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Lab 10 Nucleophilic Substitution at Saturated Carbon

Lab 10 – Studying SN1 and SN2 Reactions: Nucleophilic Substitution at Saturated Carbon

AIM

The aim of this experiment was to convert a primary alcohol (1-butanol) to an alkyl bromide (1-bromobutane) using an SN2 reaction.

INTRODUCTION

Substitution reactions occur when one atom or group of atoms replaces another. A type of substitution reaction is an SN2 reaction which happens in one single step with a nucleophilic attack and ejection of a leaving group occurring simultaneously. A nucleophile contains an unshared pair of electrons that reacts with a site in an organic molecule that has a deficiency of electrons. In this type of reaction, two chemical species – the organic reactant and the nucleophile – participate in the rate-determining step (slowest step) of the reaction. The reaction is initiated by an attack of the nucleophile on the carbon bonded to the leaving group and this back side attack produces a product in which the configuration of the carbon atom is essentially inverted. In this experiment, we used an SN2 reaction to convert 1-butanol (primary alcohol) to 1-bromobutane (alkyl bromide). For this mechanism (Figure 1), bromide serves as the nucleophile and water is the leaving group. Sulfuric acid is the catalyst in this case

TABLE OF PHYSICAL PROPERTIES

Table 1 Table of physical properties of compounds that were used in this experiment

Compound

Structure

Melting Point (ºC)

Boiling Point (ºC)

Physical Properties

Known Hazards

Butanol

C4H9OH

-89.8

117.7

A colorless liquid that is used in organic chemical synthesis, plasticizers, detergents, etc.

Flammable liquid and vapor, harmful if swallowed, causes skin and eye irritation, may cause drowsiness or dizziness.

1-bromobutane

C4H9Br

-112.4

101.3

It appears as a clear colorless liquid that is denser than water and insoluble in water.

Highly flammable liquid and vapor, causes skin, eye, and respiratory irritation

Hydrobromic Acid

HBr

-87

-67

It appears as a colorless gas with a pungent irritating odor. It is corrosive and heavier than air.

Causes severe skin burns and eye damage, may cause respiratory irritation.

Sulfuric Acid

H2SO4

10.31

337

A colorless oily liquid. It is soluble in water with release of heat. It is corrosive to metals and tissue.

Contact with eyes or skin causes severe burns and irritation, highly reactive.

PROCEDURE

Reaction Part

For the reaction portion of this experiment, a 50mL round bottom flask was used and the following was added: 2-3 boiling chips, 5.0 g (6.2 mL) of the substrate 1-butanol, and 10 mL of 48% HBr. This solution was then cooled down using an ice bath with an appropriate size beaker for the flask to rest on. While the solution was still in the ice bath after two minutes, 4 mL of H2SO4 were slowly added using a glass pipette and stirred after every 10-20 drops. In order to avoid complications, we made sure that the glassware used for this process was spotless and clean because the very strong acid easily reacts with any residue that may be present. This was done so by first rinsing the materials with water and then adding acetone to dry them. Afterwards, the solution was taken out of the ice bath and a reflux was performed. The reflux apparatus was prepared by attaching the round bottom flask to a condenser and placing it on a flask heater. We made sure that the water in and water out tubes were placed accordingly for the correct flow of water in the condenser. The solution was let to reflux for an hour once we observed boiling taking place.
Workup Part

Following the reaction procedure for this experiment, the work up portion was then conducted to purify the product. After the solution was refluxed, it was left to cool, and 10 mL of water were carefully added. A distillation apparatus was prepared using the same methods shown on Figure 4 in the methods portion of the lab manual. A test tube inside a beaker was placed towards the end of the condenser using an adapter. This test tube would essentially collect the product. Two layers were produced in the test tube after a couple of minutes, the bottom layer being the organic product and the top layer being the aqueous product. To obtain the organic product alone, a pipette was used to wash off the aqueous layer about three times with NaHCO3 and two times with water. Once dried, 1g of the drying agent NaSO4 was added into the test tube. To obtain the mass of the product, a vial was preweighed and the product was carefully transferred into this vial. The mass was measured and recorded

RESULTS AND DISCUSSION

Preweight of vial = 9.566g

Weight of vial + product = 16.329g

Weight of product = 16.329g – 9.566g = 6.763 g

Yield

6.763 g of bromobutane / 137.0 g/mol = 0.0494 mol

Theoretical Yield

5 g butanol (limiting reagent) / 74.1 g/mol = 0.0675 mol

Percent Yield

(0.0494 mol bromobutane / 0.0675 mol 1-butanol) * 100% = 73.19%

Our results indicate that 73.19% of 1-butanol was converted to 1-bromobutane using an SN2 reaction. These results are fair considering that the process we used wasn’t very precise because product might have been lost during the completion of this experiment.

ANALYSIS OF REACTION

The solvent for the reaction was water. Since the OH group of the alcohol is not a weak base and, therefore, not a good leaving group, the reaction was conducted in a strongly acidic condition using sulfuric acid. Sulfuric acid was used in to protonate the alcohol group (a bad leaving group) in 1-butanol in order to allow it to leave the molecule as water (a good leaving group). The reaction was conducted under two different temperatures, under an ice bath and then a reflux, to successfully convert the compound by SN2. The cold step is loss of the leaving group because water (the solvent) is being removed with the addition of sulfuric acid. The hot step indicates the nucleophilic attack, or bromination of the organic material because two layers are being produced through distillation. During the workup step, the organic phase was at the bottom of the test tube because since the organic solvent is now brominated, it will switch. Halogenated compounds sink to the bottom because bromine is very heavy and becomes denser than water. The aqueous phase had to be washed with a weak base NaHCO3 to neutralize any remaining acid and obtain the organic layer by itself.

CONCLUSION

The purpose of this experiment was to convert a primary alcohol (1-butanol) to an alkyl bromide (1-bromobutane) using an SN2 reaction. After carefully conducting this experiment, we managed to convert 73.19% of the 1-butanol into 1-bromobutane.  These results are fair because product may have been lost during the process due to experimental factors. For instance, the distillation step may have led to some product being lost, as well as during the washing of the test tube. The pipettes may have taken up some of the organic layer since it was hard to see the distinctive layer at times. Overall, this experiment strengthened our understanding of SN2 reactions and how they occur.

REFERENCES

“1-Butanol.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, https://pubchem.ncbi.nlm.nih.gov/compound/1-butanol.

“1-Bromobutane.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, https://pubchem.ncbi.nlm.nih.gov/compound/1-Bromobutane.

“Hydrobromic Acid.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, https://pubchem.ncbi.nlm.nih.gov/compound/Hydrobromic-acid.

“Sulfuric Acid.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine, https://pubchem.ncbi.nlm.nih.gov/compound/sulfuric_acid.

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