Orgo 2 Exam 1

resonance

stronger acid has the more stable conjugate base, due to resonance.

more resonance means stronger acid

lower pka —> less stable —> easier to break bonds and deprotonate

more p orbital sharing

more acidic proton

allylic

carbon atom next to the carbon-carbon bond

allylic position tells about reactivity

3° allylic more stable that 2° or 1° which makes it more reactive and makes the reaction faster

if something were to form at an allylic position, it is easier to do because of

the resonance at that position

sigma orbital

bonding orbital

electron density is concentrated between nuclei

sigma star orbital

antibonding orbital

electron density is not between nuclei- there is a node between them

bonding orbital lower E than

antibonding orbital

lowest to highest energy pi orbitals for C=C=C

lowest to highest E

• pi 1: red top blue bottom, red top blue bottom, red top blue bottom

  • bonding orbital

• pi 2: red top blue bottom, node, blue top red bottom

  • non bonding

• pi 3: red top blue bottom, blue top red bottom, red top blue bottom

  • antibonding

nodes

the 0 point, the mismatch point where orbital switches from red to blue

rules for bonding orbital energy diagram

• must be symmetrical (sliced vertical down middle) bc double bonds are planar

• orbitals in = orbitals out

• more nodes = higher E

  • more mismatched orbital arrangement is higher E

know how to draw 3 carbon allylic cation, radical, anion

cation: has 2e- total

radical: has 3e- total

anion: 4e- total

LUMO

HOMO

lowest unoccupied molecular orbital

highest occupied molecular orbital

reactions of allylic systems

1) radical halogenation

• radical hal with resonance

2) nucleophile substitution

• sn1 with resonance

3) addition to electrophile

• grignard with resonance

radical halogenation methods

• X2 (low conc) and ROOR/hu

• NBS and (BzO)2

• NBS and hu

• NCS and ROOR

radical halogenation with propene

• X2 (low conc)

• ROOR or hu

halogen attaches to allylic spot

nucleophilic substitution (sn1)

use ROH with heat

• leaving group leaves

• forms carbocation

• do hydride shift to make carbocation most stable

• O attaches to carbocation

• leaving group deprotonates nucleophile

if only one product forms

no carbocation intermediate, went through sn2

organometallic allylic systems- 3 types

• 3C allylic system-Br + Mg + THF —> 3C allylic system-MgBr

• 3C allylic system + nBuLi + TMEDA —> 3C allylic system-Li

• allylic-MgBr + ketone + THF + H3O, H2O —> grignard product

TMEDA

• tetramethyethylenediamine

• helps speed up/facilitate allylic system + nBuLi reaction

• connects N to Li at 2 points instead of 1

conjugation and orbitals

3C allylic system has 3 p orbitals

4C conjugate diene has 4 p orbitals

diene

molecule with 2 C=C bonds

triene

molecule with 3 C=C bonds

conjugated

1 single bond separating the double bonds

all carbons are sp2

non-conjugated

can have multiple single bonds separating double bonds

some sp3 carbons separating

allene

C=C=C with 2 planar H on one C and 2 wedge/dash H on the terminal C

know diene/triene/E/Z/trans/cis nomenclature

stability in allylic systems

conjugated structures make structures more stable (as compared to nonconjugated)

• -3.5 kcal/mol due to conjugation (less energy means more stable)

order of stability in structures with double bonds

lowest E: one double bond

middle E: 2 double bonds, conjugated

highest E: 2 double bonds, nonconjugated

energy difference between conjugated and nonconjugated allylic system

conjugated is 3.5 kcal/mol more stable

why is conjugated more stable structure than nonconjugated or non db structure (3 reason)

1. all sp2 p orbitals are parallel

• even the single bond connected the double bonds has conjugation in parallel p orbitals to help with stability

2. shortens structure compared to no db (ex butane) : single bond between the double bonds creates a shorter space between + pulls p orbitals tg

3. creates barrier to rotation: takes more E to rotate single bond, rotation inhibited

a 4 C diene has how many pi electrons

4 pi electrons

• they fill pi1 and pi2 lowest energy levels first bc aufbau

when drawing MO diagram with the energy levels, explain using nodes and when to omit orbitals

• omit orbitals when odd number of pi orbitals

• lowest E level will have 0 nodes, and will increase as E level increases

1,3-butadiene + HBr at 0°C

• 1,2 product: H, Br at opp ends of double bond (70%)

• 1,4 product: move double bond to other side bc of allylic property: H, Br on opposite ends of molecule

  • MECHANISM?

1,3-butadiene + Br2

• 1,2 product: Br, Br

• 1,4 product: Br, Br, after allylic rearrangement

hydration + TsOH + H2O, THF —>

?

1,3-buta-3-methyldiene + HBr

2 non-identical double bonds

• only 1,2 and 1,4 product at the double bond site with more stability are possible outcomes