-bromobutane, and the fourth was labeled 1-chlorobutane. Once set up, using one of the clean pipets, five drops of 2-bromo-2-methylpropane was placed in its corresponding test tube. Using a different pipet, five drops of 2-bromobutane was placed in its

corresponding. The same method was used for the third and fourth test tubes, five drops of 1bromobutane and 1-chlorobutane was obtained and placed in their corresponding tubes. Once all four test tubes contained five drops of their labeled reagent, 5 mL of a 15% solution of sodium iodide in acetone was collected and 20 drops of the solution was placed in each of the four test tubes. The exact time that the first drop of the sodium iodide and acetone solution was added to each test tube was then recorded in a notebook. After the addition of sodium iodide, the each test tube was inverted once in order to mix the contents. Once mixed, the tubes were observed and watched for the formation of any cloudiness or precipitant. The exact time of the formation of cloudiness and precipitant for each test tube was also recorded in the notebook. Once all observations were recorded the test tubes were then emptied into the halogenated waste container and cleaned ethanol in order to prepare for the second section of the experiment. Part B. Structural Effects on the SN1 Reaction To start this section of the experiment the same clean and dry test tubes and test tube rack was obtained along with four new pipets. The test tubes were labeled and five drops of each reagent was collected and in placed in their respective test tubes, as in Part A. 5mL of a 1% solution of silver nitrate in ethanol was collected and 20 drops of the solution was placed in each of the four test tubes. Like Part A, the exact time that the first drop of the silver nitrate and ethanol solution was added to each test tube was recorded in a notebook. After the addition of silver nitrate, the tubes were inverted once and observed for the formation of any cloudiness or precipitant. The exact time of the formation of cloudiness and precipitant for each test tube was also recorded in the notebook. Once all observations were written down, the test tubes were

emptied into the waste container and cleaned ethanol in order to prepare for the third section of the experiment. Part C. Solvent Effects on the SN1 Reaction To perform this section of the experiment the same clean and dry test tubes and test tube rack was obtained along with four new pipets. The test tubes were then labeled. The first test tube was labeled methanol/water, the second test tube was labeled ethanol/water, the third was labeled 1-propanol/water, and the fourth was labeled acetone/water. Once set up, using one of the clean pipets, 1 mL of a 1:1 mixture of methanol and water was placed in its corresponding test tube. 1 mL of a 1:1 mixture of ethanol and water was placed in its corresponding test tube. The same method was used for the third and fourth test tubes, 1 mL of a 1:1 mixture of 1-propanol and water and 1 mL of a 1:1 mixture of acetone and water was placed in their corresponding test tubes. Once all four test tubes contained 1 mL of their labeled reagent, three drops of 0.5M sodium hydroxide, 1% phenolphthalein, and 2-bromo-2-methylpropane was collected and added to each test tube. Like Parts A and B the exact time that the first drop of 2-bromo-2methylpropane was added to each test tube was recorded in a notebook. After the addition of the reagent, the tubes were inverted once and observed for the disappearance of the color pink produced. The exact time of the disappearance and all other observations for each test tube was also recorded in the notebook. Once all observations were written down, the test tubes were emptied into the waste container and cleaned ethanol. In the case that no reaction occurred in either of the test tubes during any particular part of the experiment, the test tube was placed in a warm water bath ranging from 50-60°C for five to six minutes.

Results: During this experiment the times and observations seen while testing the effects structure has on SN2 and SN1 reactions were recorded in two separate tables, Table 2 and Table 3. The observations seen while testing the effects a solvent has on SN1 reactions was also recorded in a table, Table 4. The reaction times for each section of the experiment was recorded in seconds. Table 2 depicts the effects structure has on S N2 reactions. The table displays the reaction times for the formation of cloudiness and precipitant for each of the alkyl halides, as they were mixed and reacted with the sodium iodide and acetone solution. The second test tube in which contained the reagent 2-bromobutane had to be heated in a warm water bath for six minutes at a temperature of 55°C in order for the formation of cloudiness to develop. This reaction also failed to produce a precipitant. Table 2. Structural Effects on the SN2 Reaction Test Tube

