ch07 - mb with parts of Chap 6 2010 fclean al 1pp

ch07 - mb with parts of Chap 6 2010 fclean al 1pp - Chapter...

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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 PRIORITY IS DETERMINED BY ATOMIC NUMBER examples
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Chapter 7 3 Relative Stabilities of Alkenes Generally Z alkenes are less stable than E 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 comparable 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 Elimination Reactions of Alkyl Halides Dehydrohalogenation Used for the synthesis of alkenes Elimination competes with substitution reaction Strong bases such as alkoxides favor elimination NB the double bond formed can be under kinetic or thermodynamic control Heat favours elimination
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Chapter 7 6 The alkoxide bases are made from the corresponding alcohols The solvents here cannot be water. WHY? H OH H H H Me Li H H H H CH 4 H OH H H H NH 2 Li H OLi H H H NH 3 + +
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Chapter 7 7 The E2 Reaction (WE ADDED SN2) E2 reaction involves concerted removal of the proton, formation of the double bond, and departure of the leaving group Both alkyl halide and base concentrations affect rate and therefore the reaction is 2nd order H Br H H H O H O H H H Compare with S N 2
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Chapter 7 8 E2 vs S N 2 Thermodynamically favoured S N 2 – lose and form σ bonds With alkene form C=C ( π bonds) and lose σ bonds E2 is favoured when SN2 is l difFcult z for steric reasons (base/nucleophile can ` t Fnd the C-X E2 favoured by heating and increased concentration Greater increase in entropy in an elimination than in a substitution
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Chapter 7 9 The E1 Reaction The E1 reaction competes with the S N 1 reaction and likewise goes through a carbocation intermediate ONLY FOR 3 o CARBOCATIONS
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Chapter 7 10 Synthesis of Alkenes via Elimination Reactions Dehydrohalogenation Reactions by an E2 mechanism are most useful FOCUS ON THIS
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ch07 - mb with parts of Chap 6 2010 fclean al 1pp - Chapter...

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