Dehydration of 2-methylcyclohexanol

Transcription

1 Reminder: These notes are meant to supplement, not replace, the lab manual. Dehydration of 2-methylcyclohexanol History and Application: Alcohols are readily dehydrated to alkenes by using an acid catalyst i. In Brazil, where ethanol is a readily available, ethylene is made by a dehydration route. The ethylene is then made into the plastic polyethylene. In this way the plastic is made not from petroleum products as is common in the rest of the world, but instead is made from grain. ii Another common alcohol dehydration is the transformation of acetaldol (2- hydroxy butanal) into crotonaldehyde (2-butenal). Crotonaldehyde is used in the manufacture of sorbic acid which is a widely used food preservative. (Read the ingredients on your snack bags). This process is used to make approximately 25,000 tonnes of sorbic acid per year iii. Safety Phosphoric acid is highly corrosive. Wear eye protection the entire time when anyone is working in the lab. Clean up all spills immediately. Never look directly down the barrel of a heating still. All of the organic materials are highly flammable and slightly toxic. Keep away from direct flames. The alkene products have a strong noxious odor. Keep products capped or in the hood to reduce vapor. Hot sand baths look cool. Unplug sand baths when finished heating. A syringe will be used to measure and dispense the phosphoric acid. Be careful when emptying a syringe, the plunger sometimes sticks then releases quickly and the liquid will be expelled rapidly, not unlike when using a squirt gun. This liquid can easily ricochet and splash onto undesired locations such as hands or face. Be very careful not to jam the thermometer to the bottom of the sand bath. The temperature is hotter there and will exceed the maximum allowable temperature of the thermometer. If this is done, the alcohol within the thermometer will boil, expand, and shatter the thermometer. This is a dangerous situation and severe injury or blindness may result. Terminology Dehydration Removing an OH from one carbon and an H from an adjacent carbon and forming a new C=C and a H 2 O molecule. Catalyst- A substance which changes the speed of a reaction, but is present in its original concentration at the end of the reaction iv. It is not stoichiometric.

2 LeChatelier s Principle v. A change imposed on a system at equilibrium will result in a shift in the equilibrium to minimize the impact of that imposed event. Zaitzev s Product. The more substituted alkene product. Hoffmann Product. The less substituted alkene product. Thermodynamic Control. When the most stable product forms in the highest percent. Kinetic Control. When the product with the lowest activation energy, and hence the fastest rate forms in the highest percent. The Experiment 1. An alcohol is dehydrated and an alkene is formed. In today s experiment, 2- methylcyclohexanol is reacted in the presence of phosphoric acid, and three different products can result. The overall reaction is shown below. 2-methylcyclohexanol methylenecyclohexane 3-methylcyclohexene 1-methylcyclohexene B.P. vi = o C 102 o C 104 o C 110 o C (cis 165.6, trans o C) This means that any single molecule of 2-methylcyclohexanol can form any of the following three molecules; 3-methylcyclohexene, 1-methylcyclohexene or methylenecyclohexane. One molecule cannot form 3 molecules. Which product alkene do you predict to be the most stable? (Remember Zaitsev s rule.) One mole of starting material can form a total of one mole of any one or all three products. The sum of the mole fractions of the products cannot exceed the moles of starting material. 2. The phosphoric acid acts as a catalyst. This means that the reaction will precede very slowly or not at all without its presence, but that the amount of product produced is not directly tied to the amount of catalyst present. It is consumed and regenerated along the course of the reaction. It is not stoichiometric. 3. The E1 mechanism describes how all three products are able to form. Phosphoric acid protonates the alcohol group and turns the poor leaving group (-OH) into a very good leaving group(-oh 2 + ). The departure of the leaving group forms a 2 o carbocation. This is a slow step and the rate determining step (RDS). The carbocation can form two of the products with the elimination of a H on an adjacent carbon, 1-methylcyclohexene and 3-methylcyclohexene. Alternatively the 2 o carbocation can undergo a rearrangement forming a more stable 3 o carbocation. This carbocation can form two products methylenecyclohexane and 1-methylcyclohexene. The product distribution can be under either kinetic or thermodynamic control. Typically the conditions in this lab favor thermodynamic control. The full mechanism follows. The dehydration of 2-methylcylcohexanol begins with the reversible protonation of the -0H by the acid, transforming it from a poor leaving group to a good leaving group. This group leaves as a water molecule, forming a 2 o carbocation in the rate determining step.

