CHEM Alkenes by Dehydration PDF

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Summary

lab for chem 141...

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Alkenes by Acid-catalyzed Dehydration of an Alcohol Introduction: The dehydration of alcohols to produce alkenes happens by heating alcohols, paired with a strong acid, at high temperatures. If the reaction is not heated enough, the alcohol will not dehydrate, but react with each other and form ethers.Tertiary alcohols dehydrate by an acidcatalyzed E1 mechanism to produce alkenes. E1 reactions always follow Zaitsev’s rule, telling us the most stable alkene and most favorable product will be the alkene that is more substituted. In this experiment, we will reflux 2-methyl-2-butanol with sulfuric acid causing it to dehydrate and produce 2 alkenes. The reaction is being done in steps, rather than all at once. The removal of an HX substituent causes an alkene bond to form. The -OH of the 2-methyl-2butanol gains a hydrogen. This water molecule is eliminated, giving us 2 product results: 2methyl-1-butene and 2-methyl-2-butene. Fractional distillation will be used to separate the acid from any remaining alcohol. Gas chromatography will be used to analyze the distillate and determine the ratios of each alkene.

Procedure: Due to the volatile makeup of the alkenes used, the apparatus must be set up first to reduce the amount of lost material. Weigh the graduated cylinder used for collection. Assemble the fractional distillation equipment. Cool the graduated cylinder in ice-water to prevent the alkenes from evaporating. Cool the flask and add 6 mL of 6M sulfuric acid. Cool the flask with its sulfuric acid solution in an ice-water bath. Slowly add 9 mL 2-methyl-2-butanol (in three batches) into a round bottom flask. Swirl between additions and add 2 boiling chips. Reinstall the flask into the distillation/reflux apparatus. Make sure water is running through the condenser, plug in the heating mantle, and set the variac to 4. Collect approximately 10 mL of alkenes, while

keeping the temperature at approximately 35℃ . Weigh the measuring cylinder and calculate the yield. Inject a sample of the alkenes into the gas chromatograph.

Results: Empty Graduated Cylinder

27.567 g

Graduated Cylinder with Contents

32.478 g

Total Moles of Product

0.070 mol

Total Moles of Starting Materials

0.082 mol

Percent Yield

85.4%

Area 2-methyl-1-butene

1.5 cm2

Area 2-methyl-2-butene

9.99 cm2

Ratio 2-methyl-2-butene to 2-methyl-1-butene

6.66:1

Ratio Total to 2-methyl-1-butene

7.66:1

Ratio Total to 2-methyl-2-butene

1.15:1

Discussion: The objective of this experiment was to produce 2-methyl-2-butene and 2-methyl-1butene by result of a dehydration reaction. From our graph we can see 2-methyl-1-butene was the less favored product, since it has a smaller area. The ratio of each alkene was found using graphs produced by gas chromatography. We found that 2-methyl-2-butene and 2-methyl-1butene had a ratio of 6.66:1. The ratio of the total to 2-methyl-1-butene was 7.66:1 and the ratio of the total to 2-methyl-2-butene was 1.15:1. The area of the peaks were calculated by multiplying height by width at half height. For the total area, we added the area of both peaks together, giving us 11.49 cm2. We used the experimental product mass, molecular weights, and densities given to calculate the moles of both the product and starting materials. These mole

values were used to calculate the percent yield, which was found to be 85.4%. Possible error in this experiment could have been a result of placing the thermometer too low. It would have read a higher temperature than in actuality and therefore we may not have evaporated all of the product. This could have been the reason for not getting a 100% yield. To improve this lab, we could run more trials or use more accurate rulers/liquid measuring techniques to produce more exact results....