Experiment No. 7 Dehydration of Cyclohexanol

Name: Arlene Jane Chang Section: A

Locker no. : 7A Date started/completed: August 31, 2010

Experiment no. 7: Dehydration of Cyclohexanol Results and Discussion: There are two keywords important in this experiment, these are: E1 and dehydration. In order to remove the OH group from cyclohexanol and convert it to cyclohexene, a reaction called dehydration of alcohols is used (McMurry, 2008). Dehydration is defined by the Encyclopedia Britannica as a reaction that is carried out by warming the alcohol in the presence of an acid catalyst. The dehydration process was called E1 because it is an elimination reaction in which the rate determining step was unimolecular (Carey, 2006). The rate determining step in this type of reaction is when the leaving group leaves to form an intermediate carbocation. Generally, the more stable the alcohol the faster the reaction will be (Carey, 2006). There are three steps in the mechanism of the E1 reaction: First, by protonating the hydroxyl group, it can leave as a water group. Next, after the protonation, a carbocation intermediate is formed. Lastly, it is stabilized again by losing the proton from a carbon atom adjacent to the positively charged carbon ion to yield an alkene (Encyclopedia Britannica). Figure 1. Three step mechanism: OH H

H +¿¿ H

OH 2

OH 2

+

H

+

H2O

H H

H 3 PO 4

Figure

Figure 2: General reaction: OH H 3 PO 4

+

H2

O

In the experiment, the lab manual dictated 8.0 g of solid cyclohexanol was to be used, but since the lab had only the liquid form, the amount to be used was calculated using density. The

density of cyclohexanol is 0.962 g/mL. By using this equation:

0.962 g 8.0 g = 1mL x mL

, the total

amount that was used was 8.32 mL. The percent yield meanwhile was only 14.42 %. According to Carey, the total yield of cyclohexene should have been around 79 to 89%. This low output is expected since it is a reversible reaction it will be subjected to the Le Chatelier’s Principle. Another reason might be

because some of the cyclohexene might have evaporated and not enough was distillated from the round bottom flask. In order to verify the presence of cyclohexene, two qualitative tests were used: Baeyer’s test to test the presence of double bond and chromic acid test to see if there are secondary alcohols in the solution (Ault, 1979). As expected, the Baeyer’s test was positive since cyclohexene contained a double bond. But the chromic test was problematic because it also gave positive result. The positive result in the Chromic Acid test suggests that there are compounds in the solution that contained secondary alcohol (Ault, 1979). The goal of this experiment was to get pure (or almost pure) cyclohexene. This indicated that an error made by the experimenter since there were still some cyclohexanol left in the solution.

References: Alcohol. (2010). In Encyclopædia Britannica. Retrieved September 1, 2010, from Encyclopædia Britannica Online: http://search.eb.com/eb/article-277693 Ault, A. (1979). Techniques and Experiment for Organic Chemistry. Boston, Massachusetts: Allyn and Bacon Carey, F. A. (2006). Organic chemistry. Boston, Massachusetts : McGraw-Hill/Higher Education. Guidote, A., Del Rosario, D.R., & Abuzo, A.L. (2005). Experiencing organic chemistry: A laboratory manual. Loyola Heights, QC: Ateneo de Manila University McMurry, John. (2008). Organic chemistry. Brooks/Cole.