First Drop

Cloudiness

Precipitant

1

0.1 seconds

8.15 seconds

482.96 seconds

2

0.1 seconds

1084.036 seconds

Did not form

3

0.1 seconds

38.87 seconds

453.0 seconds

4

0.1 seconds

9.34 seconds

962.0 seconds

Table 3 shows the effects structure has on SN1 reactions. Similar to Table 2, the table displays the reaction times for the formation of cloudiness and precipitant for each of the alkyl halides, as they were mixed and reacted with the silver nitrate and ethanol solution. Table 3. Structural Effects on the SN1 Reaction Test Tube

First Drop

Cloudiness

Precipitant

1

0.1 seconds

7.91 seconds

12.76 seconds

2

0.1 seconds

69.42 seconds

406.88 seconds

3

0.1 seconds

121.28 seconds

457.67 seconds

4

0.1 seconds

4.83 seconds

36.57 seconds

Table 4 displays the effects solvent has on SN1 reactions. Shown in the table is the reaction times for the disappearance of the pink color produced due to the addition of the alkyl bromide, as well as all other observations. Test tube one in which contained the reaction involving methanol was the only reaction to produce a precipitant. Test tube two, containing the reaction with ethanol, produced a yellow color before becoming clear and colorless. Test three, containing the reaction with 1-propanol, become cloudy before becoming clear and colorless. The final test tube, test tube four, produced no other observations.

Table 4. Solvent Effects on the SN1 Reaction Test Tube

First Drop

Color Disappearance

Other Observations

1

0.1 seconds

44.19 seconds

78.48 seconds formation of a precipitant

2

0.1 seconds

73.052 seconds

406.88 seconds produced a yellow color

3

0.1 seconds

53.93 seconds

457.67 seconds formation of cloudiness

4

0.1 seconds

86.98 seconds

none

Discussion: The reaction rates for SN2 and SN1 reactions can be effected by various factors. These factors include the alkyl and halide leaving groups and the solvent, all of which were tested

during this experiment. Part A of this experiment tested the structural effects in a SN2 reaction and involved four different reagents in which reacted with a solution containing sodium iodide in acetone. Due to the iodide ion being a strong non-basic nucleophile, once in the solvent acetone, which is non-polar, it was expected that the substitution reaction would favor an SN2 reaction1. The reaction rates seen in Part A proceeded in the following order: 2-bromo-2-methylpropane, 1chlorobutane, 1-bromobutane, and 2-bromobutane in terms of the formation of a cloudy solution. In terms of precipitate formation, the reaction rates for Part A, proceed in the following order: 1bromobutane, 2-bromo-2-methylpropane, 1-chlorobutane, and 2-bromobutane. These trends can be seen in Table 2 above and results depicted did not occur as expected. In terms of both cloudy and precipitant formation, the reaction in which involved 1-bromobutane was expected to proceed the fastest. This expectation is due to the 1-bromobutane being a primary alkyl halide and containing a stronger nucleophile and leaving group than the rest of the reagents. SN2 reactions favor primary alkyl halides over tertiary haloalkanes and strong nucleophiles, as well as strong leaving groups, over weak ones. This preference is a result of SN2 reactions’ steric hindrance. Following the 1-bromobutane reaction, the results in terms of both cloudy and precipitant formation should’ve proceeded with 1-chlorobutane, 2-bromobutane, and 2-bromo-2methylpropane. Despite 1-chlorobutane also being a primary alkyl halide as well, the bond strength between the bromide and chloride bonds differs.

The bond strength and

electronegativity between the R-group and the leaving group in 1-bromobutane is much weaker than the bond strength and electronegativity between the R-group and the leaving group in 1chlorobutane. This as a result allows 1-bromobutane to obtain a faster reaction rate.

