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Competition among SN2, SN1, E2, and E1 Reactions
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Competition among SN2, SN1, E2, and E1 Reactions
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Rate-Determining Steps Revisited
SN2 SN1 E2 E1
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Nucleophile Strength
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The Free Energy Diagram
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Hammond Postulate
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Hammond Postulate Applied to SN2 Reactions
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Nucleophile Strength Revisited
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Effective Electronegativity
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The Base Strength in E2 and E1 Reactions
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The Base Strength in E2 and E1 Reactions
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Steric Hindrance in SN2 Reactions
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Concentration of the Attacking Species
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Leaving Group and pKa
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Leaving Groups and SN2 Rates
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Leaving Groups and SN1 Rates
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Leaving Group Ability and SN2/SN1/E2/E1 Reactions
SN1 reactions are more sensitive to leaving group ability than SN2 reactions are. Excellent leaving groups favor SN1 and E1 reactions over corresponding SN2 and E2 reactions.
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SN2 Reactions and the Hybridization
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Effect of Alkyl Groups on the C Bonded to the L
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Rate of SN2 and Steric Hindrance
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E2 and Steric Hindrance
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Rate of SN1 and E1 versus Carbocation Stability
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Hyperconjugation secondary can do 2x, tertiary can do 3x
The increased stability of a carbocation as methyl or other groups are added to the C+ is due to hyperconjugation. Conjugation: p orbitals overlap. Hyperconjugation: sigma MO overlaps with p orbital. Coplanar but not parallel, so overlap not as good, stabilization not as much as normal conjugation. secondary can do 2x, tertiary can do 3x
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Summary of the Influence of the Number of Alkyl Groups on the Carbon
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Solvent Effects
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Solvation
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Solvent Effects TAB09.10_Karty1_CH09
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polar solvents especially good for SN1 (OK for some SN2)
We’ve already seen that H-bonding to a nucleophile makes it less reactive. So nucleophiles are more reactive in solvents that cannot H-bond. aprotic solvents (no H on O or N) cannot H-bond to Nu especially good for SN2
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So let’s summarize everything.
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Leaving Group Ability
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Regioselectivity in Elimination Reactions: Zaitsev’s Rule
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Another Example of Zaitsev’s Rule
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Exception to Zaitsev’s Rule
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Inter- vs Intra- molecular Reaction
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Inter- vs Intra- molecular Reaction
3 very close, entropy overcomes ring strain 4, not as close, ring strain overcomes entropy 5 and 6, not much strain so entropy helps 7 not as good, but OK larger rings, entropy not much help forms ring
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