Experiment 8 - Lab report PDF

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Experiment 8: The SN2 Reaction:Factors Affecting SN2 ReactionAshley Cook CHM 2210 S Lab Group A Teaching Assistant: Iman Keshavarz March 2, 2021Introduction: A nucleophilic substitution involves a nucleophile replacing a leaving group and forming a new bond with carbon. An SN2 reaction is a nucleoph...

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Experiment 8: The SN2 Reaction: Factors Affecting SN2 Reaction

Ashley Cook CHM 2210 S21 Lab Group A Teaching Assistant: Iman Keshavarz March 2, 2021

Introduction: A nucleophilic substitution involves a nucleophile replacing a leaving group and forming a new bond with carbon. An SN2 reaction is a nucleophilic substitution that “takes place in a single step (concerted). A strong nucleophile attacks the electrophile carbon, forcing the leaving group to leave” (Weldegirma, 2020). The strength of the nucleophile impacts the rate of the reaction. A strong nucleophile is required for an SN2 reaction. If a weak nucleophile is used, the reaction will be slow or nonexistent “polar aprotic solvents are popular choices for SN2 reactions because the nucleophile is almost “naked” in aprotic solvents, whereas in polar protic solvents it is surrounded by a cage of solvent molecules, making the reaction faster” (Walker, n.d.). The reaction will be fastest if there is no steric hindrance involved in the reaction. Figure 1.0: Below is an SN2 reaction showing the nucleophilic attack and leaving group.

SN2 reactions favor an unhindered alkyl halide (primary or methyl group) “As long as the two groups attached to the carbon being attacked are small hydrogens, the repulsions do not require much energy. If the groups attached to the carbon are larger, the transition state energy increases, the activation energy increases, and the reaction becomes much slower” (Walker, n.d.). The rate

of the reaction in an SN2 reaction depends on the nature of the alkyl halide, the reactivity order for alkyl halides is methyl > primary > secondary > tertiary. Finally, the strength of the leaving group plays an important role in SN2 reactions. If the bond between the leaving group and the carbon is weak, it will be easier to break when the nucleophile attacks. The least basic ion is preferred for SN2 reactions because it is most stable “the trends in basicity are parallel to the trends in leaving group potential – the weaker the base, the better the leaving group. Just as with conjugate bases” (Walker, n.d.). In an SN2 reaction, there may be possible undesired side reactions. An example is shown in figure 1.3 where elimination takes place instead of substitution, resulting in undesired products.

Figure 1.2: Above a nucleophile is attacking the backside of an alkyl halide and forcing the loss of a leaving group simultaneously.

Figure 1.3: Above is a possible undesired side reaction in experiment 8. This is an elimination reaction instead of a substitution reaction.

Experimental Section: Procedural Steps for Experiment 8:

Set up 3 test tubes and label them. Create a 1:5:4 ratio of solvent using acetone: Dietyl ether: Pentane. Measure 5mL of the solvent in a 10mL graduated cylinder.

Obtain the unknown substance. Measure 2mL of solvent in a 10mL grasuated cylinder. Set up a new test tube. Add the solvent to the test tube.

Using a pipette, add 20 drops of triethylamine to test tube 1. Add 20 drops of tripropylamine to test tube 2. Add 20 drops of ethyldiisopropylamine to test tube 3.

Using a pipette, add 40 drops of the unknown to the test tube. Add 20 drops of iodomethane and observe the appearence and time that the precipitate forms.

Add 10 drops of iodomethane to all 3 test tubes using a pipette. Record the time it takes for a precipitate to form and what the precipitate looks like.

After the precipitate has fully formed, collect it using vacuum filtration until it is fully dry. Crush the product with a capillary tube and collect some in the capillary tube.

Set up and label 3 new test tubes. Using a pipette, add 20 drops of triethylamine to each test tube.

Tap the capillary tube so the product moves to the bottom. Place the capillary tube in the melting point apparatus. Slowely increase the temperature and observe for when it starts to melt.

Observe the formation and time of the precipitate. Using a pipette, add 15 drops of iodoethane to test tube 1. Add 15 drops of 1-bromopropane to test tube 2. Add 15 drops of 2-bromopropane to test tube 3.

