E1 (Elimination unimolecular) Beta-H s. Objectives. E1 energy review. E1 mechanism. Last lecture

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1 1% 92% 1% eta- s ow many different eta- s does 2-methylpentan-3-ol possess? a % 1% Last lecture This lecture jectives Alkene structure & onding Alkene physical properties Alkene preparations ehydration (E1 mechanism) ehydration of alcohol with nonequivalent - s Another synthetic route to alkenes ehydrohalogenation 2 r. Kay Sanderg RS (rate determining step) Slow step is step 2 E1 mechanism dehydration of an alcohol E1 (Elimination unimolecular) E1 energy review 3 o carocations form faster alcohol protonation dissociation -deprotonation E1 (Elimination unimolecular) r. Kay Sanderg Same for S N 1 R->R alcohol protonation 3 3 acid catalyzed 3 Where S N 1 differs from E1 2 3 dissociation -deprotonation 4 Nucleophilic Sustitution (NS) vs Elimination (E) ompeting reactions NS vs E Y acts as nucleophile d NS Y Y E Y - Y acts as a onsted ase 5 ehydration of alcohols - regioselectivity ehydration regioselectivity 2-methylutan-2-ol 2 S 4 2 S 4 2-methylut-1-ene Which - was eliminated? 2-methylut-2-ene 6 1

2 1% 4% 93% 2% ehydration of alcohols - regioselectivity ehydration of alcohols - regioselectivity Which 2 S eliminated? 4 2-methylutan-2-ol ut-1-ene 2 S 4 ut-2-ene 1. methine methyl 2. methine methylene 3. methyl methylene 4. methyl methine 5. methylene methyl 1 o - E 2-methylut-1-ene Which - was eliminated? methylene methine 2 o - E 2-methylut-2-ene 7 ehydration of alcohols - regioselectivity ehydration regioselectivity 2-methylutan-2-ol 2 S 4 2 S 4 1 o - E 2-methylut-1-ene 2-methylut-2-ene Which - was eliminated? (1) (9) 2 o - E 8 ehydration of alcohols - regioselectivity ehydration regioselectivity 2-methylutan-2-ol 2 S 4 1 o - E 2 o - E 2-methylut-1-ene 2-methylut-2-ene Which - was eliminated? (1) (9) ehydration of alcohols - regioselectivity ehydration regioselectivity 1 o - E 2 o - E 2-methylut-1-ene 2-methylut-2-ene Which - was eliminated? (1) (9) 9 10 ehydration of alcohols - regioselectivity ehydration regioselectivity 1 o - E 2-methylut-1-ene 2-methylut-2-ene Which - was eliminated? (1) (9) 2 o - E Zaitsev s rule: elimination rxns of alcohols yield the most highly sustituted Zaitsev s alkene as rule the major product. disu. trisu 3 MAJR

3 Zaitsev s rule MAJR lower E arrier, faster rate LQ #1) Given Zaitsev s rule draw the MP (major organic product) formed when 2-methylcyclopentanol is treated with conc. sulfuric acid under ed conditions. disu less stale trisu more stale 13 If only - s are eliminated, then the a- must ecome a - & a g- must ecome a -, 3 ut W???? g 3 Rearrangements in dehydration g 3 a 3 g 3 P 4 3 only - s present Section % % % 3 14 Rearrangements in alcohol dehydration Methide shifts methide now a (was ) shift (was g) now 3 now (was a) % 3 o % Section 5.13 Rearrangements in alcohol dehydration - s ond Section o Methide d shift d TS 3 3 empty p orital d d 3 3 o methyl group takes the 2 electrons with it & shifts over. (methide ion for ppp) 3 2 o Methide shift 3 TS 3 3 d d ppp product prediction purposes 3 o Reactions: Preparation of alkenes Reactions LQ #2A) raw skeletal structure of the carocation intermediate that leads to MP LQ #2) raw MP 2,3-dimethylcyclohexanol 18 3

4 Reactions: Preparation of alkenes Step 1: PT Step 2: pop off water Reactions 32 o 32 o Rearrangements in alcohol dehydration S 4 Regio & stereoselectivites o % regioselective 32% 3 3 stereoselective 56% 3 20 Section Section 5.13 product ydride shifts ppp prediction purposes ydride shift % 56% trans, 32% cis stereoselectivity Rearrangements in alcohol dehydration ydride shift o regioselectivity % 21 Stailization requires a stereospecific arrangement of oritals. 3 3 Stereoselectivity The - - ond needs to e parallel with empty p-orital to e ale to smear out the misery d p- ond like 3 3 d 22 Stailization requires a stereospecific arrangement of oritals. 3 Stereoselectivity The - - ond needs to e parallel with empty p-orital to e ale to smear out the misery = 3 3 d p- ond like 3 3 d 23 Notice the spatial relationship of methyl groups Nearly anti etween Stereoselectivity gauche & eclipsed 3 d 3 3 More stale, thus there are more in this which leads to the trans configuration 3 d 3 3 d 3 3 d 24 4

