Section_5_Sub-Elim_The_rest

Section_5_Sub-Elim_The_rest - bulky strong base okay...

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1 Examples C CH 3 H 3 C CH 3 O + CH 3 -Br δ - δ + Br C H H H (CH 3 ) 3 COH δ - δ + bulky strong base okay nucleophile C CH 3 H 3 C CH 3 OC H 3 + Br Na + Br C H H + (CH 3 ) 3 COH carbanion much stronger base CH 3 O Na methyl + C H H H C H H C H H Br + CH 3 OH α β + small unhindered strong base great nucleophile δ - δ + electrophile (acid) + Br CH 3 CH 2 CH 2 OCH 3 CH 3 CH=CH 2 a b c a S N 2 C H H C H H C H H Br For an E2 reaction you must have a good leaving group C H H C H C H H Br + H stronger base even stronger base b E2 major minor c S N 2 E2? γ A) S N 2 B) S N 1/E1 C) E2 Not possible Not possible Section 5: Substitution and Elimination Text-sub elim-36-37
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2 CH 3 CH 2 O Na + C H H H C H Br C H H H + CH 3 CH 2 OH α β C H 3 C H C H H H 3 CC H OCH 2 CH 3 CH 3 + small unhindered strong base great nucleophile δ - δ + electrophile (acid) C CH 3 H 3 C CH 3 O Na + (CH 3 ) 3 COH C H H H C H H C H H Br + α β bulky strong base δ - electrophile (acid) a b CH 3 CH 2 CH 2 OC(CH 3 ) 3 CH 3 CH=CH 2 a S N 2 b E2 major minor + + Br major minor δ + Examples a b a S N 2 b E2 poor nucleophile + Br β A) S N 2 B) S N 1/E1 C) E2 Section 5: Substitution and Elimination Text-sub elim-38-39
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3 Na + C H H H C H Br C H H H + CH 3 CO 2 H α β C H 3 C H C H H H 3 CC H O CH 3 C O CH 3 + weak base fair nucleophile δ - electrophile (acid) major minor δ + Examples a b a S N 2 b E2 C H 3 C O O + Br C H H H C H Br C H H H CH 3 OH + weak base poor nucleophile 80° C C H H H C H C H H H δ - δ + + + Br CH 3 OH α β β β C H 3 C H C H H minor b E1 H 3 H OCH 3 CH 3 major a S N 1 a b + H + A) S N 2 B) S N 1/E1 C) E2 Section 5: Substitution and Elimination Text-sub elim-40-41
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Section 5: Substitution and Elimination 4 3 o alkyl halides form carbcations and undergo a unimolecular reaction (S N 1/E1), however, in the presence of a strong base deprotonation occurs quickly and an E2 reaction occurs, WHY? Sterics are not an issue in Bronsted acid-base reactions and the transition state energy is much lower for the E2 reaction than the formation of a carbocation. a b C H H H C C Br C H H H H H H CH 3 CH 2 OH weak base poor nucleophile C H H H C C C H H H H H H + C H 3 C H 3 C C H H H 3 CC CH 3 CH 3 OCH 2 CH 3 major a S N 1 + + Br minor b E1 A) S N 2 B) S N 1/E1 C) E2 Examples Text-sub elim-42
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5 CH 3 CH 2 O Na + C H H H C C Br C H H H H H H + CH 3 CH 2 OH α β C H 3 C H 3 C C H H H 3 CC OCH 2 CH 3 CH 3 CH 3 + small unhindered strong base great nucleophile δ - electrophile (acid) major minor δ + S N 1 E2 + Br β β 25° C No S N 2 At higher temperatures or by increasing the concentration of ethoxide (CH 3 CH 2 ONa), no S N 1 products would form because it is a slow endergonic process and the fast exergonic E2 process would dominate.
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This note was uploaded on 09/13/2011 for the course CHEM 228 taught by Professor Jacobfischer during the Spring '09 term at Waterloo.

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Section_5_Sub-Elim_The_rest - bulky strong base okay...

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