Chem lab 5 - Lab 5 Preparation of Cyclohexene From...

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Lab # 5 Preparation of Cyclohexene From Cyclohexanol
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Introduction Organic synthesis is very important in that it allows the experimenter to make new compounds from compounds that might be more readily available. Sometimes a synthesis reaction is easy to do and other times great effort and care must be given. This experiment will require good technique, as the cyclohexene product is volatile and can be easily lost by evaporation thus lowering your yield. The reaction that was done was an acid-catalyzed dehydration of cyclohexanol to give cyclohexene, which is an example of an E1 reaction. Cyclohexanol is a secondary alcohol, which can be protonated by a strong acid. The protonated alcohol then loses a water molecule to form a secondary carbocation. Loss of a hydrogen atom on an adjacent carbon atom results in the formation of cyclohexene. It is possible that a substitution reaction could compete in this synthesis. To minimize this, a strong acid is used so that the accompanying anion is a weak nucleophile. The acids of choice for this dehydration reaction are concentrated sulfuric acid and concentrated phosphoric acid. We used concentrated phosphoric acid in this experiment. Recall that this reaction is a reversible one in that water can be added to an alkene in the presence of an acid to give an alcohol. Hence, we should be mindful of LeChatelier’s principle in order to move the equilibrium in the direction we want. If you look at the balanced equation it becomes obvious that water must be avoided to minimize the undesired reverse reaction. This is why concentrated acids are used. Concentrated phosphoric acid is 85% pure whereas concentrated sulfuric acid can be obtained in 100% purity. So, phosphoric acid may be slightly less advantageous than sulfuric acid. Since this reaction is not immediate, the reagents must be heated for a period of time to allow significant amounts of product to form. Reaction times are shortened as the temperature is increased. So the higher the temperature, the shorter the reaction time. The limit we are able to achieve is the boiling point of the solution. In order to allow the reaction to proceed at its boiling point, we must condense and collect the distillate back into the reaction vessel. This is called reflux. We will do a modified reflux by using a fractional distillation setup. At the beginning of the heating process, the distillate will condense and flow back into the reaction vessel. Over time, the hot vapors will reach the thermometer and side arm of the distilling head and begin to distill over. The success of this method lies in the fact that cyclohexene has a low boiling point (83°C) compared to cyclohexanol (bp of 160-161°C) and will distill over first. If the mixture is not overheated, no cyclohexanol will distill. Method:
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This note was uploaded on 09/24/2011 for the course CHEM 201 taught by Professor John during the Spring '10 term at University of Guelph.

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Chem lab 5 - Lab 5 Preparation of Cyclohexene From...

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