11a Alkenes by Elimination

11a Alkenes by Elimination - 11a Alkenes by Elimination...

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11a Alkenes by Elimination Author: Holly Polk Instructor: Oleksandr Zhurakovskyi Organic Chemistry Lab 243A, Section 010 Date Work Performed: April 16, 2009 Date Submitted: April 23, 2009 Abstract In this lab, both the dehydration of 2-butanol and the dehydrohalogenation of 2- bromobutane were performed. For dehydration, the area of the peaks yielded the following: 1-butene 5%, trans-2-butene 60%, and cis-2-butene 35%. For dehydrohalogenation, the area of the peaks yielded the following: 1-butene 21%, trans-2- butene 57%, and cis-2-butene 22%. The amount of products differed between the acidic dehydration conditions and alkaline dehydrohalogenation conditions.
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Introduction In this lab, alkenes are prepared by elimination, with one student running the base-catalyzed dehydrohalogenation of 2-bromobutane, and the other student running the acid-catalyzed dehydration of 2-butanol (Padias, 85-86). There are four key components; pronation, formation of a carbocation, formation of a “good” leaving group, and depronation. This is a classic example of an electrophile and a nucleophile, with 2- butanol acting as the nucleophile. This is considered a Bronsted-Lowry interaction in which substances are readily accepting and donating protons. In addition, the excess acid within this reaction created an environment rich in hydronium ions. This allowed 2- butanol to be pronated by the addition of H+ creating a good leaving group (H 2 O) from OH-, a poor leaving group. In doing so, a carbocation was formed leaving three possible products: 1-butene, trans -2-butene, and cis -2-butene. The product that forms is based on Zaitsev’s rule, that the most highly substituted alkene will be formed (Padias, 85). Knowing this, one can assume that trans configuration will be the most stable, followed by cis formation within both the dehydration of 2-butanol and the dehydrohalogenation of 2-bromobutane — under thermodynamic conditions bimolecular reactions are favored.
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Physical Constants Physical Constants Name Formula (structure) m.w., g/mol Density, g/ml m.p., °C b.p. , °C Quantity Mass (volume ), g (ml) ν, mm ol Equi v. Phosphoric
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11a Alkenes by Elimination - 11a Alkenes by Elimination...

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