Exam One

Alcohol/Carboxylic Acid

OH pka = 15

Carboxylic pka = 5

Allylic

Radical on carbon next to carbon with double bond

• resonance stabilizes and rxn proceeds easier

Allyl cation, radical, and anion

2 pi e-, 3 pi e-, 4 pi e-

• have same molecular orbital diagram but different homo and lumo

Molecular Orbital Rules

1. number of orbitals in = number of orbitals out

2. adding nodes increases the amount of energy

3. when reading L —> R the orbitals should be symmetrical

4. when you have a weak nucleophile that would form no reaction, it can form a cation if it is allylic (good nucleophile will do sn2)

Radical Halogenation

• Low Concentrations X₂ with RO-OR or Hv reacts at allylic spot → adds one X 

• NBS/NCS with Peroxide adds TINY Br/Cl

Mechanism:

1. Initiation: Br₂ homolytically cleaves

2.  homolytic cleavage of H at allylic spot (weakest bond) + bond formation with radical from initiation 

   1. Draw possible resonance structures

3. Propagation two: homolytic cleavage of Br₂ and bond formation with radical 

Product: starting material with a Cl/Br at allylic position and Cl/Br radical

Possible Peroxides with NBS/NCS

(tBuO)₂

(BzO)₂

Reactions of Allylic Systems

1. Radical Halogenation: NBS/NCS w/ peroxide

2. Nucleophilic Substitution: sn1 with poor nucleophiles

3. Addition to electrophiles:

   1. addition of Mg and THF to form organometallic

   2. H attached to allylic carbon reacting with nBuLi and TMEDA to replace H with Li

TMEDA

nBuLi deprotonates H at allylic position and replaces it with Li

• nBuLi with TMEDA

• TMEDA boosts the reaction by coordinating to Li⁺ through two nitrogen lone pairs, increasing the reactivity of n-BuLi

• C-H bond is ~40 while C-Li bond is ~50 so reaction will still proceed, but slowly and less effectively without TMEDA

Conjugation

• 3 sp² p orbitals in a row

• lowers energy of molecule and stabilizes

• single bond separates the double bond

Diene

2 C=C bonds

Triene

3 C=C bonds

Alternating single and double bonds

Allene

Two adjacent double bonds

• since adjacent, one orbital is up and down while the other is in and out

E vs Z

E is when high priority groups are on opposite sides

Z is when high priority groups are on same sides

Measuring stability + result of conjugated systems

△H°hydrogenation: energy released when pi bond breaks into sigma bond

• One double bond reacting with H2/Pt = -30 kcal/mol

• two double bonds reacting with H2/pt = -60 kcal/mol

• Conjugated system reacting with H2/pt = -57 kcal/mol

Conjugation: Stabilizes starting material due to sp² hybridization and p orbitals of adjacent bonds overlapping, allowing delocalization of pi electrons resulting in lower energy arrangement and increasing stability.

Characterization of Conjugated systems

• Single Bond Shortens: single bond between two double bonds has overlap of p orbitals allowing delocalization resulting in a partial double-bond character on the single bond

  • Makes bond strong and shorter than normal c-c bond

• Barrier to Rotation: Rotation around central bond would break p-orbital overlap, disrupting conjugation

  • conjugations lowers energy, breaking this system requires more energy

  • inhibits rotation —> higher rotational energy barrier than normal single bond

Bonding Orbitals

π3* = Antibonding - High energy

π2 = nonbonding - Neutral energy

π1 = bonding - Low energy

3 carbon conjugated system - MOD

π3* = red/blue : blue/red : red/blue = 2 nodes = highest energy state

π2 = red/blue: blue/red = 1 node = neutral energy state

π1 = red/blue, red/blue, red/blue = 0 nodes = low energy state

MOD is same for 2,3,4 pi e- allylic systems - HOMO/LUMO changes

4 carbon conjugated system MOD - Diene

2 double bonds = 4 pi bonds

no nonbonding orbitals in even number pi orbitals

π4* = red/blue : blue/red : red/blue : blue/red = 3 nodes = high energy

π3* = red/blue : blue/red : blue/red : red/blue = 2 nodes = high energy

π2 = red/blue : red/blue : blue/red : blue/red = 1 node = high energy

π1 = red/blue, red/blue, red/blue, red/blue = 0 nodes = lowest energy

Thermodynamic Product

• Most stable form (most substituted double bond)

  • compare all resonance forms

• Higher E_a + low energy product

• favored at high temperatures

Kinetic Product

• Fastest forming product

• Lowest E_a

• Favored at low temperatures

• Can form at high temperatures

  • not favored due to ability to reverse reaction ability

Impact of Increasing Conjugation

• More double bonds = more resonance = more stable

  • due to increasing # of overlapping adjacent p orbitals

• Lower E_a —> lots of possible products (more resonance intermediates)

• Polymerization side rxn increases —> due to stable intermediate formation —> reacts with itself

Benzene

• Aromatic functional group

• conjugated

• not a triene

Diels-Alder Reaction

4+2 Cycloaddition between diene (4πe-) and dienophile (2πe-) with heat

Diene

Electron Rich —> Electron Donating Group (EDG)

• R group

• :O:R

• :NR₂

• Me:O: (at position 3)

• CH₃ (at position one)

• Hexane ring (connected at position 2/3) —> e- donating alkyl groups

Dienophile

Electron Poor —> Electron Withdrawing Group (EWG)

• CF₃

• NO₂

• C≡N

• -CR=O

Inductive Effect

Electronegativity acting at a distance

• CH₃ donates e- via hyperconjugation

  • 3- in C-H sigma bond overlap with pi system, pushing e- density into pi system

  • stabilizes positive charges —> carbocation has empty p orbital, CH₃ sigma bonds overlap with empty p orbital, partially delocalizing positive charge, stabilizing it

Resonance

Stronger effect than inductive effect

• EDG: more e- density into π system

• EWG: less e- density in π system

How do you know if you are working with a diels-alder reaction?

If you see a cyclohexene with at least one π bond

Diels-Alder Reaction Mechanism

Concerted Reaction!

1. Arrow from C1-C2 Diene π bond to C2-C3 σ bond

2. Arrow from C3-C4 Diene π bond to space between diene/dieneophile

3. Arrow from C1-C2 π bond from Dieneophile to upper space between diene/dieneophile

Diene Conformations - trans vs cis

s-trans: Thermodynamically favored and more stable

• unreactive

s-cis: required for diels-alder reaction

• Heat used to push this reaction

• reactive