Expt 8 - iiiiiiiiiiiiiiiiiiiiiiiiiii PDF

Preview

Summary

iiiiiiiiiiiiiiiiiiiiiiiiiii...

Description

Patricia Archambault Experiment 8- The SN2 Reaction: Factors affecting SN2 reaction

Introduction An important concept to understand organic chemistry fully, and for scientists to have full access to is the SN2 reaction. A SN2 reaction is a second order Nucleophilic Substitution. During nucleophilic substitution, a nucleophile replaces a leaving group, which is separated from the carbon. The Nucleophile then forms a bond to the carbon. 1 In a SN2 reaction, these two steps happen simultaneously. 1 In the reaction, the tertiary amine acts as the nucleophile, while the halide ion acts as the leaving group. 1 This reaction occurs using a backside attack, in which the nucleophile attaches to the carbon atom on the opposite side of the leaving group, subsequently pushing out the leaving group. 2 Since in this reaction, the two steps occur at the same time, the nucleophile cannot attach to the side of the molecule that the leaving group is in, as it would block the nucleophile from attaching to the carbon. There are three factors that influence the SN2 reaction: the nucleophilicity, the nature of the leaving group, and the steric hinderance. Nucleophiles are electron rich groups, that are essentially donating electrons to pair to the carbon atom when the leaving group escapes. The strength of this nucleophile can play a

role on the SN2 reaction. The stronger the base, the stronger the nucleophile. 3 As you may recall, the periodic trend for basicity goes from the right of the period table to the left, increasing as you move. Hydrogen would be the most basic element according to this periodic trend. Having a basic, strong nucleophilic nucleophile will make the nucleophile more electron dense, making it more strongly attracted to the electrophile. 3 When the strong nucleophile forms a bond to the carbon, the leaving group leaves more quickly, aiding in the rate of the SN2 reaction. In this experiment, an example of a strong nucleophile would be triethylamine. Nucleophiles can also be affected by steric hinderance. Steric hinderance occurs when there is a blockage from the backside, where the nucleophile is supposed to attach. 2 In a reaction that has steric hinderance, the reaction is slowed down greatly, and will likely not occur. 4 Without steric hinderance, the nucleophile can attach to the backside like normal, and the reaction will proceed. Having a strong nucleophile and no steric hinderance creates the fastest SN2 reaction rate. When there is a methyl, or a primary alkyl halide present, there is the lowest likelihood for steric hinderance to occur.4 In this experiment, an example of a primary alkyl halide is iodoethane, meaning it has no steric hinderance. The final concept that influences the rate of SN2 reactions is the nature of the leaving group. A good leaving group is a stable,

weak base, that is attached to an electrophile. 5 On the periodic table, as we move from left to right, basicity decreases. This again makes Hydrogen the most basic, as previously mentioned. Likewise, as we move down the periodic table, basicity decreases. Therefore, for a leaving group to be classified as a good leaving group it most be a weak base, as it is the most “electron happy” and therefore the most stable. 5 In this experiment Iodomethane makes a good leaving group, due to the Iodine ion. Shown below, the SN2 reaction takes place when a nucleophile (trimethylamine in this case) forming a back side attack on the halide, which will produce a quaternary ammonium salt. 1 Figure 1: The SN2 reaction mechanism +

N

+

R

X

N

R

+

X

Unfortunately, an unwanted side reaction may occur when there is steric hindrance present. This reaction is shown below. Figure 2: Possible unwanted side reaction

Procedure A) Round 1:

Obtain 3 clean and dry test tubes

Watch for a reaction in each test tube

In test tube 1 add 20 drops of triethylamine

Place 5 ml of solvent into each test tube

In test tube 3 add 20 drops of ethyldiisopropylamine

In test tube 2 add 20 drops of tripropylamine

B) Round 2

Obtain 3 clean and dry test tubes

Watch for a reaction

Add 20 drops of triethylamine to each test tube

In test tube 3 add 15 drops of 2bromopropane

In test tube 1 add 15 drops of iodoethane

In test tube 2 add 15 drops of 1bromopropane

C) Unknown Obtain 1 clean and dry test tube

Add 40 drops of the unknown

Add 20 drops of iodomethane

Add 2ml of solvent and mix. Watch for a reaction

Table of Chemicals Table 1- Chemicals used Triethylamine

Tripropylamine

Ethyldiisopropylamin

Iodoethane

C6H15N

C9H21N

e C8H19N

C2H5I

Molar Mass: 101.19g/mol Melting point: -114.70℃ Boiling point:89℃ -nervous system toxicity - skin/eye irritation -nose and throat irritation when inhaled

