Lab Manual- Experiment 9

Lab Manual- Experiment 9 - 8/26/2010 Experiment 9:...

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8/26/2010 E x p er i m e n t 9: S YN T H E S I S AN D R E AC TI O N S O F A L K E N E S : 4 - Me t h y l c y c l o h e x e n e f r o m 4 - Me t h y l c y c l o h e xa n o l R e f e r e n ce s - Pavia: 179-183; 464-467; 8 7 1 - 8 7 2 ; Technique 25, Pavia. Solomons & Fryhle, chapters 2, 7 & 8; Wade pages 306-309, 342-345, 358-359, 480-484, 508-537. Alkenes can be prepared by a variety of methods, of which the dehydrohalogenation of alkyl halides and the dehydration of alcohols are perhaps most common. These general reaction types are illustrated below. Both of these equations represent ELIMINATION reactions, in which small molecules such as HX and H 2 O are eliminated from the larger starting material. However, the actual mechanism by which this occurs can vary depending on the structure of the starting material, the conditions under which the reaction is carried out and the nature of the other reagents used. The two major classes of Elimination reactions involve E1 and E2 mechanisms, whose rates follow unimolecular and bimolecular kinetics respectively. E1 reactions occur in two steps with a positive carbocation intermediate being formed in the rate- determining step after the departure of a suitable “leaving group”. This is followed by the removal of a proton from the β-carbon adjacent to the positive charge and the formation of a double bond using the pair of electrons that it leaves behind. Since only the starting material is involved in the slow, rate-determining step, the overall rate of the reaction is only dependent on this species, hence the name E1 for Unimolecular. Because a carbocation intermediate is formed, species undergoing E1 reactions are also prone to rearrangement if this will result in the formation of a more stable carbocation. An example of an E1 mechanism: In contrast, E2 reactions occur in a single, concerted step when there is a strong enough base present to remove a β–hydrogen, forcing the leaving group to depart, before the relatively slow 1
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8/26/2010 carbocation formation has the chance to occur. The reaction is, therefore, Bimolecular since the rate is dependent on both the starting material and the strong base involved. An example of an E2 mechanism: Further details about the mechanisms of elimination reactions and the factors that affect them are beyond the scope of this manual. However, they are covered in detail in the Organic Chemistry Lecture, as well as in the references listed on the previous page. You will also need to be familiar with the type of arrow- pushing mechanistic drawing used in these and other examples. GENERAL GUIDELINES FOR THE EXPERIMENT This experiment follows the procedure outlined in E xp e r im e nt 22 of your Pavia text, on pages 179 to 183. 4-Methylcyclohexene will be made by dehydration of 4-methylcyclohexanol in the presence of an acid catalyst. The mechanism involves the initial protonation of the alcohol by a strong
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Lab Manual- Experiment 9 - 8/26/2010 Experiment 9:...

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