Steric Hindrance Effects on SN2 Reactions: Experimental Study and Analysis

Introduction

In a substitution reaction a nucleophile replaces a leaving group off of a carbon atom. The nucleophile uses its lone pair of electrons to from a bond to the carbon atom. There are two types of nucleophilic substitution reactions, Sn1 and Sn2. Sn2 differs from Sn1 in that Sn2 is one step, not two. Sn2 has the nucleophile replace the leaving group, create a bond, and the leaving group also leaves all in the same step, while in Sn1 it happens in two steps.

Sn2 reactions also invert the stereochemistry configuration. Sn2 occurs with methyl, primary, and secondary carbons. A secondary carbon is a carbon attached to two other carbons, a tertiary carbon is a carbon attached to three other carbons.

Strong nucleophiles will increase the rate of the Sn2 reaction, some of the strong nucleophiles include HO-, CN-, I-, etc. Iodomethane has a strong nucleophile which would favor an SN2 reaction. Weak nucleophiles would favor an Sn1 reaction. A primary alkylhalide like 1-brompropane would favor an Sn2 reaction because it is primary and only attached to one carbon.

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Steric hindrance and the nature of the substrate are directly related and both impact what kind of reaction will occur. The objective of this experiment is to perform an SN2 reaction and evaluate the effects of steric hindrance on the rate of the reaction, nucleophilicity of the amine, and the nature of the leaving group.

Experimental Procedure

Obtain a test tube rack and 6 test tubes. Label the first three test tubes and add 5 mL of a mixture of an acetone: diethyl ether and pentane in a 1:5:4 ration to each test tube.

To test tube 1, add 20 drops of triethylamine. To test tube 2, add 20 drops of tripropylamine. To test tube 3, add 20 drops of Ethyldiisopropylamine. Add 10 drops of Iodomethane to each test tube and record when a precipitate forms. Discord all the solutions in the quaternary salt formation waste jar. Dry the other three test tubes and label them. Add 20 drops of triethylamine to each of the three test tubes. To test tube 1, add 15 drops of iodoethane and record when a precipitate forms. To test tube 2, add 15 drops of 1-bromopropane and record when a precipitate forms. To test tube 1, add 15 drops of 2-bromopropane and record when a precipitate forms.

Obtain an unknown amine. Dry a clean test tube and add 40 drops of the unknown to the test tube. Add 20 drops of iodomethane, constantly mixing. Add 2 mL of the acetone:diethyl ether and pentane solvent to the test tube. Mix the contents of the tube and let it stand for 15 minutes. Place the test tube in an ice-water bath. After 15 minutes a precipitate should have formed, collect it by vacuum filtration and wash the precipitate with the solvent. Allow the solid to dry and determine its melting point. Use the table of Quaternary Ammonium Salts melting points to determine to identify your unknown.

Table of Contents: Chemicals

AcetoneC3H6O 58.08 g/molBp: 56C Mp: -95C

Colorless liquid. Extremely flammable. Can irritate eyes. Use water spray to extinguish. Do not put near an open flame. Wear goggles, gloves, scrub pants, and a lab coat.

Triethylamine

C6H15N 101.19 g/molBp: 89C Mp: -114.7C

Clear colorless liquid. Toxic on skin. Do not swallow. Use water spray to extinguish a fire. Can explode. Wear gloves, goggles, a lab coat, and scrub pants.

Tripropylamine

C9H21N 143.274 g/molBp: 156C Mp: -93.5C

White liquid. Do not ingest. Do not get on skin. Do not breathe in. Use water spray or carbon dioxide to extinguish a fire. Wear gloves, goggles, a lab coat, and scrub pants.

Ethyldiisopropylamine

C8H19N 129.247 g/molBp: 126.5C Mp: -46C

Colorless/light yellow. Highly flammable and corrosive. Do not inhale. Use water spray to extinguish a fire. Wear goggles, gloves, scrub pants, and a lab coat.

IodomethaneCH3I 141.94 g/molBp: 42C Mp: -110C

Colorless liquid. Do not swallow, inhale, or ingest. Do not let it touch your skin. Use water spray or carbon dioxide to extinguish a fire. Wear gloves, goggles, scrub pants, and a lab coat.

IodoethaneC2H5I 155.97 g/molBp: 72C Mp: -110C

Colorless liquid. Combustible and flammable. Do not swallow, do not get on skin. Use water spray to extinguish a fire. Do not get in eyes. Wear gloves, goggles, scrub pants, and a lab coat.

1-bromopropaneC3H7Br 122.993 g/molBp: 59C Mp: -89C

Clear and colorless liquid. Can irritate eyes and skin. Do not inhale. Flammable. Use water spray, carbon dioxide, or dry chemical to extinguish. Wear gloves, goggles, scrub pants, and a lab coat.

2-bromopropaneC3H7Br 122.993 g/molBp: 59C Mp: -89C

Clear and colorless liquid. Can irritate eyes and skin. Do not inhale. Flammable. Use water spray, carbon dioxide, or dry chemical to extinguish. Wear gloves, goggles, scrub pants, and a lab coat.

Discussion

In test tube 1 the iodomethane reacted with triethylamine in under 30 seconds. Iodomethane is a primary alkylhalide so an SN2 reaction is favored, which is why it occurred so quickly. A primary alkylhalide has little steric hindrance due to it being attached to only one carbon, which also makes an SN2 reaction favored. The nucleophile for the reaction in test tube 1 is also a strong nucleophile which favors makes an SN2 reaction. The test tube with tripropylamine had a slower reaction rate than the triethylamine, and the Ethyldiisopropylamine took the longest time to form a precipitate. The triethylamine with iodoethane formed a precipitate in under a minute, while the 1-bromopropane made a precipitate in about a minute.

The triethylamine with 2-bromopropane formed a precipitate in about 17 minutes and 50 seconds, though it never got as cloudy as the others, and barely formed a precipitate. Test tube 1 had a primary alkylhalide which increased the rate of the SN2 reaction. A good leaving group is also favorable for substitution reactions, having a good leaving group would be more favorable and increase the time until a precipitate formed. In order to identify our unknown, we created a precipitate and collected it through vacuum filtration. We found the melting point of our unknown and used the table of Quaternary Ammonium Salts melting points to determine to identify our unknown.

Conclusion

The theoretical background and the results we observed are connected because we were able to perform an SN2 reaction and evaluate the effects of steric hindrance on the rate of the reaction, nucleophilicity of the amine, and the nature of the leaving group. We also were able to record the times and rates each solution gave yielded a precipitate. We also were able to determine what our unknown was through a reaction and its melting point.

The experiment revealed that our unknown had a similar melting point as (C6H5)CH2N(CH3)3+ I-, allowing us to conclude that this was our unknown. Precipitation reactions can be useful in water treatment, to take out contaminants in the water. Hydroxide ions can be placed in the water to take out metals in a precipitation reaction. The experiment did accomplish what it set out to do as we were able to create a precipitate out of each reaction and determine our unknown.

References

1. Acetone. https://pubchem.ncbi.nlm.nih.gov/compound/acetone (accessed June 13, 2019).
2. Admin. Precipitation Reaction - Examples & Definition | Precipitation Reaction Equation. https://byjus.com/chemistry/precipitation-reaction/ (accessed Jun 13, 2019).
3. Libretexts. 7.12: Comparison of SN1 and SN2 Reactions.
4. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map:_Organic_Chemistry_(Wade)/07:_Alkyl_Halides:_Nucleophilic_Substitution_and_Elimination/7.12:_Comparison_of_SN1_and_SN2_Reactions (accessed Jun 13, 2019).