Molar mass: 144.27g/mol Melting Point: -93℃ Boiling point: 158.1 ℃

Molar mass: 129.24g/mol

Molar mass: 155.97g/mol

Melting Point: -46℃

MP: -108 ℃

Boiling Point 126.5℃

BP: 72.90℃

-Eye/ skin irritation

-Aquatic toxicity

-Flammable

-Skin/eye irritatiom

-Organ damage

-Flammable Corrosive

-Skin/eye irritation

-Inhalation toxicity -Organ Toxicity

1-bromopropane C3H7Br

2-bromopropane C3H7Br

Acetone C3H6O

Iodomethane CH3I

Molar mass: 122.992g/mol MP: -110℃ BP: 71.7℃ -Flammable -Eye/skin irritation -Poisonous -Neurological damage

Molar mass: 122.992g/mol MP: -89℃ BP: 59℃ -Eye/skin irritation -Neurological damage -Poisonous

Molar mass: 58.08 g/mol MP: -95 ℃ BP: 56 ℃ -Respiratory irritation - Skin/eye irritation -Skin burns

Molar mass: 141.94g/mol MP: 42℃ BP: 42℃ -Skin irritation -Flammable -Acute toxic

Results Table 2: results from round 1 Test tube 1:

Test tube 2:

Test tube 3:

Fastest

Medium

Slow

Table 3: results from round 2 Test tube 1:

Test tube 2:

Test tube 3:

Fast

Medium

Slow

Table 4: Unknown Fast precipitation formation

Melting Point: 178℃-179℃

The identification of the unknown amine is: Triethylamine and the salt formed is (C6H5)CH2N(CH3)3+I(Methyltributylammonium Iodide)

Discussion

When observing the reactions in table 2, the results from round one, it is shown how steric hinderance as well as nucleophile strength play a role in SN2 reactions. In this round, Iodomethane was combined with three different amines: triethylamine, tripropylamine and ethyldiisopropylamine. Iodomethane acts as a primary halide, which is ideal for the SN2 reaction. Iodomethane is also a good leaving group due to the weak basicity of Iodine. In test tube 1, the fastest reaction occurred. In this test tube was the combination of Iodomethane and triethylamine. Since we know that there is a primary halide as well as a strong leaving group already present, that means that this reaction was not sterically hindered, and that Triethylamine must be a strong nucleophile. In test tube 2, Iodomethane and Tripropylamine were combined. A precipitate formed, but slower than the previous reaction. This means that while Tripropylamine is still relatively nucleophilic, it is less nucleophilic than triethylamine. This also indicates that there is some steric hinderance, since Propyl groups are slightly bulky. The last test tube,3, had Iodomethane combined with ethydiisopropylamine. This reaction occurred extremely slowly. This indicates that Ethyldiisopropylamine is a weak nucleophile, and there is a great level of steric hinderance present. Isoropyl groups are even bulkier than propyl’s, which negatively influenced the attachment of the nucleophile to the electrophile in this case.

