Competition among SN2, SN1, E2, and E1 Reactions
Competition among SN2, SN1, E2, and E1 Reactions
Rate-Determining Steps Revisited SN2 SN1 E2 E1
Nucleophile Strength
The Free Energy Diagram
Hammond Postulate
Hammond Postulate Applied to SN2 Reactions
Nucleophile Strength Revisited
Effective Electronegativity
The Base Strength in E2 and E1 Reactions
The Base Strength in E2 and E1 Reactions
Steric Hindrance in SN2 Reactions
Concentration of the Attacking Species
Leaving Group and pKa
Leaving Groups and SN2 Rates
Leaving Groups and SN1 Rates
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.
SN2 Reactions and the Hybridization
Effect of Alkyl Groups on the C Bonded to the L
Rate of SN2 and Steric Hindrance
E2 and Steric Hindrance
Rate of SN1 and E1 versus Carbocation Stability
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
Summary of the Influence of the Number of Alkyl Groups on the Carbon
Solvent Effects
Solvation
Solvent Effects TAB09.10_Karty1_CH09
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
So let’s summarize everything.
Leaving Group Ability
Regioselectivity in Elimination Reactions: Zaitsev’s Rule
Another Example of Zaitsev’s Rule
Exception to Zaitsev’s Rule
Inter- vs Intra- molecular Reaction
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