L12a - Section 5.3 1 E 2 Z EBror Z H Lo H E 43 57 Last lecture Alkene structure bonding Lo H C C,H,H C C,C,H Hi 1 Z This lecture Alkene

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1 1 E or Z? E Z 57 % 43 % 1. E 2. Z Section 5.3 Br © Dr. Kay Sandberg ( ? )-3-(2-bromoethyl)penta-1,3-diene H Hi Lo C C : C,H,H : C,C,H σπ H H H Hi Lo 2 ©Dr. Kay Sandberg Last lecture This lecture Alkene structure & bonding Alkene preparations 3 ©Dr. Kay Sandberg Alkene representation convention sp 2 hybridization makes for planar bonds Keep the bonds to the double bond in the plane of the paper/screen. trans -2-butene NO! NO! NO! At this point the only time you should use wedge/dashed bonds is to show configurations of rings. 4 ©Dr. Kay Sandberg Ring configuration representation convention -1-isopropyl-3-propylcyclohexane NO! NO! NO! Only the 2 bonds to the ring are wedge/dashed 5 FC? +1 +1 -1 -1 1% 1% 1% 98% 1. +1 +1 2. +1 -1 3. -1 +1 4. -1 -1 Leaving group takes the bonding electrons and leaves C C H LG C C H LG + C LG Select the resulting formal charges LG = leaving group Please send login again “Menu” “Login” check ID “2” 6 Leaving group takes the bonding electrons and leaves C C H LG C C H LG + Leaves behind a “screaming” carbocation There are 2 ways to “shut up” a “screaming” carbocation ©Dr. Kay Sandberg “screaming” = very electrophilic = high E LG = leaving group “sexteted” carbon (give it access to an octet of valence electrons & thereby lower its energy.)
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2 7 H C H C H H H Nucleophilic Substitution (NS) vs Elimination (E) C C H CC + H Y + X - NS E Competing reactions Y + Y acts as nucleophile (Lewis base) Y acts as a Bronsted base δ - δ+ Hyperconjugation: β -H δ+ δ+ H Y ©Dr. Kay Sandberg 8 Reactions: Preparation of alkenes Section 5.8 X Y α β + X Y Elimination Reactions H H H H H H H H H H + HH H H C H H H H H CH 3 H + H H H 750 o 750 o Dehydrogenation ©Dr. Kay Sandberg 9 Reactions: Preparation of alkenes Section 5.8 H OH + H OH acid - Elimination Reactions: Dehydration of alcohols De hydro halogenation of alkyl halides H X + H X base Remove elements of water ©Dr. Kay Sandberg 10 - Elimination Reactions: Dehydration of alcohols Section 5.9 H OH + H OH H + H 3 C C H 2 OH H 2 SO 4 heat H 2 H 2 + H OH ©Dr. Kay Sandberg π –e lectrons: higher E σ lower E Δ G = Δ H - T Δ S Δ Gibb’s Free energy = Δ enthalpy - T Δ entropy 2 products 11 - Elimination Reactions: Dehydration of alcohols Section 5.9 H OH + H OH H + H 2 SO 4 heat OH + H OH Key point: a β -H is removed!
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This note was uploaded on 03/29/2009 for the course CH 221 taught by Professor Melander during the Spring '07 term at N.C. State.

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L12a - Section 5.3 1 E 2 Z EBror Z H Lo H E 43 57 Last lecture Alkene structure bonding Lo H C C,H,H C C,C,H Hi 1 Z This lecture Alkene

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