Observe the melting point range and compare it to the literature values to determine the identity of the unknown substance.

Figure 1.4: Above is a flow chart listing all the steps to successfully perform and SN2 reaction experiment.

Table of Chemicals: Physical and Chemical Properties of Chemicals used for Experiment 8: Chemical

Chemical

Molar

Boiling

Melting

Solubility

Appearance

Hazards

Name

Formula

Mass (g/mol)

Point ( ℃)

Point (℃ )

Acetone (2-Propanone)

CH3COCH3

58.04

56

-95

Soluble

Colorless liquid, sweet odor

Diethyl Ether

C4H10O

74.12

34.6

-116.3

Slightly soluble

Colorless liquid, aromatic

Pentane

C5H12

72.15

36.1

-130

Slightly soluble

Clear/ colorless liquid, gas odor

Triethylamine

(CH3CH2)2 N

101.19

89

-114.7

112 g/L in water

Clear/ colorless liquid, aminelike smell

Tripropylamine

C9H21N

143.27

156

-93.5

Slightly soluble

Liquid, colorless, amine-like smell

Ethyldiisopropyl -amine

C8H19N

129.25

127

-46

N/A

Liquid, light yellow

Iodomethane

CH3

141.94

42.43

-66.5

Soluble

Liquid, colorless, pungent odor

Iodoethane

C2H5I

155.97

72.2

-111

Soluble

Liquid, light yellow, ethereal smell

1-Bromopropane

C3H7Br

122.99

71

-110

2.5 g/L @ 20℃

Light brown liquid, pleasant odor

2Bromopropane

C3H7Br

122.99

60

-89

0.3g/ 100mL

Clear, Colorless liquid

Flammable, eye irritant, avoid inhalation Flammable, respiratory irritant, internal damage Flammable, toxic to aquatic life, fatal if swallowed Flammable, toxic, eye damage, skin irritant, toxic to aquatic life Flammable, eye damage, skin and respirator irritant, toxic Flammable, corrosive, toxic, eye damage, respirator irritant Toxic, skin/ eye/ respirator irritant, cancer causing Combustibl e, skin/ eye/ respiratory irritant, genetic defects Flammable, toxic, skin and eye irritant Flammable, eye irritant,

Trimethylphenyl Ammonium iodide

263.12

N/A

187

N/A

Light yellow powder

277.14

N/A

178-179

N/A

White powder

(C3H7)3CH3 NI

315.28

N/A

201

N/A

White powder

C13H30IN

327.295

N/A

184-189

N/A

White powder

(C

organ toxicity Skin/ eye/ respirator irritant

6

H 5

) N( CH 3

) 3 +

I BenzyltrimethylAmmonium Iodide MethyltripropylAmmonium Iodide MethyltributylAmmonium Iodide

C9H14NI C10H16IN

Figure 1.5: Above lists all the physical and chemical properties of all the chemicals used in experiment 7.

Skin/ eye/ respiratory irritant Skin/ eye/ respiratory irritation Skin/ eye/ respiratory irritation

Results: Results from Experiment 8: Part 1: Iodomethane + Amines Test Tube # 1

2

3

Part 2: Triethylamine + Alkyl Halides Test Tube #

Contents of Test Tube Triethylamine + Solvent + Iodomethane Tripropylamine + Solvent + Iodomethane Ethyldiisopropylamin e + Solvent + Iodomethane

Contents of Test Tube

Precipitate Form? Yes

Color of Precipitate Milky White

Time to Form Precipitate 6 Seconds

Yes

Cloudy White

23 Seconds

Yes

Hardly any precipitate. Liquid still clear

35 Seconds

Precipitate Form?

Color of Precipitate

Time to Form Precipitate

1

Triethylamine + Iodoethane Triethylamine + 1Bromopropane

2

3

Part 3: Unknown Amine + Iodomethane Precipitate Form? Yes

Yes

Cloudy White

14 Seconds

Yes

Slight Precipitate. Liquid still clear Slight Precipitate. Liquid still clear

30 Seconds

44 Seconds

Triethylamine + 2Bromopropane

Yes

Color of Precipitate

Time to Form Melting Point Identity of Precipitate Range Unknown 15 Seconds 178-179℃ Benzyltrimethyl - ammonium Iodide

Milky White

Figure 1.6: Above is a table with all the results from experiment 8, including the identity of the unknown substance.

