Exploring Alcohol Dehydration and Gas Chromatography Analysis

Objective

The purpose of this experiment was to make a microscale E1 dehydration reaction. E1 dehydration reaction was done with a secondary alcohol in order to synthesize a gaseous product. Sulfuric acid was used as a catalyst to prepare three isomeric butenes. And in order to achieve gas chromatography, the sample was then prepared and the GC instrument was then used to obtain a gas chromatograph for analysis.

Experimental Procedure

1. Calibrate the gas supply via fixing one end with the elastic septum and rearrange and including 3 mL of water and marking the level with a sharpie.

At that point include another 1 mL of water and mark that level too. Set up an external thermometer 4mL of distilled water was measured with a graduated cylinder then added

2. Gas collection tube needs to be Filled all the way to the top with water, then put finger over the open end, to prevent water from dropping when the tube gets invert and lower into a 250 mL beaker filled with water.

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3. U sing automatic deliver pipet, add around 100 μL of 2-butanol then dry 1.0 mL conical vial with a boiling chip. Then 50 μL of concentrated sulfuric acid need to be added to using a 9” Pasteur pipet. A 9” pipet was already marked and its been used instead of an automatic deliver pipet to add 100 μL of 2-butanol and 50 μL of sulfuric acid. Liquid has a color of a light orange/brown

4. Cap/seal the tapered vial to the gas conveyance cylinder and position the straight end of the cylinder under the water in the container with the end goal that the gas will be conveyed into the gas store.

Set the funnel shaped vial in a sand shower. Clip the vial. It was difficult to set up and clamp the apparatus

5. Heat the reaction while it in the sand bath and maintain the temp around 140◦C. Wait till 3-4 cm3 of gas has been collected, then stop the heating but allow to reaction vessel to remain in the sand bath. Sand bath was warmed while device was being set up so once the raction blend was cinched into the sand bath, bubbles were seen rapidly. Sand bath was at 115◦C when first air pockets were seen

6. Now carefully start removing the gas delivery tube form the water bath. Be sure to NOT remove the reservoir or allow the open end to break the surface of the water. Reservoir was removed from apparatus carefully and carried to GC room without allowing the open end to break the surface of the water

7. Remove the delivery tube form the water before the conical vial cools too much.

8. Withdraw a 0.5 cm3 sample of the isomeric products using a gas-tight syringe for GC analysis. 0.5 cm3 of the sample was withdrawn for GC

9. Interpret the gas chromatogram. Ascertain the mole division of every isomer in the blend. Recognize every item utilizing Zaitev's Rule as the 'secure' determinant of request of elution. Took a little over 2 minutes for GC to show any peaks. Three peaks appeared, each larger than the one before

Results and Analysis

Table: Product Distribution of Isomeric Butenes

The theoretical yield was calculated based on the 1:1 stoichiometry between 2-butanol and the products, resulting in 1.09 mmol. The GC analysis revealed that trans-2-butene was the major product, followed by cis-2-butene and 1-butene, aligning with Zaitsev's rule which predicts the formation of more substituted (and thus more stable) alkenes.

Discussion

In this experiment, by using sulfuric acid as a catalyst to prepare three isomeric butenes (1-butene, cis-2-butene, trans-2-butene) a microscale E1 dehydration reaction was performed on a secondary alcohol to synthesize a gaseous product. The sample was prepared for gas chromatography and then gas chromatograph was obtained for analysis.

The experiment procedure was very quick once the apparatus was completely set up. The sand bath was on the hot plate warming up the entire time while the rest of the apparatus was being set up. Once the reaction mixture was clamped into the sand bath, gas bubbles began to collect in the reservoir very quickly, almost immediately. The temperature reached about 115◦C and the reservoir filled with gas after only a few minutes. The reservoir was carefully removed to keep it under the line of water, so the gas could not escape, then taken to the room with the GC instrument.

T hree isomeric butenes were formed in this experiment,. This is because the secondary carbocation that is formed when the sulfuric acid deprotonates the 2-butanol has two different types of beta protons. If a terminal beta proton gets deprotonated, then it will result in the terminal alkene. If an internal beta proton gets deprotonated, then it will result in a trans or cis alkene. According to Zaitsev’s rule, the more substituted alkenes are the more stable alkenes.

This would mean that the trans and cis products would be the major products and the terminal alkene would be the minor product. Since the trans and cis 2-butenes are both disubstituted, steric strain is what determines which would have the higher yield. Cis-2-butene has both of its methyl groups on the same side of the double which causes the double bond to have a lot more steric strain than the trasn-2-butene, which has its methyl groups on opposite sides of the double bond. This would make the cis-2-butene less stable and have a smaller yield, so the trans-2-butene would have the largest yield of all three isomers.

And in order to complete the separating and analyzing compounds that vaporize without decomposing, a gas chromatography was used. Once a sample is injected into the GC, it is transported through the column by a mobile phase which was helium in this experiment. The helium is inert and just aids in transportation. The stationary phase was a non-polar substance and interacts with the sample that is injected. This is how gas chromatography can be used to separate the isomeric alkenes of butene in this experiment. There are two factors that determine retention time in the GC which are boiling point and polarity.

Compounds with a lower boiling point will go through the GC faster because they will spend more time in the gas phase and vice versa. The yields, however, are more dependent on each compounds polarity and its interaction with the stationary phase. Due to the column being non-polar, less polar compounds will have more interaction with the stationary phase than more polar compounds, and therefore will have longer retention times. This would explain why the order of elution in the gas chromatogram would be based on decreasing strength in polarity- the most polar, 1-butene, followed by cis-2-butene, and then the least polar trans-2-butene.

The Zaitsev rule can be observed in the fractions of each isomer when looking at the gas chromatogram. The most stable and most substituted alkene would have the highest percentage (87.23%), the second having the middle percentage (11.3%), and the least substituted and least stable having the lowest percentage (0.6%). This explains why 1-butene is peak A which is the smallest, cis-2-butene is peak B which is in the middle, and trans-2-butene is peak C which is the largest. The major, minor, very minor products are as follows: trans-2-butene, cis-2-butene, and 1-butene.

Conclusion

The microscale E1 dehydration reaction of 2-butanol successfully synthesized isomeric butenes, with gas chromatography offering detailed insights into the product composition. The experiment highlights the practical application of GC in chemical analysis and reinforces the theoretical principles governing the stability and formation of alkene isomers. This study not only demonstrates the dehydration mechanism but also illustrates the effectiveness of GC in distinguishing and quantifying similar compounds, making it an invaluable tool in organic chemistry research.