Nucleophilic Substitution Lab PDF

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Nucleophilic Substitution Lab...

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Chem-314, H3 July 20, 2021

Nucleophilic Substitution Purpose: The purpose of this lab is to see which halides undergo an SN1 reaction, an SN2 reaction, both or none. We are also looking at the effects of the structure (primary, secondary, etc.) and seeing if this will play a part in the mechanism it will follow.

Introduction: Nucleophilic substitution is the reaction of an electron donor (nucleophile) with an electron pair acceptor (electrophile). The electrophile must have a leaving group in order for the reaction to take place. Nucleophilic substitution reactions depend on the nature of the alkyl halide, the nature of the nucleophile, the nature of the leaving group, temperature, the concentration of the nucleophile and the nature of the solvent. Nucleophilic substitution has two mechanisms: SN1 and SN2 reaction. SN2 is biomolecular, this is when two reacting species are

involved in the rate determining step. The rate law of SN2 is:

−¿ ¿ Nu ❑ [R ─ L] . The Rate(SN 2)=k ❑SN 2 ¿

stronger the nucleophile the faster the reaction. The Nu ─ C bond forms at the same time the C ─ L bond breaks. SN2 takes place in a single step:

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In an SN2 reaction, the decrease in steric hindrance increases the reactivity. This means a primary halide is more likely to react than a tertiary halide. Another way to speed up a reaction in an SN2 reaction is by having the perfect leaving group. Good leaving groups are typically weaker bases. For example, Iodine is considered a far better leaving group than Chlorine. When there is a higher concentration of the nucleophile, the rate also increases. The reaction works best for SN2 if it has a polar aprotic solvent. The second mechanism is the SN1 reaction. SN1 is unimolecular, this is when only one reacting species is involved in the rate determining step. The rate law of SN1 is: Rate(SN 1)=k ❑SN 1 [ R ─ L] . SN1 is independent of the type of nucleophile. SN1 takes place in two steps:

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In an SN1 reaction, the higher the stability of carbocation the more likely it is to reacte. Tertiary carbons are known for being very stable, this means a tertiary halide is more likely to follow the SN1 path than a primary halide. The leaving group follows the same trend as SN2, the weaker the base the higher the rate. A polar protic solvent is good at stabilizing the carbons, so polar protic solvents work best for SN1 reactions.

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Table of Reagents: 1-chlorobutane 2-chlorobutane 1-bromobutane 2-bromobutane 2-chloro-2bromobenz methylpropane ene

Boiling Point (℃) Density (g/m³)

78℃

68℃

102℃

91℃

51℃

156℃

0.880g/m³

0.873g/m³

1.27g/m³

1.26g/m³

0.84g/m³

1.5g/m³

Procedure + Observations(*):

Procedure Experiment 1 1

Obtain six test tubes for each of the six alkyl halides: 1-chlorobutane, 2-chlorobutane, 1bromobutane, 2-bromobutane, 2-chloro-2-methylpropane, and bromobenzene. Put three drops of each halide in the separate tubes.

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Using 6 mL of the sodium iodide in acetone solution, place 1mL of the solution in each test tube. Once you have put the solution in the tube, start a five minute timer for all of the test tubes.

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Start a 50℃ warm water bath while you wait. After the five minutes are up, check on the solutions to see if a visible reaction has occurred. *1-bromobutane has a lot of white precipitate at the bottom of the tube and the solution has turned to a pale yellow. None of the other five tubes have precipitate in it. Total time for 1-bromobutane: 5 minutes.

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Label all the test tubes that showed no reaction and place them in the warm water bath to see if a reaction will occur. Let these solutions sit for another five minutes. .

* After the five minutes were done, the 1-bromobutane and the 2-

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bromobutane are a little opaque. Still no precipitate from the five test tubes. 5

Place 1-chlorobutane and 2-chlorobutane in an ice bath for 2 minutes. *After two minutes, both solutions form a solid white precipitate at the bottom of the tube. 2-chlorobutane also becomes a vibrant yellow. Total time for 1-chlorobutane and 2-chlorobutane: 12 minutes. Experiment 2

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Obtain six test tubes for each of the six alkyl halides: 1-chlorobutane, 2-chlorobutane, 1bromobutane, 2-bromobutane, 2-chloro-2-methylpropane, and bromobenzene. Put three drops of each halide in the separate tubes.

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Using 6 mL of silver nitrate in ethanol solution, place 1mL of the solution in each test tube. Once you have put the solution in the tube, start a five minute timer for all of the test tubes. *2-bromobutane and 2-chloro-2-methylpropane immediately react and become opaque.

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After the five minutes are up, check on the solutions to see if a visible reaction has occurred.