Discussion: Part one of the experiment demonstrates the importance of having no steric hindrance in an SN2 reaction. In an SN2 reaction “The nucleophile attacks the electrophile from the side that is opposite to the leaving group, this means that the three other groups attached to the reactive carbon in the electrophile face towards the nucleophile as it approaches. If these three groups are small (e.g., all H's), then the nucleophile can approach easily, which can help the reaction proceed quickly” (SN2, 2016). If the groups are too big, the nucleophile will struggle to get close to the electrophile because of steric hindrance. The smaller the groups attached to the electrophile, the faster the SN2 reaction will be. Triethylamine has the least amount of steric hindrance when compared to tripropylamine and Ethyldiisopropylamine (the most), allowing it to have the fastest reaction among the three mixtures. This also means triethylamine is the strongest nucleophile among the three due to its lack of steric hindrance. The stronger the

nucleophile, the faster the reaction. Part 2 of the experiment deals with the substrate and effective the leaving group is in the reaction. 2- Bromopropane is a secondary substrate, which means it does not react as fast as a primary substrate in an SN2 reaction (or not at all). This is seen in Figure 1.6. Iodoethane and 1-Bromopropane are both primary substrates, which mean they react the fastest in an SN2 reaction. Iodoethane has iodine as a leaving group and can stabilize the charge over a larger area when compared to bromine in 1-Bromopropane. This is seen in Figure 1.6, Iodoethane is the fastest reaction and 1-Bromopropane is the second fastest reaction. In part 3 of the experiment, an unknown substance was identified by looking at the melting point and comparing it to the literature values. The precipitate was collected through vacuum filtration and inserted into a melting point apparatus. The melting point apparatus read the melting point range of the compound as 178-179℃ . This value exactly matched the given literature value for benzyltrimethylammoniumiodide of 178-179℃. Conclusion: The theoretical background information and results from the experiment are directly connected. Factors that impact the rate of an SN2 reaction played a major role in this experiment. Steric hindrance was present in the experiment. In part one triethylamine had the least steric hindrance, thus it was the strongest nucleophile and had the fastest reaction time. In part two, iodoethane was the primary substrate with the best leaving group, thus had the fastest reaction time when compared to a primary substrate with an inferior leaving group and a secondary substrate. The precipitate from the unknown substance was extracted through vacuum filtration and the melting point was measured. The melting point was found and the data revealed that the unknown substance was benzyltrimethylammoniumiodide. SN2 reactions can be used for many things. SN2 reactions can be used to build different functional groups from alkyl halides. Things

like alcohols, ethers, sulfides, thiols, nitriles, azides, halides, esters, and acetylenes can be formed. This is very useful when creating consumer products and medicines (Says, 2020). Overall, this experiment was a success. The identity of an unknown compound was found using a melting point range and vacuum filtration. Students observed the process of an SN2 reaction and how different things impact the rate of an SN2 reaction.

References: Says, K., Says, J., & Says, O. SN2 reaction examples: How to use it & make various functional groups; Master Organic Chemistry, 2020. https://www.masterorganicchemistry.com/2012/07/11/why-the-sn2-reaction-is-powerful/ (accessed March 01, 2021).

SN2: Electrophile, LEAVING group, and Nucleophile; The University of British Colombia; Vancouver, 2016. http://chem123chirp.chem.ubc.ca/sn2-electrophile-leaving-group-andnucleophile/#:~:text=Steric%20hindrance%3A%20In%20an%20S%20N%202%20reaction %2C,nucleophiles%20react%20more%20quickly%20than%20more%20bulky %20nucleophiles (accessed March 01, 2021).

Walker, M. Organic chemistry 1: An open textbook; Lumen. https://courses.lumenlearning.com/suny-potsdam-organicchemistry/chapter/8-3-factorsaffecting-rate-of-nucleophilic-substitution-reactions/ (accessed March 01, 2021) Weldegirma, S. Experimental Organic Chemistry,9th ed.; University of South Florida: Florida, 2020; pp 46-48....