Organic Reactions and Mechansims Flashcards
DKS mechanistic priority rules
- Electron flow must start from an electron rich site
- Electron flow must move towards an electron poor site
- Electron flow must end up somewhere good
- Charge is conserved
- This may not be the end of the process
Electron rich sites
- Negative charge (Cl⁻, OH⁻)
- Lone pair (R-NH₂, H₂O)
- Double bond (alkene, aromatic ring
- singe bond, especially if weak (O-O)
Electron poor sites
- Carbocation or H⁺
- Atom adjacent to positive heteroatom (positive hetero atoms cannot be attacked directly - alrady have 8e-)
- Onwards towards a positive charge (e.g. if carbocation cannot be attacked directly, hetero atom attacked and electrons from bond pushed onto carbocation)
- Partially positive charge
- Partially positive charge at an alternating distance (may be dictated by different reagents)
‘Good’ areas for electron flow to end up
Electronegative atoms
Nucleophile
Electron rich species that will react with C⁺ or Cδ⁺
Base
Electron rich species that will react with H⁺ or Hδ⁺
Electrophile
Electron poor atom that will react with a nucleophile.
Often C⁺ or Cδ⁺.
Acid
Electron poor species that will react with a base.
Often H⁺ or Hδ⁺.
Chemoselectivity
When several different sites could react, one does so in preference
How does electronegativity affect nucleophile reactivity?
Less electronegative is better - the less electronegative the atom, the more likely it is to donate its electrons in nucleophilic attack
How do mesomeric effects affect nucleophile reactivity?
More resonance is worse - electrons that are shared in resonance conjugation are not so easily donated during nucleophilic attack
How do sterics affect nucleophilic reactivity?
Steric hindrance reduces nucleophile reactivity as is prevents donation of electrons
Why are bases less suscpetible to steric hindrance than nucleophiles?
Bases attack exposed hydrogens, rather than carbon atoms (which are more crowded).
Impact of inductive effects on electrophile reactivity
+I groups push electron density onto the electrophile (C+ or Cδ+) , reducing reactivity.
-I groups withdraw electron density from electrophile, increasing reactivity
Remember, where mesomeric effects are also present, these will take dominance
Impact of mesomeric effects on eletrophile reactivity
+M effect pushes electron density onto electrophile - reduces reactivity (positive charge weakened).
-M effect withdraws electron density from electrophile - increases reactivity (positive charge strengthened).
Remember, resonance conjugation from less electronegative atom (e.g. N) has greater +M effect (less electronegative atom shares electrons in resonance more readily)
