Cyclohexanol to Cyclohexene

Lab Report: Cyclohexanol to Cyclohexene

Discussion:

Dehydration is an elimination reaction of an alcohol: The elimination reaction involves the loss of an OH from one carbon and an H from an adjacent carbon. Overall, this amounts to the elimination of a molecule of water, resulting in a pi-bond formation of an alkene or alkyne. The loss of water from a molecule is called dehydration.

In many cases alcohol dehydration require an acid catalyst and heat. Phosphoric acid (H3PO4) and sulfuric acid (H2SO4) are the most commonly used acid catalysts.

When more than one elimination product can be formed, the major product is the more substituted alkene-the one obtained by removing a proton from the adjacent carbon that has fewer hydrogens (Recall Zaitsev’s rule). The more substituted alkene is the major product because it is the more stable alkene, so it has the more stable transition state leading to its formation.

Dehydration of 2-methyl-2-butanol produces primarily 2-methyl-2-butene, a tri-substituted alkene, rather than 2-methyl-1-butene, a di-substituted alkene:

To prevent the alkene formed in the dehydration reaction from reforming back the alcohol, the alkene can be removed by distillation as it is formed, because it has a much lower boiling point than the alcohol. Removing a product displaces the reaction to the right.(Recall Le Chatelier’s principle).

In this experiment, Cyclohexanol is dehydrated to cyclohexene according to the following reaction:

Tb = 160-161oC                                                           Tb = 83oC

Because the OH group is a very poor leaving group, an alcohol is able to undergo dehydration only if its OH group is converted into a better leaving group.

One way to convert an OH group into a good group is to protonate it. Thus, in the first step of dehydration reaction, protonation changes the very poor leaving group –OH into  a good leaving group –OH2+.

In the second step, water departs, leaving behind...