Organic theory Flashcards
1
Q
Nucleophilic addition to carbonyl
* Key orbitals
* Trajectory, why?
* What affects reactivity?
A
- HOMO - Nu- l.p. orbital
- LUMO - C=O pi-star
- Burgi-Dunitz trajectory (107deg) - max overlap, minimise repulsion w/ C=O electrons
- Electrostatics - hard Nu better, more +ve C
- Sterics reduce reactivity
2
Q
Nucleophilic addition to carbonyl
* Key orbitals
* Trajectory, why?
* What affects reactivity?
A
- HOMO - Nu- l.p. orbital
- LUMO - C=O pi-star
- Burgi-Dunitz trajectory (107deg) - max overlap, minimise repulsion w/ C=O electrons
- Electrostatics - hard Nu better, more +ve C
- Sterics reduce reactivity
3
Q
What affects hydrate/acetal/hemiacetal formation?
A
- Steric crowding reduces reactivity
- Ring strain increases reactivity
- e-withdrawing groups increase reactivity
4
Q
Nucleophilic substitution to carbonyl
* Key orbitals
* What affects reactivity?
A
- HOMO - Nu- l.p. orbital
- LUMO - C=O pi star
- Better if good leaving group (low pKa)
- Hard Nu-
- Dominated by electrostatics
5
Q
Reactivity of carbonyls
* Sequence
* Explanation
A
- Acid chloride
- Anhydride
*3. Aldehyde - Ketone*
- Ester
- Amide
* Conjugative effect stabilises carbonyl (delocalise)
* Inductive effect increases reactivity
* Beware COOH often reacts as acid!
6
Q
What affects nucleophilic substitution at saturated carbon?
A
- Substrate - decides Sn1/2
- Nucleophile - (Sn2 only)
- Leaving group ability - in RDS
- Solvent (polar protic Sn1, polar aprotic Sn2)
- Dominated by MO overlap
7
Q
What affects Sn1?
A
- Favoured by steric crowding
- Stabilisation of carbocation (sigma donation < pi delocalisation < lone pair donation)
- Good leaving group
- Polar protic solvent solvates cations/anions
8
Q
What affects Sn2?
A
- Favoured by minimum steric crowding
- Stabilised TS by pi system
- Good leaving group
- Soft Nu-: smaller HOMO/LUMO gap (driven by orbitals)
- Polar aprotic solvent (solvate cation NOT Nu-)
9
Q
E1
* Key orbitals
* What affects reaction?
A
- 2 HOMO/LUMO interactions
- Base l.p. HOMO w/ C-H sigma-star LUMO
- C-H sigma HOMO with empty carbon p orbital LUMO
- Favoured by more stable carbocation
- Good leaving group
10
Q
Sn1/Sn2 or E1/E2?
A
- High temp favours elim (entropy)
- Steric hindrance (substrate and nucleophile) favours elim
- Strong base favours elim
- Sn1 analogous to E1, Sn2 to E2
11
Q
E2
* Key orbitals
* What affects reaction?
A
- 2 HOMO/LUMO interactions
- Base l.p. HOMO w/ C-H sigma-star LUMO
- C-H sigma HOMO with C-Br sigma star LUMO
- C-H sigma and C-Br sigma-star must be antiperiplanar (180deg) to each other
- Favoured by strong non-nucleophilic base
- Good leaving group
12
Q
What is a soft/hard nucleophile?
A
Soft Nu-: low HOMO, lower pKa, low charge density
Hard Nu-: hi HOMO, higher pKa, high charge density
13
Q
Epoxidation geometry
A
- cis C=C gives cis epoxide
