5.2_substitution_unimolecular_psot

5.2_substitution_unimolecular_psot - Vollhardt: Chapt. 7...

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5.2 - SN1 & Eliminations 1 Unimolecular S N 1 Reactions (CH 3 ) 3 C-Cl + OH¯ (CH 3 ) 3 C-OH + Cl¯ H 2 O acetone Double [tert-butyl chloride], keep [OH¯ ] constant rate of reaction doubles Double [OH], keep [tert-butyl chloride] constant rate stays the same Rate is proportional [(CH 3 ) 3 C-Cl] or rate = k[(CH 3 ) 3 C-Cl] overall first order. Conclusion the reaction is unimolecular S N 1 substitution nucleophilic unimolecular The order of reactivity is very different to a S N 2 reaction, thus a different mechanism must be occuring Vollhardt: Chapt. 7
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5.2 - SN1 & Eliminations 2 S N 1 Reactions An S N 1 occurs through a series or steps and forms intermediate species. The rate of the reaction is determined by the slowest step. The slow step is called the rate-limiting step or the rate determining step.
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5.2 - SN1 & Eliminations 3 S N 1 Mechanism (CH 3 ) 3 CCl + 2 H 2 O Æ (CH 3 ) 3 OH + H 3 O + + Cl¯ C CH 3 Cl CH 3 H 3 C Overall reaction δ - δ+ + C CH 3 Cl CH 3 H 3 C transition state C CH 3 CH 3 H 3 C What is the hybridization or the carbon? + Cl Step 1 1 st intermediate A) sp B) sp 2 C) sp 3 Rate determining step
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5.2 - SN1 & Eliminations 4 + C CH 3 CH 3 H 3 C + Cl Step 2 1 st intermediate S N 1 Mechanism electrophile (acid) nucleophile (base) O H H C CH 3 H 3 C H 3 C OH 2 + C CH 3 H 3 C H 3 C OH C CH 3 H 3 C H 3 C O HH + O H H Step 3 acid base C H 3 C CH 3 H 3 C O H H + 2 nd intermediate Transition State C H 3 C H 3 C CH 3 O H H O H H + + H 3 O + + Cl¯
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5.2 - SN1 & Eliminations 5 S N 1 Mechanism What happens at a stereogenic carbon? C H 3 CH 2 C H 3 C CH 2 CH 2 CH 3 Br H 2 O acetone C H 3 CH 2 C H 3 C CH 2 CH 2 CH 3 + O H H A B C H 3 CH 2 C H 3 C CH 2 CH 2 CH 3 OH A -H + C H 3 CH 2 C H 3 C CH 2 CH 2 CH 3 OH B -H + C H 3 CH 2 C CH 2 CH 2 CH 3 OH (R)-3-bromo-3-methylhexane S R C H 3 CH 2 C CH 2 CH 2 CH 3 OH C CH 2 CH 3 CH 3 H 3 CH 2 CH 2 C OH optically inactive a racemic mixture 50:50 use instead of last step optically active f l i p (R)-3-methyl-3-hexanol (S)-3-methyl-3-hexanol
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5.2 - SN1 & Eliminations 6 Solvolysis Reactions Notice that in all of the reactions we looked at so far the solvent has been the nucleophile (CH 3 ) 3 C- Br + H 2 O (CH 3 ) 3 C- OH + H Br (CH 3 ) 3 C- Br + CH 3 CH 2 OH H 2 O Æ hydrolysis CH 3 CH 2 OH Æ ethanolysis (CH 3 ) 3 C- OCH 2 CH 3 + H Br (CH 3 ) 3 C- Cl + HCO 2 H + H Cl (CH 3 ) 3 C- OCH O
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5.2 - SN1 & Eliminations 7 Polar Protic Solvents C CH 3 H 3 C H 3 C + + Cl Cl H 3 C H 3 C 3 C 3 H 3 C H 3 C δ δ + Δ G 1 + Free Energy Reaction coordinate Δ G 2 R + R Less polar solvent (slower) more polar solvent (faster) The polar transition state in a S N 1 reaction is stabilized by a polar solvent as well as the intermediate that is formed. Δ G 1 Δ G 2 > more polar solvent less polar solvent
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5.2 - SN1 & Eliminations 8 Like S N 2 Reactions the rate of reaction is influenced by the Leaving Group Order the compounds from most reactive to least reactive in a S N 1 reaction.
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5.2_substitution_unimolecular_psot - Vollhardt: Chapt. 7...

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