[Objectives]
The goal of this experiment is to carry out the dehydration of 2-methylcyclo-hexanol and study the orientation of C=C bond in the final alkene products. We will also find Zaitsev's rule to be tested.
[Principle & Procedure]
* Procedure
(A) Dehydration
First, we will put 9.0mL of 2-methylcyclohexanol and 2.3mL of 85% phosphoric acid into 25mL round bottom flask. Then we will proceed steam distillation. This step is to make alcohol to be dehydrated, which means eliminating water. 85% phosphoric acid will act as catalyst. As this experiment is about alcohol's E1 mechanism, its rate equation is v=k[ROH]. It means its reaction rate is determined only by the concentration of 2-methylcyclohexanol, not the concentration of 85% phosphoric acid. Then we will check the temperature of the first drop and major drop. We will also check the amount of water layer and organic layer. This step is about azeotrope, which is a heterogenous mixture of immiscible liquid. It can be distilled by steam distillation. Then we will move those liquid into separatory funnel and take out the water layer. The density of water layer is higher than organic layer, so we will be able to take out easily as it exists.
Then, we will do extraction procedure with saturated NaHCO3. It is to remove water and we will get rid of excess acid as form of CO2 and H2O. Then we will do brine wash to dry the organic layer. After that, we will dry organic layer with anhydrous Na2SO4 then cotton filtration to get pure product.
(B) Baeyer Test
We will put 1mL of our 1~2 drop of potassium permanganate in acetone in test tube. Then we will put 1mL of our product and shake gently. We will check the change of color. If our experiment is successful, the color would change purple into brown as oxidation number of Mn changes +7 into +4 by reduction. (Product will be oxidated at the same time)
* Principle
Alcohols can be dehydrated by using an acid. Water will be eliminated from the molecule and C=C bond will be formed. If the alcohol is not symmetric, then a mixture of products containing alkene isomer is possible. According to Zaitsev's rule, the more substituted C=C is favored in the product in a reaction following E1 mechanism.
E1 indicates an elimination, unimolecular reaction, of which rate=k[R-leaving group]. This implies that the rate determining step of the mechanism depends on the decomposition of single molecular species. Overall, this pathway is a multi-step process with the following two critical steps.
(반응물보다 생성물의 개수가 더 많아져 용질이 늘어나므로 라울의 법칙에 의해 vapor pressure가 감소함을 알 수 있음)
(1) Loss of leaving group to generate a carbocation intermediate.
(2) Loss of a proton.
H+ from the carbocation and form the pi-bond. (Base's effect)
In this case of 2-methylcyclohexanol, 1-methylcyclohexene should be formed as a major product.
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