organic chemistry

Topics: Chemistry, Hydrochloric acid, Distillation Pages: 17 (2747 words) Published: October 8, 2014
NameNaza Moore
Lab instructorsAlecia Palmer and Donald Burke
Course InstructorMaureen Wilson
Lab TitlePreparation of cyclohexene from cyclohexanol
AimPreparation of an Alkene by dehydration of an alcohol in the presence of a catalyst.
Calculate the percentage recovery of products.
Test for purity and identification of products.

Cyclohexene and cyclohexanol are both colourless aromatic compounds. The major difference between the two is the presence of the hydroxyl group present in cyclohexanol. This hydroxyl group can be removed by a process known as dehydration which results in the production of cyclohexene at the end of the dehydration process in the presence of a catalyst which, in this case, was 85% phosphoric acid. Dehydration can be defined as any chemical reaction which involves the removal of a water molecule from a substance. During the experiment, anhydrous calcium chloride was provided as a drying agent to ensure that pure cyclohexene was obtained, calcium chloride works by binding itself to the water molecule thus removing it in the process. After the dehydrating process was completed, a few tests using Potassium Permanganate solution and Bromine which were purple and Reddish-Orange respectively was conducted. The tests showed a positive result which indicated that the product was indeed pure cyclohexene. As a result of this, it was concluded that the activity was indeed a success as cyclohexene was produces from cyclohexanol.

On a general level, alkenes can be formed from the dehydration of an alcohol in the presence of a strong acid. The acid simply acts as a catalyst and as such increases the reaction time but while doing so does not affect the overall stoichiometry. The usual mole ratio of such reaction is therefore 1:1 which means that the theoretical yield of Alkene in mole is equal to the amount of alcohol used in the reaction. Below are the reaction and the overall mechanism of it.

Essentially what this diagram is saying is that the mechanism of the reaction can be viewed in three different steps or stages. The first step of the mechanism is the protonation of the alcohol group by the acid, this mechanism is slightly exothermic. The second step of the mechanism is where water is lossed to form the carbocation. This step of the mechanism is highly endothermic and the third and final step of the mechanism is where the beta-hydrogen is removed by the base, which in this case is the water to form the Alkene. This step of the mechanism is exothermic. In looking back at the overall reaction it can be observed that these reactions are usually endothermic. These reactions are usually carried out by the use of a fractional distillation apparatus. During this process, as the alcohol and the acid are heated, Alkene and water is produced and are co-distilled into a collection vial. A common error associated with this experiment is the loss of some of the product as a result of hold-up in the apparatus which will result in a reduction in the yield of the product. The necessary precaution required to overcome this problem and to ensure that maximum amount of the product is distilled and yielded, is to add a substance that has a higher boiling point to the distillation flask. These solvent are known as chaser solvents and the purpose of them is to ensure that the temperature rises well above that of the product. Dehydration in chemistry is a chemical reaction which involves the loss of a water molecule from a reacting molecule. In the experiment to prepare cyclohexene from cyclohexanol, the dehydration process is a very important stage of it. In these experiments, anhydrous calcium chloride pellets are used as the drying agent. Calcium chloride pellets are inorganic drying agent that binds strongly with water. As a result of this property, they are able to remove any trace of water from a solution. Cyclohexanol:

Cyclohexanol is a...
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