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4 4. Alkenes with the highest degree of alkyl substitution are the lowest energy and most stable. When a reaction is under thermodynamic control the most energetically favored product is formed preferentially. vii The stability of alkenes is directly tied to the degree of substitution. Knowing this, which of the three products is predicted to be present in the largest amount if this reaction is under thermodynamic control? If this reaction is instead under kinetic control, which product would be predicted to be the major product? (Think about the easiest hydrogen to remove from the initial carbocation.) 5. If a reaction is under thermodynamic control it means the product with the lowest energy (most stable) forms in the largest yield. If a reaction is under kinetic control it means the product with the lowest activation energy (easiest to form) is present in the largest yield. The kinetic and thermodynamic product may be the same or different depending on the exact reaction energetics. Look at the two reaction coordinates below. In the reaction coordinate on the left, the product C is the thermodynamic product because it is lower energy than B. The product B is the kinetic product because it has a lower activation energy and hence needs less energy to form than C. In the reaction coordinate on the right, the kinetic product is E and the thermodynamic product is also E. The dehydration reaction perfomed in this lab is most similar to the reaction coordinate on the left. In the alkene formation from the 2 o carbocation, the 1-methylcyclohexene is the thermodynamic product because it is the most stable. The 3-methylcyclohexene is the kinetic product because it is easier to remove either of the two hydrogens on the methylene carbon (-CH 2 -) than the hydrogen on the tertiary carbon (C-H). Procedure The reaction will be carried out in a Hickman still. These are custom manufactured and relatively expensive pieces of glassware. The Drierite, phosphoric acid, and 2-methylcyclohexanol are contained in the bottom portion of the still. viii

5 Only the lower 1/3 of the still bottom should be buried in the sand. Some Drierite should be above the level of the sand. The Drierite will absorb the water from the 85% phosphoric acid and absorb the water that is liberated during the dehydration. Remember what you learned in general chemistry about LeChatlier s principle. In this reaction 2-methylcyclohexanol is undergoing a reaction to form an alkene and a molecule of water. What is expected to the reaction equilibrium if the water is removed from the reaction by absorption into the Drierite? The Drierite also acts as a boiling stone and a fractionation column. The temperature of the sand should not be allowed to rise above the boiling point of the 2-methylcyclohexanol. As the products form, they will vaporize (boiling points o C), and rise in the still. They will condense on the glass in the upper portion of the still and run down the glass, collecting in the ring. Why doesn t the starting alcohol also vaporize, condense and collect in the ring? As the reaction is proceeding, the products are being removed by distillation. How is this expected to impact the equilibrium? (Again think of LeChatlier s principle.) Analysis The starting alcohol and product will be analyzed using IR. The IR of 2- methylcyclohexanol will have a distinct OH stretching absorbance near 3400cm -1. This peak is absent from the IR spectrum of pure alkene products. Water present in the product will show a typical absorbance in the 3400cm -1 region and may be difficult to distinguish from the alcohol stretch. The IR spectrum of the desired products contains a sp 2 =C-H stretch around 3050cm -1. This peak appears as a shoulder on the normal sp 3 C-H absorbance. It is not always clearly discernible. A C=C stretch may also be observed around 1620cm -1. An IR of pure 2- methylcyclohexanol is on the left, and that for a methylcyclohexene is on the right. ix

6 Revised: September 7, 2016, S.L. Weaver References i Weissermen, K, Arpe, H.-J., Industrial Organic Chemistry, 3 rd Completely Revised Edition, VCH, New York, 1997 p185 Heaton, A., An Introduction to Industrial Chemistry, 3 rd, Blackie Academic and Professional, London, 1996 Chenier, P.J., Survey of Industrial Chemistry, 3 rd, Kluwer Academic, New York, 2002 ii (January 28, 2010) iii September 10, September 17, 2009 iv Hackh s Chemical Dictionary, McGraw Hill, 1969, p138 v Hill, Petrucci, McCreary, Perry, General Chemistry, 4 th Edition, Pearson-Prentice Hall, New Jersey 1996 p589 vi vi CRC Handbook of Chemistry and Physics, 65 th ed, CRC Press, Bocca Raton,1985 pp C-249, C-250, C-253 vii D. Klein, Organic Chemistry, Wiley, New York, 2012,p344 viii Ibid, p 250 ix SDBSWeb : (National Institute of Advanced Industrial Science and Technology, September28,2013