5 Stereoselectivity R cis- trans- Summary: Transformations involving alcohols Alkene formation: alcohol dehydration E1 a Alcohol reactions 3 3 reaction progression 3 LQ #3) A) most, ) middle, ) least product when pentan-2-ol is dehydrated Alkyl halide formation R R In oth (for 2 o & 3 o R), RS is dissociation of alkyloxonium ion (unimolecular) forming 2 and. New way to make alkenes SN1 26 ehydrohalogenation of alkyl halides ehydrohalogenation a ase alkyl halide a ppp product prediction purposes Na ase promoted (not ase catalyzed) Na Na cyclohexyl chloride ehydrohalogenation of alkyl halides Reagent systems for ase a typical reagent system sodium ethoxide l Na , 55 o ethanol (conjugate acid of salt anion) ehydrohalogenation S d slam d a gra d- oot Mechanism for ase S LQ #4) raw MP formed when 4-romo-4-propylheptane is treated with sodium isopropoxide in isopropyl alcohol and. d S d a TS for ase d S S - ond makeage - ond reakage p ond makeage d - ond reakage

6 d S d a Energy hill for ase : elimination imolecular Transition State d S d S R & S - TS Go Rxn progression ehydrohalogenation d S d a Rate law expression ase : elimination imolecular 1) Rate is second order: 1st order in R & 1st order in - R = k[r][ - ] 2 nd order kinetics S ehydrohalogenation d S d a R rate ase : elimination imolecular 2) Rate depends on halogen of alkyl halide The weaker the - ond S the easier to reak the faster the rxn RF << Rl < R < RI Iodide is the est leaving group 33 - s ond 3 ase LP d rital description s ond 3 d 3 - p ond - s ond 3 LP 34 3 Stereoelectronic aspect 3 and are anti d 3 3 d and are anti Anti coplanar The formation of the p ond requires that the oritals forming p ond e parallel (or nearly so) Stereoelectronic aspect 3 3 Section and are eclipsed Syn coplanar 36 6

7 Anti coplanar More stale: staggered onds Stereoelectronic effect 3 3 Stereoelectronic aspect Syn coplanar Less stale: eclipsing onds 3 Stereoelectronic aspect Anti coplanar Section Syn coplanar Section Section 5.16 Which isomer has the faster alkene formation rate: 1) cis or Which 2) trans? is to react? 1. cis 2. trans - :ase - :ase 41 % 59 % Section 5.16 Which isomer has the faster alkene formation rate: cis Stereoelectronic or trans? aspect & are gauche & are anti (anti coplanar ) Reacts 500 faster K( 3 ) 3 ( 3 ) 3 39 Experimental evidence for this stereoelectronic effect? Stereoelectronic effect 40 ehydrohalogenation slam S d d a gra d- oot Mechanism for ase S o you see anything good that results? good corresponds to lower energy o you see anything ad that results? ad corresponds to higher energy 41 d S d a S TS for ase Good? Look at the species with the negative charge. ad? s-electrons are converted to p-electrons. er ase higher E () s electrons: lower E () G = - TS p electrons: higher E () Weaker ase lower E () Gi s Free energy = enthalpy - Tentropy 3 products 42 7

8 Reactions: Preparation of alkenes : elimination imolecular S d ases do not handle negative charge well (relatively speaking) i E d a d- E2 summary d S ase S d Anti Great charge trade!! Increased S Weak ases (l -, -, I - ) do handle negative charge well (relatively speaking) Lo E 43 ehydrohalogenation of alkyl halides Practical route to terminal alkene 3 ( 2 ) l 1-chlorooctadecane K( 3 ) 3 3 ( 2 ) 15 = 2 or octadec-1-ene (86%) ( 3 ) 3 MS yield of dehydration of 1 o alcohol (R) 12% vs yield of of 1 o alkyl halide (R) 86% S 44 Regioselectivity of ehydrohalogenation of alkyl halides ehydrohalogenation of alkyl halides MP? K , 70 o MP? 2-methylut-1-ene 2-methylut-2-ene regioisomers utene 2. 2-utene K , 70 o 29% 71% 2-methyl-1-utene MP? 2-methyl-2-utene 46 Regioselectivity of ehydrohalogenation of R follows Zaitev s Rule Regioselective K , 70 o 29% 71% 2-methylut-1-ene 2-methylut-2-ene regioisomers 47 Stereoselectivity of ehydrohalogenation of alkyl halides K non-4-ene Stereoisomers cis-non-4-ene trans-non-4-ene Stereoselective 77% 23% 48 8

9 cis 71 % 29 % trans ehydrohalogenation of alkyl halides MP? Stereoselective romocyclodecane Stereoselectivity of Stereoselective ehydrohalogenation of alkyl halides romocyclodecane K cyclodecene 1. cis 2. trans K cyclodecene cis-cyclodecene trans-cyclodecene 49 cis-cyclodecene trans-cyclodecene 85% 15% 50 Alkene Preparations Section 5.12 NaR conc. Alkene preparation flash 2 S 4 cards R, E2 E1 Na Reactions regioselective? Not for this particular one. LQ #5A no carocation no rearrangements!!! ehydration of alcohols R R acid catalyzed R ehydrohalogenation of alkyl halides a ase R 2 onjugate acid of ase halide ion 51 r. Kay Sanderg 2 S 4 LQ #5 MP regioselective LQ #5 mop carocation watch out for rearrangements!!! 52 9