10-DehydrationMethycyclohexanolF08 - Dehydration of...

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Dehydration of 2-methylcyclohexanol Topics and techniques: Alcohol dehydration, Zaitsev rule, Alkene stability, Gas chromatography, Mass Spectrometry Reading assignment: a) Fessenden, R.J., Fessenden, J.S., Feist, P. Organic Laboratory Techniques , 3 rd ed.; Brooks/Cole: Pacific Grove, 2001, pp 93-96, 141-151. b) Loudon, G.M. Organic Chemistry, 4 th ed.; Oxford University Press: New York, 2002, 376, 408-411. Introduction In chapter 9 we learned that elimination of alkyl halides is a good way to synthesize alkenes (figure 1). The elimination may go through a unimolecular (E1) or bimolecular (E2) transition state. H 2 C C CH 3 Br CH 3 H E1 C H 2 C CH 3 CH 3 H H 2 C C CH 3 CH 3 Br H 3 O + H 2 O O H H ! H 2 C C CH 3 Br CH 3 H E2 Br t- BuO CH 3 C CH 3 H 3 C O H 2 C C CH 3 CH 3 ! CH 3 C CH 3 H 3 C O H Figure 1. E1 and E2 elimination. In lab today you will perform an acid-catalyzed elimination of an alcohol (figure 2). This reaction is referred to as a dehydration because a molecule of water is eliminated. H 2 C C CH 3 OH CH 3 H H 2 SO 4 (aq) ! H 2 C C CH 3 CH 3 H 2 O Figure 2. General example of an acid-catalyzed elimination of an alcohol. The dehydration reaction parallels what we have learned for elimination of alkyl halides, except that an alkyl halide has a great leaving group built in (X , a weak base) while the hydroxyl group of an alcohol is a poor leaving group because it leaves as a strong base ( OH). Before an alcohol can participate in
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an elimination (or substitution) reaction, the hydroxyl group must be converted into a better leaving group. A strong acid catalyst, such as phosphoric or sulfuric acid, is commonly used for this purpose. The acid protonates the Lewis basic oxygen to give a positively charged oxonium ion intermediate. In the process the hydroxyl group has been converted into a great leaving group, water. H 2 C C CH 3 O CH 3 H H H 2 C C CH 3 CH 3 H 2 O OSO 3 H H H 2 C C CH 3 O CH 3 H H H OSO 3 H E1 or E2 Figure 3. Conversion of the hydroxyl group to a better leaving group. In today’s lab you will investigate the dehydration of 2-methylcyclohexanol (figure 4, 1). The reaction produces four isomeric alkenes: 1-methylcyclohexene (2), 3-methylcyclohexene (3), methylenecyclohexane (4), ethylidenecyclopentane (5) and water. CH 3 OH CH 3 CH 3 CH 2 H 3 PO 4 H 2 O 1 2 3 4 5 ! CH
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This note was uploaded on 05/13/2010 for the course CHEM 151L taught by Professor Roy,christopher during the Summer '08 term at Duke.

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10-DehydrationMethycyclohexanolF08 - Dehydration of...

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