Conjugated pi systems and Pericyclic Reactions

Dienes

A compound that posses two c=c bonds

Cumulated Diene

Allenes, the pi bonds are next to one another

Conjugated Diene

The pi bonds are exactly one bond away, think resonance

Isolated Dienes

The pi bonds are separated by two or more bonds

Why are conjugated pi bonds special?

They contain one continuous system of overlapping p orbitals

What is the most common reagent used to prepare dienes?

t-buOK, any sterically hindered base and allylic halides will do

Which bond is longer, one in a conjugated system or one in a non-conjugated system?

The bond in the non-conjugated system is longer. The sp2 hybridized orbitals hold electrons closer to the nucleus, and therefore will have shorter bonds

Stabilization energy of a conjugated diene

15 kj/mol

What happens when the c2-c3 bond in 1,3 Butadiene is rotated 90 degrees?

The stabilizing effect of conjugation is lost. the cis and trans conformers continuously invert, and act as isolated pi bonds

a single bond in a conjugated diene is ______ than the normal C-C bond

Shorter

The least stable alkene will have the _______ heat of hydrogenation.

highest

Order of stability for dienes

allene < non-conjugated diene < conjugated diene

Kinetic control

The "manual" addition of a halide in a cyclic reaction. Not the most stable product, but the most convenient (ie 1,2 addition)

Thermodynamic control

1,4 addition of a halide. Forms the most energy convenient structure.

Which forms faster, 1,2 addition or 1,4?

1,2 addition occurs faster due to proximity effect, however, when equilibrium takes control, the thermodynamic product is favored.

Major product of a electrophilic addition reaction at 0 degrees?

The 1,2 addition

Pericyclic Reactions

Reactions that do not involve ionic or radical intermediates. They include:
- Cycloaddition
- Electrocyclic reactions
- Sigmatropic Reactions

characteristics of Pericyclic Reactions

- proceed via concentrated process ( all bonds are formed and made in the same step)
- Involves a ring of electrons moving in a closed loop
- Has a cyclic transition state
- Polarity of solvents have little effect, suggesting very little charge on the transition state

Cycloaddition adds ____ sigma bonds and loses ___ pi bonds

+2, -2

Electrocylic adds ____ sigma bonds and loses ___ pi bonds

+1, -1

sigmatropic adds ____ sigma bonds and loses ___ pi bonds

0,0

Diels Alder is also called

[4+2] cycloaddition

True or False: High temps favor Diels Alder

False, moderate temps favor diels alder product, but high temps disfavor the product formation and can cause a retro diels alder reaction

Dienophile

starting material for diels alder

Do diels alder reactions proceed with higher yields with or without an electrion withdrawing group?

They proceed faster with an electron withdrawing group

Can triple bonds function as dienephiles?

Yes

Cis alkenes form _____ disubstituted rings, trans alkenes form _____ disubstituted ring

cis, trans

Cyclopentane is locked in what conformation? Does it favor diels alder?

s-cis, yes it proceeds very quickly, even with itself

When reacting cyclopenadiene, which position is favored for the substituents?

Endo, the transition state of the exo product makes it to have a IMR

True or False: Electrocyclic Reactions have different reactions to heat and light

True, heat favors syn additions, while light favors anti addition

Thermal electrocyclic addition

Favors disrotatory rotation when system has 6 pi electrons, favors conrotatory when system has 4 pi bonds

Photochemical electrocyclic addition

Favors conrotatory when system has 6 pi electrons, favors disrotatory when system has 4 pi electrons

sigmatropic rearrangement

A cyclic reaction where the pi bonds change location, making a net change in bonds of 0

UV spectroscopy

Passes UV light to conjugated pi systems to display their excited state.

pi -> pi*

A photon that has equivalent energy to the energy gap between MOs, is absorbed by a pi electron in the HOMO and is promoted to a higher energy state. This excitation is called pi to pi star.

Lambda max

the maximum wavelength of absorption on a UV spectrum

Molar absorptivity

Measures the amount of light absorbed by Lambda max

True or False: Lambda max indicates the extent of conjugation

True

Chromophore

Region of molecule that is conjugated

Auxochromes

Region that is not conjugated

Conservation of Orbital symmetry

Requires that MOs have same phase to constructive overlap.

Woodward-Fieser rules

Rules for simple predictions of lambda max for UV spectra