Alkenes by Acid-catalyzed Dehydration of an Alcohol (Sully) PDF

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Lab #8 for Chem 141...

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CHEM 141 Experiment #8 November 13th Alkenes by Acid-catalyzed Dehydration of an Alcohol Experimental 6ml of 6M sulfuric acid was measured out and added to a 50ml round bottom flask and set onto ice to cool. 9ml of 2-methyl-2-butanol was then added to the flask in three equal batches, and the flask swirled between additions. Two boiling chips were then added and the flask in a standard reflux/distillation apparatus that had been set up. The graduated cylinder placed to collect the product was also weighed first. The heating mantle was then set to a variac setting of 40, and the reaction allowed to proceed until 40ºC was reached and 2.59ml product had been collected. The graduated cylinder was then weighed to obtain the yield(44.968%). A sample of the alkene product was then injected into the GC, and those results used to determine the ratio of the two alkenes produced (6:1). Results and Discussion Reaction Data Volume of added 2-methyl-2-butanol (ml)

9.0

Mass Empty Graduated Cylinder (grams)

28.247

Mass Full Graduated Cylinder (grams)

30.840

Height 1 (cm)

2.55

Height 2 (cm)

11.7

Half Height 1 (cm)

1.275

Half Height 2 (cm)

5.85

Width at Half Height 1 (cm)

0.7

Width at Half Height 2 (cm)

0.925

Area 1 (cm2)

1.785

Area 2 (cm2) UPDATE

11.115

Ratio (2-methyl-2-butene:2-methyl-2-butene)

6:1 (6.063:1)

Crude Yield (%)

44.968

Secondary and tertiary alcohols will dehydrate via E1 reaction mechanisms to form alkenes. E1 reactions are elimination reactions that display first order chemical kinetics and are unimolecular reactions. These mechanisms are composed of two main steps, the loss of a leaving group to form a carbocation intermediate, and the deprotonation by the solvent to form an alkene. An alcohol, 2-methyl-2-butanol, in the presence of a strong acid, sulfuric acid, can be dehydrated to form an alkene. This works by protonating the OH group, a poor leaving group, to H2O, which is a far better leaving group. Once the water has left, the alkene is left. There are often two alkenes formed from these reactions, and the more substituted one is favored due to its greater stability, and thus it is the main product. This concept is known as Zaitsev’s rule. 2-methyl-2-butanol reacts to form 2-methyl-2-butene and 2-methyl-1-butene. Both of these products have much lower boiling points than the reactant, and thus fractional distillation could be used to reflux and then seperate then. The reactants were heated allowing energy to enter the reaction as the formation of the carbocation is an endothermic reaction, and the reflux accomplished just this. The calculated ratio between 2-methyl-2-butene and 2-methyl-1-butene was found to be 6:1, which makes a lot of sense given that was the predicted main product. This was easily done given the printed graph from the GC that allowed for the area ratios, which are in turn product ratios. The crude percent yield for this experiment was low at 44.968%. One reason for this is that some of the product was spilled slightly before being weighed. There was also most likely product that had still not distilled off for collection when collection was stopped, and some of the reactant most likely did not react to form the product. All of this would decrease the yield in the experiment, and thus explain the low yield. To mitigate this the reaction could have been brought to distillation temperature slightly slower, or held there for longer, allowing for greater reaction time prior to distillation. Once the product began to distill, the slower temperature climb would have allowed for more complete distillation. Both of these would help increase the yield. Conclusion When heated with a strong acid catalyst alcohols typically undergo elimination reactions, specifically E1, generating an alkene and water. These are known as dehydration reactions due to the removal of a water molecule. To provide a concrete example of this, 2-methyl-2-butanol was dehydrated into 2-methyl-2-butene and 2-methyl-1-butene by being heated with concentrated sulfuric acid in a fractional distillation apparatus. The crude yield of the experiment was only 44.968%, but was explained by loss of product, and both unreacted reactant and undistilled product with reactant. Using a GC the ratio of the products was found to be 6:1, with 2-methyl-2-butene being the main product as expected, so this experiment can be called a success. To help improve the yield of the experiment it could have been refluxed at a slightly lower heat, allowing for the solution to reach fractional distillation temperature after a longer period of time, and so that the wanted alkene has more time to distill off for collection. Dehydrations are often used outside of the lab setting as well, in producing acid anhydrides. This

dehydration can convert alcohols into ether polymers, and alcohols to alkenes, as we have seen....