Neil Keddie- Lecture 2 Nucleophilic Substitutions Flashcards
What is an SN2 reaction?
The rate of the reaction is proportional to the concentration of the nucleophile and electrophile.
Both species are in the rate determining step.
The nucleophilic attack and loss of the leaving group are simultaneous
What are the orbital requirements of an SN2 reaction?
The electron pair on the nucleophile must overlap with the σ* orbital of the C-X electrophile.
Maximal overlap occurs when the nucleophile is 180° from the electrophile.
Trigonal bipyramidal transition state with δ charges as far apart as possible.
New -ve charge on leaving group (electrophile)
More-O’Ferall-Jenks (MOJ) diagrams
Describe an SN1 reaction.
The rate is proportional to the concentration of the electrophile only.
One species is involved in the rate determining step.
What is the structure of the carbonation in an SN1 reaction?
Planar, sp2 hybridised with an empty p orbital. Attack is possible from either face.
Compare the energy profile of an SN1 reaction to an SN2 reaction.
The profile of an SN1 reaction has two peaks and two transition states. An SN2 reaction only has one of each.
What factors affect an SN1 reaction?
Substrate structure
Leaving group
Solvent
What factors affect an SN2 reaction
Substrate structure
Leaving group
Nucleophile
Solvent
What carbocations are stable enough for SN1 reactions?
Secondary
Tertiary (good)
What carbon can the leaving group be bonded to for an SN2 reaction?
Methyl (good)
Primary (good)
Secondary
What is hyperconjugation?
The weak donation of σ bond electrons from groups on the carbons adjacent to the empty p-orbital.
How do carbocations rearrange themselves?
They will rearrange to more stable tertiary forms if possible by moving electrons within the molecule.
Why does conjugation increase stability?
It allows the positive charge to be delocalised across several atoms. Requires π bonds or lone pairs.