In round 2, the effect of the leaving group was tested. In this round, triethylamine was combined with three different alkyl halides: iodoethane, 1-bromoproane and 2-bromopropane. From the previous round, we determined that Triethylamine was a strong nucleophile with no steric hinderance. When we combined triethylamine with iodoethane in test tube 1, the reaction occurred fast. Iodoethane is a primary alkyl halide, which the SN2 reaction favors, and the iodide ion in Iodoethane makes it a good, stable leaving group. In the second test tube, triethylamine was combined with 1-bromopropane. Although 1-bromopropane is still a primary alkyl halide and is still being added to a strong nucleophile, 1bromoproane is not as good of a leaving group as Iodoethane. This is due to the 1-bromoproane having a bromide ion, which is a stronger base than iodine, making it less stable and less favorable as a leaving group. The final reaction in test tube 3 was a combination of trimethylamine and 2-bromopropane. This reaction occurred extremely slowly. Even though a strong nucleophile was present through the triethylamine, 2-bromopropane is not a primary halide. We know that the SN2 reaction favors primary halides over all other structures, which will create steric hinderance and therefore will stop the reaction from performing as rapidly as expected.

The final part in the experiment was to identify an unknown amine when it is combined with iodomethane. When the unknown and the amine were combined, a precipitate formed very rapidly. Once the precipitate was vacuum filtrated, it was determined that the quaternary ammonium salt had a melting point of 178℃179℃ . When comparing these values to the literature values, it is found that the salt that was formed is (C6H5)CH2N(CH3)3+I- which is Trimethylammonium Iodide, which can be formed by the amine triethylamine. Knowing this, it makes sense that the reaction occurs fast, since triethylamine is a strong nucleophile with little hinderance and iodomethane is strong leaving group and a primary alkyl halide.

Conclusion The experimental data and the theoretical background support each other. It is confirmed that the stronger the nucleophiles, the more likely it is that the reaction will occur rapidly, that if steric hinderance is present that the rate of the reaction is slowed down, and that the more favorable the leaving group, the faster the reaction. Looking at the results from table 2, it is obvious that steric hinderance and nucleophile strength have a huge impact on the SN2

reaction rate. The reaction that occurred the fastest was when iodomethane was combined with triethylamine, which possesses little hinderance, and represents a strong nucleophile due to it being a strong base. The second fastest was when iodomethane was combined with tripropylamine. The propyl group attached to the substrate created some steric hinderance, which made the reaction occur slower. Tripropylamine is also not as strong of a nucleophile due to its slightly weaker basicity. The last reaction od iodomethane combined with ethyldiisoproplamine was very slow, which was due to the extreme bulkiness of isopropyl groups causing steric hinderance. Ethyldiisoproplamine is also not a very strong nucleophile. The data also supports the theory that the nature of the leaving group has a major effect on the rate of the SN2 reaction. In round 2, Triethylamine (strong nucleophile with little hinderance) was combined with different amines. The first reaction with the amine Iodoethane occurred the fastest since the iodine in Iodoethane makes it act as a great leaving group and is also a primary halide which the SN2 reaction favors. The second reaction with 1bromopropane did not occur as fast, due to the Bromide ion making it less favorable than Iodoethane as a leaving group. The final reaction with 2-bromoproane occurred very slowly, as 2bromopropane is a secondary halide, which SN2 reactions do not

favor, thus making them have more steric hinderance and not reacting as rapidly. The experiment did accomplish its goal. The factors that affect the SN2 reaction were experimented and supported. The reactions were successfully conducted, and an unknown amine was identified form its corresponding quaternary ammonium salt produced. Outside of this lab, SN2 reactions can also be useful. For example, the SN2 reaction that occurs in the human body that are catalyzed by SAM, dependent methyltransferase enzymes.6 In this reaction, a methyl group is transferred from SAM to the adenosine’s amine group. 6

References 1. Weldegirma, S. Experimental Organic Chemistry, 9th ed.; University of South Florida: Tampa, 2021. 2.

The SN2 Reaction. (2020, August 10). March 4, 2021, https://chem.libretexts.org/@go/page/31506

3. Structure and SN2 Reactivity: The Nucleophile. (2019, December 30). Retrieved March 4, 2021, from https://chem.libretexts.org/@go/page/201150 4. Steric Hindrance is Like a Fat Goalie. Master organic chemistry (accessed March 4 ,2021) 5. What makes a good leaving group? Master Organic Chemistry (accessed March 4, 2021) 6. Morsch, L. (2019, June 5). Application: Useful SN2 Reactions. Retrieved March 4, 2021, from https://chem.libretexts.org/@go/page/28176...