The RE or RSE is usually the difference in enthalpy (heat within the system) between a strictly bonded resonance contributor and its resonance hybrid (where the bond is floating back and forth between possible bond sites). When 2 or more contributors have unequal E's the RE will be the difference between the most stable contributor and the hybrid (and the value is smaller)

1. Only pi electrons and lone pairs move. 2. An sp3 C can't accept electrons since it has a full octet and it can't undergo hyperconjugation due to energy constraints. 3. Don't break sigma bonds

Separation of charges is not a good thing for resonance contribution but it can be improved under polar conditions

It will barely contribute to resonance at all

Two oxygens will be the stabilest while no groups will be the least stable. Nitrous groups will be less stable than oxygen but they're better than nothing

When phenol is undergoing its various resonance stages it comes across one stage where there is a negative charge opposite of the double-bonded oxygen; this is a delocalized anion which results in a very strong acid

If you're treating a substance with HX the H will bond pretty much as far away from the more complex end as possible (where it can bind) and the X will bind pretty much as close to the more complex end as possible to stabilize the resonance area

Sp2 carbons in sequence are stabler than their sp2-sp3 counterparts

It is cross-conjugated and the third double bond can't stabilize the system through resonance. Since penta-1,4-diene has a specific heat of 30.4 and 4-(eth-2-ene)-propyl-1,3-diene has a specific heat of 28.9, 1.5 is atrributable to the hybridization change

Allenes < alkynes < isolated dienes < conugated dienes

Conjugated dienes are dienes which have 2 double bonds separated by a single bond. A good example of this is 1,3-butadiene

They can be separated. Although flawed this is conceptually useful

Pi is p1+p2 when looking at a graph of the free energy (pi in this case is symmetric or S). Pi* is p1-p2 which turns out to be asymmetrical or A

Isolated dienes undergo additions without influencing each other. If you were to treat them directly with HX they would react to the more reactive double bond

Conjugated dienes can undergo both 1,2 and 1,4 additions. The 1,2 adduct is considered the "normal addition" whereas the 1,4 adduct is considered the "conjugate addition"

It means that the reaction was under kinetic control

It means that the reaction was subject to thermodynamic control

Let's say that you react penta-1,3-diene with HCl. The double bond from the 1 location will attack the H and bond to it. The Cl will likely bond at the 2 site but may travel to the 4 site. Since it's so much easier to remain at the 2 site the 1,2 product will be the major product

Ion pairing is a proximity effect in which ions will effectively pair with each other but in some cases they will choose somewhat random pairs. The proximity effect says that the ions are very likely to bond if close together

The entropy (delta-S) of the DAR is negative so therefore the reverse reaction (called the retro-DAR) becomes feasible at high T

1. Only interactions of filled and unfilled orbitals lead to net stabilization. 2. The stabilization is greater if the delta E (change in free energy) between the filled and unfilled orbitals is smaller. Therefore interaction between HOMOs and LUMOs is key

HOMO dienophile - LUMO diene is greater which means that the other way around is the controlling factor. It should be noted, though, that the delta E(HOMO diene - LUMO dienophile) for hydrocarbons is too large for a fast reaction and therefore will, in most cases, act as the rate determining step in a reaction

Add some electron donating groups onto the diene or add some electron withdrawing groups onto the dienophile. Some common EDGs include OR, OSiR₃ and NR₂; some common EWGs include CN, CO₂R, C(O)R and NO₂

The 1,4 adduct will be the dominating product. The issue here, just like with the 1,2 vs. 1,3 adduct, is that in the 1,3 adduct the negative charge is off of the EWG whereas with the 1,4 adduct the negative charge is on the EWG. This property is crucial in determining the regioselectivity of the DAR product

The 1,3 adduct is always disfavored for adducts such as the 1,2 and 1,4

The transition states in this case will all give the same meso product from z-alkene and addition will maintain the orientation of the halide substituents

The enantiomeric exo and endo syn transition states give separate diastereomeric racemates from Z-alkene. The result is an overall exo adduct and an overall endo adduct