The Synthesis of Alkenes: The Dehydration of Cyclohexanol
In Organic Chemistry, many different methods are used to synthesize organic compounds from various components. In this lab, cyclohexanol was dehydrated to cyclohexene through an elimination reaction. In order to separate the cyclohexene product from the cyclohexanol starting component, previously learned lab techniques such as extractions and simple distillation were used.
The formation of the product was verified by performing a Bromine test as well as an analysis using IR Spectroscopy. A percent yield of 8.33% was obtained. Introduction:
In an elimination reaction, two substituents are removed from a molecule in either a one or two-step mechanism. There is a removal of a leaving group and a proton to form alkenes. The proton is often called an alpha hydrogen, which is the hydrogen atom attached to the alpha carbon. Even though it is possible to lose any alpha hydrogen in an elimination reaction, the most substituted product is the major product, and the most substituted double bond will predominate.
There are two types of elimination reactions: E1 and E2. Good leaving groups are needed in these two reactions. E2 reactions are a bimolecular concerted reaction, which means that it is a reaction involving two molecular entities and bonds are forming and breaking at the same time. The two steps in this reaction happen simultaneously in one transition step. For this reason, both the concentration of the substrate and the base are present in the rate law equation; rate = k[substrate][base]. In an E2 reaction, a strong base is used in order to deprotonate the molecule causing a transfer of the electron density to form a pi bond between two carbons. Examples of a strong base are KOH or OH-. A good leaving group is also required which includes I-, Cl-, and Br-. There are other good leaving groups aside from those previously listed. E2 reactions usually work best with a secondary or a tertiary substrate. Primary or methyl substrates typically undergo SN2 reactions, but could undergo E2 if a bulky base is used.
The other type of elimination reaction is E1. E1 reactions are unimolecular and instead of being concerted like E2, the process happens in steps. During the first step of E1 elimination, the leaving group leaves and a carbocation is formed. This step is the rate-determining step, which is the slowest step. For this reason, the rate equation for this reaction is: rate = k[substrate]. The rate-determining step depends on how favorable the formation of a carbocation is. The formation of tertiary carbocations if the most favorable followed by secondary. Primary carbocations do not form. E1 elimination will not happen for primary substrates of methyl substrates because the formation of the carbocation is not favored due to the fact that it cannot be stabilized through hyper conjugation. Once the carbocation is formed, a weak base can deprotonate the alpha hydrogen and the electron density will move to form a pi bond to get rid of the positive charge created when the leaving group left. This reaction should involve a weak base, a good leaving group, and a polar protic solvent. A polar protic solvent is a solvent that has a hydrogen atom bound to an oxygen or nitrogen atom. This type of solvent is good for an E1 reaction because it will be able to help the leaving group leave and stabilize the carbocation.
When a bad leaving group such as –OH, -NH2, or R-O- is present, certain reagents must be used to remove them because they cannot leave on their own. For example, OH can be removed using dehydration and an acid. The OH can be protonated by a strong, non-nucleophilic acid. Water is created which is then the leaving group. The E1 reaction will then proceed. In this lab, cyclohexanol undergoes an E1 elimination in order to form cyclohexene because it contains an OH as a...
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