orgo 2 - conjugation & pericyclic reactions

conjugation

exists when 3 or more atoms with the ability to resonate are adjacent to each other in a molecule, allowing for delocalization of pi electrons

cumulated diene

pi bonds are adjacent

conjugated diene

pi bonds are separated by a single bond, continuous system of overlapping p orbitals

isolated diene

pi bonds are separated by more than one single bond

preparation of conjugated dienes

elimination reaction, alkyl halide or dihalide (needs 2 eq.) with a bulky base

bond length of conjugated dienes

the single bond in a conjugated diene is shorter than a typical C-C bond because of the overlap of 2 sp² hybridized orbitals

sp² bonds have more…

s character, meaning atomic orbitals are held closer to the nucleus, resulting in a shorter and stronger bond length

stability of conjugated dienes

conjugated double bonds are more stable than isolated double bonds, stabilization energy = 15 kJ/molhea

heat of hydrogenation

heat released when adding H₂ across double bonds

more substituted double bond =

more stable (hypoconjugation)

when ranking compounds in order of stability

more conjugation = more stability, more substituted double bond = more stability, isolated double bond = not as stable

why is s-trans a more stable conformation?

lower in energy, less steric hinderance

MO theory

use atomic orbitals to describe probable regions that those electrons could reside in

bonding MO

combine (constructive overlap) to form a low energy bonding MO

antibonding MO

2 lobes of opposite phases cancel out (destructive overlap) to form a high energy antibonding MO

lowest energy, 0 node in MO

pi electrons fill orbitals starting from lowest energy level

HOMO

highest occupied MO

LUMO

lowest unoccupied MO

frontier orbitals

HOMO and LUMO, most important MOs to consider

electrophilic addition

addition of H-X to a C=C double bond (markovnikox addition), 1,2 adduct and 1,4 adduct

1,2 adduct is favored at

cold temp, due to lack of thermal energy to reverse, fastest reaction wins, C2 is closer than C4, under kinetic control - major product is the faster one

1,4 adduct is favored at

high temp, because reaction is reversible and reaches equilibrium, more stable product dominated, under thermodynamic control - major product is the more stable one

pericyclic reactions

proceeds via a concerted process, invovles a ring of electrons moving around in a closed loop, occurs through a cyclic transition state, polarity of a solvent generally has no effect on the reaction rate, no ionic or free radical intermediates, no charge buildup

cycloaddition

change in sigma bonds: +2

change in pi bonds: -2

electrocyclic

change in sigma bonds: +1

change in pi bonds: -1

sigmatropic rearrangment

change in sigma bonds: 0

change in pi bonds: 0

diels-alder reaction

[4,2] cycloaddition, chemical reaction between a conjugated diene and alkene (dienophile), most are favored at low and moderate temperatures - product stability dominates

retro diels-alder reaction

reverse dield-alder reaction, favored at high temperatures - entropy is the dominant factor

dienophiles and EWG

the dienophile needs to possess an electron withdrawing group, or else the reaction with require high temp which does not favor product formation

diels-alder reactions can only occur when the diene is

in the s-cis conformation because they are close enough to react

cis diene chain + cis disubstituted dienophile

6 membered ring, both wedges or dashes

cis diene chain + trans disubstituted dienophile

6 membered ring, one wedge one dash

cis cyclic diene + cis disubstituted dienophile

endo favored bicyclic

cis cyclic diene + trans disubstituted dienophile

bicyclic compound, one endo (down) one exo (up)

both diene and dienophile are unsymmetrical

major and minor products, (1,3 vs. 1,4 - major)

why does the endo product form faster?

endo product is lower in energy

both diene and dienophile are symmetrical

one regiochemical outcome

why does cyclopentane react so rapidly in diels-alder reaction, while 1,2 dimethylenecyclohexane fails to react?

cyclopentane is permanently locked into s-cis conformation by its 5-membered ring, while 1,2 dimethylenecyclohexane is trapped in s-trans

electrocyclic reaction

pericyclic process where a conjugated polyene undergoes cyclization, pi bond coverted to sigma bond

disrotatory rotation

one rotates clockwise and other other counterclockwise, gives the cis product, conjugated systems with 6 pi electrons - (opposite in photochemical conditions)

conrotatory rotation

both rotate clockwise the same direction, gives the trans product, conjugated systems with 4 pi electrons - (opposite in photochemical conditions)

sigmatropic rearrangements

a reaction in which one sigma bond is replaced with another

cope rearrangement

a [3,3] sigmatropic reaction in which all 6 atoms in the cyclic transition state are carbon atoms, equilibrium favors more substituted alkenes

claisen rearrangement

a [3,3] sigmatropic reaction and is the oxygen analogue of a cope rearrangement, occurs with allylic vinylic ethers and allylic aryl ethers, favors formation of the C=O bond (more stable)

compounds with conjugated pi systems absorb

UV or visible light, the necessary energy to excite an electron from HOMO to LUMO falls within the UV or visible region of the spectrum

more conjugation =

smaller energy gap, longer wavelength

Woodward and Fieser

developed rules to predict max wavelength for chromophore starting with butadiene as the base

highly conjugated compounds absorb

light in the visible region of the spectrum