The Synthesis Of Alkenes Dehydration Of Cyclohexanol

The Synthesis Of Alkenes Dehydration Of Cyclohexanol -...

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Takamasa Tobita CHEM 0330: Organic Chemistry Laboratory The Synthesis of Alkenes: The Dehydration of Cyclohexanol Jonathan Lippert 3/3/2014 Signature ________________________________________
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Abstract The purpose of this lab was to synthesize cyclohexene by the dehydration of cyclohexanol. A bromine chemical test was performed to verify the formation of cyclohexene product, and an Infrared Spectrum of the product was also analyzed for further confirmation of the cyclohexene production. The bromine test was positive, a peak with the absorption of 150.17 1/cm showed presence of alkene, and the percent yield for cyclohexene was 21.36%. Introduction
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Elimination reactions can occur in two different ways, either in one or two steps. An E1 reaction, also known as unimolecular elimination reaction, is a two-step process that is dependent on only the concentration of the substrate. The rate can be determined by the equation: Rate = k * [substrate] E2 reaction, or bimolecular elimination reaction, is a one-step process that is dependent on both the concentration of the substrate and the nucleophile. The rate can be determined by the equation: Rate = k * [substrate] * [base] These two reactions can yield products that contain an alkene functional group (a carbon double bonded to another carbon). The main difference between E1 and E2 is that E2 is a concerted process: the breaking of bonds and making of other bonds occurs simultaneously. E2 reaction starts when a strong base, acting as a nucleophile, takes a hydrogen atom attached to a carbon that is connected to another carbon with a certain leaving group. During this single transition state, the lone electron pair of the carbon - which its hydrogen atom is taken - is pushed to form a pi bond with the adjacent carbon, forming a carbon-carbon double bond. This results in the leaving group to leave (Figure 1). E2 reactions favor strong bases and hindered substrates.
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Figure 1. Basic E2 Reaction Mechanism Conversely, E1 reactions lead to formations of a carbocation intermediate. Thus, it is a two-step process. First, the leaving group is heterolitically cleaved from the carbon, creating the carbocation. Then, a base attacks a hydrogen atom of a carbon that is neighboring the carbocation. The attack from the base pushes the lone electron pairs to form a pi bond between the two carbons, leading to the E1 product (Figure 2). Figure 2. Basic E1 Reaction Mechanism To create alkene products, organic chemists can use the technique of dehydration of an alcohol. An example is the dehydration of cyclohexanol to form cyclohexene (Figure 3).
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Figure 3. Basic Dehydration Reaction of Cyclohexanol to form Cyclohexene Dehydration reactions yield a water molecule – “dehydrates” the molecule - and an alkene product through an E1 or E2 pathway. Using a strong acid, such as Phosphoric Acid, as the reagent, the acid donates a proton to the oxygen of the hydroxyl group, forming H2O + on the carbon. The dehydration of cyclohexanol continues by an E1 pathway. The H2O
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