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
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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
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3
Q

What affects hydrate/acetal/hemiacetal formation?

A
  • Steric crowding reduces reactivity
  • Ring strain increases reactivity
  • e-withdrawing groups increase reactivity
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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
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5
Q

Reactivity of carbonyls
* Sequence
* Explanation

A
  1. Acid chloride
  2. Anhydride
    *3. Aldehyde
  3. Ketone*
  4. Ester
  5. Amide
    * Conjugative effect stabilises carbonyl (delocalise)
    * Inductive effect increases reactivity
    * Beware COOH often reacts as acid!
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6
Q

What affects nucleophilic substitution at saturated carbon?

A
  1. Substrate - decides Sn1/2
  2. Nucleophile - (Sn2 only)
  3. Leaving group ability - in RDS
  4. Solvent (polar protic Sn1, polar aprotic Sn2)
  • Dominated by MO overlap
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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
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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-)
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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
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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
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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
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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

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13
Q

Epoxidation geometry

A
  • cis C=C gives cis epoxide
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