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ch07 - Chapter 7 Alkenes and Alkynes I Properties and...

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Chapter 7 Alkenes and Alkynes I: Properties and Synthesis Elimination Reactions of Alkyl Halides
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Chapter 7 2 The ( E )-( Z ) System for Designating Alkene Diastereomers The Cahn-Ingold-Prelog convention is used to assign the groups of highest priority on each carbon If the group of highest priority on one carbon is on the same side as the group of highest priority on the other carbon the double bond is Z (zusammen) If the highest priority groups are on opposite sides the alkene is E (entgegen)
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Chapter 7 3 Relative Stabilities of Alkenes Generally cis alkenes are less stable than trans alkenes because of steric hinderance Heat of Hydrogenation The relative stabilities of alkenes can be measured using the exothermic heats of hydrogenation The same alkane product must be obtained to get accurate results
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Chapter 7 4 Heats of hydrogenation of three butene isomers: Overall Relative Stabilities of Alkenes The greater the number of attached alkyl groups ( i.e . the more highly substituted the carbon atoms of the double bond), the greater the alkene’s stability
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Chapter 7 5 Synthesis of Alkenes via Elimination Reactions Dehydrohalogenation Reactions by an E2 mechanism are most useful E1 reactions can be problematic E2 reaction are favored by: Secondary or tertiary alkyl halides Alkoxide bases such as sodium ethoxide or potassium tert -butoxide Bulky bases such as potassium tert -butoxide should be used for E2 reactions of primary alkyl halides
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Chapter 7 6 Zaitsev’s Rule: Formation of the Most Substituted Alkene is Favored with a Small Base Some hydrogen halides can eliminate to give two different alkene products Zaitzev’s Rule: when two different alkene products are possible in an elimination, the most highly substituted (most stable) alkene will be the major product This is true only if a small base such as ethoxide is used
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Chapter 7 7 The transition state in this E2 reaction has double bond character The trisubstituted alkene-like transition state will be most stable and have the lowest G Kinetic control of product formation: When one of two products is formed because its free energy of activation is lower and therefore the rate of its formation is higher This reaction is said to be unde r kinetic control
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Chapter 7 8 Formation of the Least Substituted Alkene Using a Bulky Base Bulky bases such as potassium tert -butoxide have difficulty removing sterically hindered hydrogens and generally only react with more accessible hydrogens (e.g. primary hydrogens)
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