Order of stability of radicals ?
allyl radical > 3 > 2 > 1 > vinyl
What is conjugation ?
occurs whenever p orbitals (sp or sp2) are located on 3+ adjacent Cs (all the way around in ring) (allows for overlap and e- to delocalize = stability)
What is the true structure of an allylic radical ?
hyrbid: instead of a double bond on right or left of C, it’s one bond each with dashed line as the second “bond” for each and a 1/2 on both Cs = radical/e- are being shared b/w each C
What is the difference b/w traditional substitution and radical substitution ?
traditional is at low temps and halogens add across double bond
radical at high temps and halogen replaces an H
What is used as a continuous supply og Br during allylic bromination ?
NBS
What are the steps of allylic substitution ?
- initiation (make X radical by splitting the X-X into 2 .X)
- propagation 1 (make allylic radical by removing an H and making it bond with one X)
- propagation 2 (make allylic X by joining radical left on C with radical b/w X-X)
How to count the number of e-/arrows ?
count pi bonds (2), radicals (1), and negative charges (2) and add
What is HOMO ? LUMO ?
- highest occupied molecular orbital/that still contains e-
- lowest unoccupied mo/lowest able to accept e- (max is 2)
What are the different orbital types ?
pi3 = antibonding w/ 2 nodes (3 diagonal)
pi2 = nonbonding w/ 1 node (2 diagonal)
pi1 = bonding/0 nodes (all faces align)
How to do MO when in excited state ?
1 e- jumps from HOMO to LUMO
How does stability of allylic carbocations go ?
3 > allylic carbocation (same or more than 2 but less than 3) = 2 > 1
What are the 3 types of dienes ?
- 1,3-butadiene = conjugated (2 double bonds separated by 1 single bond) (most stable due to lower heat of hydrogenation)
- 1,4-pentadiene = isolated (2 double bonds separated by at least 1 sp3 C)
- 1,2-propadiene = cumulated (2 double bonds sharing a common C)
What is unique about 1,3-butadiene’s double bonds ?
central bond is shorter than normal bonds since it’s b/w 2 sp2 Cs instead of sp3 (sp2 has more s character = holds e- closer = short bond) and C2 and C3 orbitals overlap w/ pi system
What are the 2 possible conformations of rotation of single bond b/w the 2 double bonds in a 1,3-butadiene ?
s-cis and s-trans (more stable)
*this is just rotating the bond, not breaking
What are the 2 products of 1,3-butadiene and what gives rise to them ?
- when it reacts with HX (AND NOTHING EXTRA:
- 1,2 addition product: H on C1, X on C2
- favored by kinetic control (low temps) - 1,4 addition product: H on C1, X on C4
- favored by thermodynamic control (high temps)
- steps: break double bond to H and kick out X –> H on C1 and + on C2 –> resonance where double bond b/w 2 and 3 and + ends up on C4 –> hybrid product (can now be 1,2 product or 1,4)
What is a Diels-Alder rxn ?
addition rxn b/w a 1,3-butadiene and an alkene (dienophile) to form a new 6-membered ring
Diels-Alder rxn steps ?
3 curved arrows show bonds breaking and forming all at the same time:
- 1 bond of double bond of dienophile forms bond w/ top C of diene
- double bond of C1 (diene) moves to the middle (b/w C2 and C3)
- 1 bond of double bond of diene forms bond w/ bottom C of dienophile
result = 6 membered ring with 1 double bond (at least)
Diels-Alder rxn rules ?
- even the simplest of this is at very low yield and requires high energy (dienophile needs e- withdrawing group)
- diene can react only when s-cis (prevented in trans cyclic alkenes but very reactive in cis)
- stereochemistry (trans or cis) carries over to product
- cyclic dienophile = bicyclic product forms
- cyclic diene = bridged bicylic product forms (rings share non-adjacent Cs
- endo (Z/substituent of dienophile ends up on same side as outside groups of diene (trans vs cis)/usually dashed and it’s always the major product) vs exo products ((Z/substituent of dienophile ends up on opp side pf outside groups of diene and usually wedged)
- same with cyclic dienes (dashes point down/wedges up)
What to look for when finding where to put radicals ?
look at sp3 Cs
How to rank reactivity of dienophiles ?
- e- withdrawing group = best (the more the better)
- e- donating group = bad (the more the worst)
- plain = plain
How to rank reactivity of dienes ?
- locked cis ring = #1 (trans = last)
- e- withdrawing group = second to last
- e- donating group = second best
- plain = plain
What’s an electron withdrawing group ?
double bond to electronegative atoms (or C of it) and halogens
(we are looking at the outgroups of it not the two set double bonds in dienes- doesnt apply to dienophiles)
What’s an electron donating group ?
lone pair/electronegative atom alone
(we are looking at the outgroups of it not the two set double bonds)
