Lesson22 - Reactions at sp3 Carbon Leaving Groups Other...

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Reactions at sp 3 Carbon: Leaving Groups Other Than Halogen In this lesson we will reason by analogy to predict the outcome of new reactions ( e.g., predictions of leaving group ability ) Breaking C–O bonds: –OH is a poor leaving group and does NOT directly participate in [S N 1], [S N 2], [E1] or [E2] reactions. Why is –OH a poor leaving group?
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Why –OH is a Poor LG -10.0 -9.0 15.7
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Protonation of an Alcohol Improves the Leaving Group Ability ___ _____________ ____________ LG Conjugate Acid pK a of Conjugate Acid H 2 O H 3 O + -1.7
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Protonated Alcohol as a Leaving Group
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Examples - Nucleophilic Substitution
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Examples - Water Elimination (dehydration)
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Activated Alcohols Based on PBr 3 PBr 3 is an electrophile δ ± δ ± δ ± δ + Nucleophilic attack on phosphorus by oxygen OH, a poor leaving group is converted into this good leaving group
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Activated Alcohols Based on SOCl 2 Nucleophilic attack on sulfur by oxygen δ ± δ ± δ ± δ +
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Activated Alcohols Based on Sulfonate Esters sulfonyl chloride sulfonate ester -1.0
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Common Sulfonyl Chlorides δ + δ ± (tosyl abbreviated Ts) (mesyl abbreviated Ms) (triflyl abbreviated Tf)
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Ethers and Epoxides Like alcohols, ethers are poor leaving groups. They can be activated by acids or _______ (epoxides are strain-activated ethers) a 3-membered cyclic ether strain pKa = ? -3.6 Protonation
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Reactions of Protonated Ethers [S N 2] or [S N 1] pathway depending on: • steric accessibility of nucleophile • stability of carbocation If a stable carbocation can form, the reaction will follow the [S N 1] pathway (otherwise [S N 2]) [S N 2] [S N 1]
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Summary: Converting Alcohols, Ethers and Amines
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Lesson22 - Reactions at sp3 Carbon Leaving Groups Other...

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