SL - Nucleophilic Substitution Flashcards
What type of atoms do nucleophiles commonly appear as?
Highly electronegative atoms
What type of atoms do electrophiles commonly appear as?
Nucleophiles
Why can the nucleophile not add before the leaving group departs?
Pentavalent carbon would form which is IMPOSSIBLE
Describe the SN1 mechanism?
Two-step mechanism
1. Leaving group departs (SLOW)
2. Nucleophile attacks (FAST)
-RDS is unimolecular
Describe the SN2 mechanism?
One-step mechanism
1. Leaving group departs as nucleophile attacks
-RDS is bimolecular
What does SN1 favour?
Stable carbocation intermediates
What does SN2 favour?
Electrophiles with fewer R-groups, avoids steric hinderance
What is the optical acitivity of SN1 mechanisms?
No optical activity
- Forms a racemic mixture as nucleophilic attack occurs on planar carbocation (attacks from either side)
What is the optical activity of SN2 mechanisms?
Has optical activity
- Nucleophilic attack occurs on opposite side of leaving group forming an inverted chiral product
How are appropriate leaving groups determined?
The anion product with greater stability is favoured
- Stability of anions is configured by determining their acid strength (Lower pKa value on reaction with H+)
How are alcohol groups substituted?
H2O is a much better leaving group than OH
- Protonation of oxygen is required
What 3 factors determine nucleophile strength?
- Delocalisation (Increased delocalisation- weaker nucleophile) Lone pair is shared
- Electronegativity (Increased electronegativity- weaker nucleophile) Lone pair is less available
- Heavier Elements (Heavier elements- stronger nucleophile) More shielding electrons
How does adrenaline form?
SN2 mechanism
1. Methionine + ATP –> S-adenosylmethionine (SAM) + P₃O₁₀⁵⁻
2. SAM + noradrenaline –>
S-adenosylhomocysteine (SAH) + Adrenaline
