My unknown was cyclohexanol and the first boiling range was 96-99 C and the seco
ID: 104688 • Letter: M
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My unknown was cyclohexanol and the first boiling range was 96-99 C and the second distillation boiling range was 83- 87 C. The post lab Q2 is to explain this difference? Experiment 13: Dehydration of Alcohols adaptod froe Gircen Organic Chemistry, by Dossee ond Hatchinsn Question to be answered in this experiment: How can an alkene be efficiently synthesized from an alcohol? BACKGROUND INFORMATION Alkenes are frequently synthesized by inducing the elimination of water (dehydration) from alcohols. This reaction is typically i catalyst: Ht + H20 R=alkyl or H In this experiment you will be assigned one of three alcohols-cyclchexanol 3-methyl-3- pen tanol, or 2-octanol - to use to synthesize the corresponding alkene. This is an example of an El reaction. In lecture, you have learned that El and Swl products would likely be formed, but in this case, the Swl product is the starting alcohol, as the only nucleophile present is the water formed by the reaction. This is a reversible reaction, so the reaction is driven to the right by distilling the product (and depending on the boiling point of the product, some water) from the reaction flask. Strong acids such as sulfuric acid are usually used to catalyze the dehydration of alcohols, but two limitations make this catalyst less useful. Concentrated sulfuric acid is extremely corrosive and it can also lead to extensive charring of organic compounds. This experiment will use a milder acid 85% phosphoric acid, as the catalyst. This greener reagent is less corrosive and therefore safer and much less likely to destroy your starting material. Another way this experiment is demonstrating green principles is that there is no organic solvent used, as the liquid reagents mix well together. Required Reading: Review Mohrig, Chapter 12, Sections 3 and 4; IR and NMR Spectroscopy Before you come to lab: Pepare a pre-lab with introduction, safety data for the chemicals (including your product) used in this experiment, a balanced equation for the reaction of your assigned alcohol, a table of reactants and products (See Appendix I to review this) and a detailed procedure, including the calculated values for the amount of alcohol you will use. Be sure to look up the boiling points of your alcohol and your product. The phosphoric acid is 85% by weight, density 1.70 g/mL PROCEDURE 1. Add 0.075 moles ofyour assigned alcohol, 1.75 mL of 85% phosphoric acid and a boiling chip to a 50-mL round-bottom flask. If your drawer has a 25-mL flask, get a 50-mL flask from the box on the front counter. Also obtain a mercury thermometer, a Vigreaux column and a vacuum take-off adapter 2 Clamp your flask and set up for fractional distillation (Figure 12.17, page 191), using a 25- ml round-bottom flask as your receiving vessel. If you do not have a 25-mL flask in your drawer, obtain one from the front counter. Use a second Lab-Jack to support your receiving vessel. 3. Turn on your Variac to about 60 or 70 until the mixture begins to boil gently. About 5 minutes after boiling begins, turn the Variac a little higher in order to distill the mixture Experiment 13-Dehydration of Alcohols 91Explanation / Answer
This is because when you are making the first distillation you have ciclohexanol that is transforming into ciclohexene, water and some impurities (crude product), in the first distillation the products will be changed into the gaseous form according to the boiling points, the 96 - 99 C is the boiling point of the water that remains in the distillation flask (also some unreacted alcohol and side products) this water is also pushing the products of the reaction into the receiving flask
In the second distillation you have removed the water (when you removed the aqueous layer) and you mainly have cyclohexene wich has a boiling point of 83 C (the boiling point of your second distillation)
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