Part B of the experiment tested the structural effects in a SN1 reaction and involved the same four reagents as in Part A. The reagents in this section of the experiment was reacted with a solution containing silver nitrate in ethanol. Due to the absence of a strong nucleophile, the dissociation of the carbon-halogen bond is preferred1. This preference is seen through the formation of the silver bromide precipitant and favors an SN1 reaction1. The reaction rates seen in Part B proceeded in the following order: 1-chlorobutane, 2-bromo-2-methylpropane, 2bromobutane, and 1-bromobutane in terms of the formation of a cloudy solution. In terms of precipitate formation, the reaction rates for Part B, proceeded in the following order: 2-bromo-2methylpropane, 1-chlorobutane, 2-bromobutane, and 1-bromobutane. These trends can be seen in Table 3 above and the results depicted did not give the expected results. In terms of both cloudy and precipitant formation, the reaction in which involved 2-bromo-2-methylpropane was expected to react the fastest. This is due to the reagent being the most substituted and the cations in the carbocation intermediate produced during SN1 reaction, preferring to translocate to the most substituted nucleophile. This preference is a result of SN1 reactions’ inductive effect and hyperconjugation. Following the 2-bromo-2-methylpropane, the results in terms of both cloudy solution and precipitant formation should’ve proceeded with 2-bromobutane, 1-bromobutane, and 1-chlorobutane. Part C of the experiment tested the solvent effects in a SN1 reaction and involved four different solvents in which were reacted with sodium hydroxide, phenolphthalein, and 2bromo-2-methylpropane. Due to the dielectric constants within in particular solvents, the rate of a reaction and the strength of the carbocation intermediate produced in SN1 reactions can be greatly influenced. The larger the value of a solvent’s dielectric constant, the better it dissolves,

and the shorter the interaction between oppositely charged ions is2. This in turn leads to a faster reaction rate. The reaction rates seen in Part C proceeded in the following order: methanol, 1propanol, ethanol, and acetone. This trend is seen in Table 4 and above and the results depicted did not give the expected results. In terms of the rate of the disappearance of the pink color produced, the reaction in which involved methanol was expected to react the fastest followed by ethanol, acetone, and 1-propanol. As a result of SN1 reactions favoring polar protic solvents, the most common solvents known are water, formic acid, methanol, ethanol, and acetic acid2. Methanol, due to its high dielectric constant value, causes reactions to proceed rapidly. Despite this particular part of the trend being presented in the results collected, the order of the remaining reactions failed to proceed as expected. Various errors could have occurred during this experiment. These errors include: miss counting the number of drops of each reagent needed as well as failing to record the exact time of the required observations. Such errors could lead to a misrepresentation of the reactions and the collection of inaccurate results. Conclusion: In this experiment the various factors in which impact the reaction rate of SN2 and SN1 reactions were studied through serval semi-quantitative tests. The experiment was separated into three sections in which the first section compared the reaction rates of four alkyl halides as they reacted with a solution containing sodium iodide in acetone. The reactions performed were SN2. The second section of the experiment compared the reaction rates of the same four alkyl halides as they were mixed with a solution containing silver nitrate in ethanol. The reactions performed

were SN1. The final section of the experiment tested the impact altering solvents has on SN1 reactions. The results obtained from this experiment fail to fully correlate with the expected results. According the results gathered from Part A of the experiment, 2-bromo-2-methylpropane, a tertiary alkyl halide in terms of the formation of a cloudy solution, is favored by SN2 reactions. This idea is false. SN2 reactions favor methyl, primary, and secondary alkyl halides as a result of their steric hindrance. The reaction in which involved 1-bromobutane, a primary alkyl halide, was expected to proceed the fastest. In terms of precipitant formation however, for Part A, the gathered results had little correlation to the expected results. 1-bromobutane obtained the fastest reaction time, which is in direct correlation of the expected results however, the order of reactivity for the remaining reactions are not parallel to the results expected. Following the 1bromobutane reaction, the results in terms precipitant formation should’ve proceeded with 1chlorobutane, 2-bromobutane, and 2-bromo-2-methylpropane. According to the results gathered from Part B of the experiment, 1-chlorobutane, a primary alkyl halide, in terms of the formation of a cloudy solution, is favored by SN1 reactions. This idea is false. SN1 reactions favor tertiary and secondary alkyl halid...