The Synthesis of Alkenes: Dehydration of Cyclohexanol
Organic Chemistry Lab 1
Tuesday 8:00 11/08/11
The goal of this experiment was to form cyclohexene from cyclohexanol through a dehydration reaction. Cyclohexene was successfully synthesized according to the bromine test performed and the IR spectra. There was a percent yield of cyclohexene of 76.1%. Introduction:
Alkenes, hydrocarbons containing at least one carbon-carbon double bond, are important functional groups in natural and synthetic compounds. One method that they can be synthesized is through elimination reactions, which form alkenes with the net loss of a leaving group and an H+ on an alpha carbon. Atoms must be attached to adjacent carbons in order to be eliminated. Elimination reactions occur most commonly through E1 and E2 mechanisms. The specific mechanism that occurs depends on various conditions. The various conditions include leaving groups, substrate structure, the nature of the base, and the solvent. Both mechanisms will occur in the presence of a good leaving group. Examples of good leaving groups include I- and Br-, where a poor leaving group includes F- and OH-.
An E2 reaction works best with a secondary or tertiary carbon because primary carbons have more competition for SN2 reactions. Additionally, the E2 pathway occurs in one step and is referred to as a bimolecular concerted reaction. The concentration of substrate and the concentration of the base are important in determining the rate of an E2 reaction. An E1 reaction works best with a primary or secondary carbon. E1 rates are determined by the concentration of substrate and are not affected by the concentration of the base. The rate determining step is the intermediate step in which a carbocation is formed. The leaving group substrate bond is broken during the rate determining step.
If the leaving group involved in an elimination reaction is a water molecule, the reaction will be a dehydration reaction. Since –OH is a poor leaving group; it is protonated in the first dehydration step in order to convert it into a better leaving group, -OH2+. After this, the water molecule can depart and an alkene can be formed. For example, cyclohexene can be synthesized from cyclohexanol in the presence of a strong acid by the following dehydration reaction:
The formed alkenes can be removed from the solution by distillation. Since cyclohexene has a lower boiling point than the other components of the solution, it will distill out of the solution before the other components and it can be collected. To check that the alkene synthesis was successful, a bromine test can be performed on the sample. Bromine adds across a double bond, allowing the solution to turn colorless. Therefore, if the solution turns colorless, the alkene synthesis was successful. The product of the reaction can be characterized by infrared spectroscopy, or IR. IR is useful for detecting the presence of functional groups in an organic material. The wavelength and corresponding frequency absorbed by a molecule can be related to a specific functional group and bonding patterns in order to characterize the product. A molecule will absorb IR when the vibration frequency of one of its bonds matches that of the radiation. The IR not absorbed by the molecule passes to the detector of an IR machine, which measures the percent of IR reaching the detector. The detector records the information on a chart as percent transmittance versus wave number. The chart can be compared to a standard in order to characterize the product.
| Molecular Formula
| Molar Mass (g/mol)
| Desity (g/mol)
| Melting Point (°C)
| Boiling Point (°C)
| Amount to be used
| Amount used
| 10 mL
| 10.1 mL
| Phosporic acid
| 12 mL
| 12 mL
| Sodium sulfate (anhydrous)
References: Organic Lab Lecture
Ericka Huston, Chong Liu. CHEM 0330 Organic Chemistry I Laboratory Manual
Padias, Anne B. Making the Connections, A How-To Guide for Organic Chemistry Lab Techniques. Second ed.
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