* 2-bromobutane and 2 -chloro-2-methylpropane have formed precipitate at the bottom of the tube. The 2-bromobutane has more of a yellowish tinted precipitate rather than pure white. 1-bromobutane is cloudy in color but does not show any solid precipitate. Total time for 2-bromobutane and 2 -chloro-2-methylpropane: 5 minutes. 4

Label the 4 tubes without precipitate and put them in a 50℃ warm water bath for five minutes. * After five minutes in the warm water solution, 1-bromobutane and 2chlorobutane become opaque and form precipitate. In the 1-bromobutane, along with the white precipitate, a solid yellow has also formed. There is still no reaction for the 2-chlorobutane and the bromobenzene. Total time for 1-bromobutane and 2-chlorobutane: 10 minutes. Experiment 3

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Using a new test tube, but three drops of 2 -chloro-2-methylpropane. This will be mixed with silver nitrate as a 1:1 ethanol and water mixture solution. Shake the tube to mix after

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the solutions have been combined. * Reaction immediately becomes an opaque white and forms a lot of white solid. 6

Let it sit for five minutes and then check the results again. * Now there are a lot of white solids at the bottom of the test tube and the solution remains an opaque white. Total time for 2 -chloro-2-methylpropane: 5 minutes.

Post-Lab Questions: 1. What would be the effect of carrying out the sodium iodide in acetone reaction with the alkyl halides using an iodide solution half as concentrated? If the iodide solution were half as concentrated, the SN2 reaction would occur at half the rate it normally would, and only half as much precipitate would form. 2. The addition of sodium or potassium iodide catalyzes many SN2 reactions of alkyl chlorides or bromides. Explain. The sodium or potassium ion is very positively charged, and this can help the negatively charged leaving group actually leave. Since SN2 reactions occur in one step, if the leaving group leaves easily, then the nucleophile can attack easily. 3. In SN1 reactions, the intermediate carbocations can eliminate a proton to yield alkenes or react with the solvent to yield ethers. Draw the structures of the by- products of this type that would be derived from the reaction of the carboca- tion derived from 2-bromo-2-methylbutane in ethanol.

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Discussion: In the first experiment with sodium iodide in acetone solution, the only alkyl halides to undergo an SN2 reaction were 1-bromobutane, 1-chlorobutane, and 2-chlorobutane. In the second experiment with ethanolic silver nitrate solution, the only alkyl halides to undergo an SN1 reaction were 2-bromobutane, 2-chloro-2-methylpropane, 1-bromobutane, and 2chlorobutane. Bromobenze was the only alkyl halide that did not undergo an SN1 or SN2 reaction. The alkyl halides to undergo both SN1 and SN2 reactions were 1-bromobutane and 2chlorobutane. Primary alkyl halides undergo SN2 mechanisms more easily because primary substrates have little steric hindrance to nucleophilic attack and their carbocations are relatively unstable. This is why you could see the reaction occur in 1-bromobutane, 1-chlorobutane. Secondary halides can go either way when it comes to SN1 and SN2. The stable carbocation in secondary halides favors SN1 while the little steric hindrance of a secondary halide also makes it

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possible for it to undergo SN2. This is why you see 2-chlorobutane undergo both reactions and the majority of the secondary halides follow the SN1 reaction. One halide that should not have undergone an SN1 reaction was 1-bromobutane. Bromobenzene is neither of these so it did not undergo any of these reactions.

For the SN2 reaction, the fastest reaction happened in 1-bromobutane. While sitting in place, this reaction occurred in five minutes. After an extra seven minutes of being in a warm bath and an ice bath, 2-chlorobutane and 1-chlorobutane both tie at a total of twelve minutes. Iodine ions make for a great leaving group compared to all the other halides, so because primary halides have less steric hindrances it makes it easier to bond. For the SN1 reaction, the fastest reactions were between 2-bromobutane and 2-chloro-2-methylpropane which happened in five minutes. The next two halides, 2-bromobutane and 2 chlorobutane, reacted in a total of 10 minutes after being placed in a warm water bath for an extra five minutes. When silver nitrate combines with a halogen it results in a considerably weaker carbon-halogen bond, meaning it can better participates in substitution reactions. With secondary or tertiary alkyl halides, the result is usually formation of a carbocation. Silver halides are insoluble in water, so if water is chosen as solvent, loss of AgCl is irreversible. The carbocation is then trapped by the water solvent. Water has more dielectric constant value compared to ethanol so the more the dielectric constant value the more polar the solvent is . 100% ethanolic solution is less polar than 50% ethanol and 50% water, the more polar the solvent, the more solvolysis. The sovolysis, the faster the SN1 reaction occurs. This is why it reacted very quickly when the ratio of ethanol and water was 1:1. An unexpected result was 1-bromobutane undergoing a SN1 reaction considering it is a

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primary halide. For 1-bromobutane to undergo SN1, that means a primary carbocation would be generated, but this is extremely unfavorable. It is possible that there might have been cross contamination.

Equations and mechanisms for each reaction that should theoretically have produced precipitate:

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Equation and mechanism for 2-chloro-2-methylpropane with silver nitrate in ethanol/ Water:

Conclusion:

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In conclusion, this experiment was conducted to see if alkyl halides would undergo SN1 or SN2. Tests were also done to see if there was an influential trend on the mechanism due to structure. It is clear that evidence was found to prove that structure does place a vital part in each mechanism. In the second experiment, testing SN1, there was a false positive for 1-bromobutane which could have been due to many reasons. Besides 1-bromobutane, all the reactions proved to operate according to their theories....