Organic Chemistry II Exam 2

Dienes

Compounds that possess two C=C bonds.

Cummulated Dienes (Allenes)

C=C bonds are adjacent. Pi bonds are perpendicular —> no conjugation.

Conjugated Diene

C=C bonds are separated by exactly one single bond. Continuous system of overlapping p orbitals.

Isolated Diene

C=C bonds are separated by two or more single bonds. Pi bonds are separated —> no conjugation.

Conjugated Dienes: Bond Lengths

Single bonds that are part of a conjugated pi system are shorter than typical single bonds. Due to the overlap of sp2 orbitals vs sp3.

More s Character —>

electron density closer to the nucleus —> shorter sigma bond length

Lower the heat of hydrogenation

More stable compound (lower energy, more stable)

Higher the heat of hydrogenation

Less stable compound (higher energy, less stable)

The more stable the compound:

more conjugated and more substituted

Conformations of Buta-1,3-diene

Generally, sigma bonds freely rotate. The two most stable conformations are cis and trans. P orbitals are conjugated only in these two rotamers. Easily interconvert at room temp.

Electrophilic Addition

First Step: Protonation gives most stable carbocation. Second Step: Nucleophile attack on carbocation.

Electrophilic Addition to Conjugated Dienes

Gives the 1,2-Addition and 1,4-Addition.

Electrophilic Addition to Conjugated Dienes: Protonation Step

H+ always adds to C1 or C4 positions (two ends) —> produce allylic carbocations.

H+ does not end to C2 or C3 positions —> do not produce allylic carbocations

Electrophilic Addition to Conjugated Dienes Mechanism

First Step: Protonation creates the most stable, resonance-stabilized allylic carbocation.
Second Step: Nucleophile attack at either electrophilic carbon atom.

Lower Temperature

The reaction is under kinetic control, and the second step is irreversible. The product that forms faster is the major product. The 1,2-addition has a lower activation energy —> fast reaction due to a proximity effect, so it will be the major product.

Higher Temperature

The reaction is under thermodynamic control, the second step is reversible, and the formation of the products is equilibrating. The product that is more stable will be the major product. The more substituted alkene will be the major product.

If the lower temperature reaction mixture allowed to warm

1,4-product favors

Pericyclic Reactions

Occur without iconic or free radical intermediates. The mechanism involves a ring of electrons moving around a closed loop.The transition state is cyclic. The polarity of the solvent has no effect on the reaction rate.

Diels-Alder Reaction

Also called a [4+2] cycloaddition because the reaction takes place between two different pi systems. One has 4 pi e- and one has 2 pi e-.

Diene

Four pi electrons. Needs to be s-cis and generally use electron rich dienes. The presence of electron donating groups enhance the reactivity like the alkyl group (-R), alkoxy group (-OR) or the -NHR.

Rate for Dienes

The diene must be in s-cis confirmation. A fixed s-cis conformation makes for a faster Diels-Alder reaction. 

The Dienophile

Usually an alkene or alkyne. Will possess an electron withdrawing group or the reaction will require a high temp. A good one has one or more electron withdrawing groups (carbonyl, cyano, or nitro).

How to predict Diels-Alder Products

Line up the ends of the diene with the dienophile, then draw the dotted lines from the two ends of the diene to the ends of the dienophile. Draw 3 arrows and draw the product.

A diene with an alkene

Makes a cyclohexene ring

A diene with an alkyne

Makes a cyclohexadiene ring

Endo and Exo Products

When bicyclic systems form, the terms endo and exo are used to describe the functional group positioning. Major product—> electron-withdrawing substituents in endo position. Often the endo product is the only observed product.

Endo Rule

The p orbitals of the electron-withdrawing groups on the dienophile have a secondary overlap with the p orbitals of the diene.

Intramolecular Diels-Alder

Both the diene and the dienophile are derived from the same molecule. Work best when separated by 3 or more carbon atoms. 

Diels-Alder Regiochemistry

Only one regiochemical outcome if the diene or dienophile is symmetrical.

Retro Diels-Alder

Very high temp —> reverse Diels-Alder reaction takes place. Produce starting diene and dienophile.

Electrocyclic Reactions

A conjugated polyene undergoes cyclization. 

Photochemical Condition: light, hv

Thermal Condtion: heat, triangle

Overall a pi bond is converted to a sigma bond.

HOMO

Determines the product outcome. The outermost lobes of the HOMO rotate so like phases overlap.

Disrotatory

One rotates clockwise and the other counterclockwise.

Conrotatory

Both rotate clockwise or counterclockwise.

Woodward Hoffman Rules

Thermal: CDC

Photochemical: DCD

Sigmatropic Rearrangements

Molecule rearrange. One sigma bond break and new sigma bond is formed. The location of the pi bonds changes as well, but the total number of sigma bonds and pi bonds remain unchanged.

Cope Rearrangements

A [3,3] sigmatropic reaction in which all 6 atoms in the cyclic transition state are carbon atoms. Equilibrium favors the more substituted alkene(s).

Claisen Rearrangements: Allylic Vinylic Ethers

The oxygen analouge of a cope rearrangement. Occurs with allylic vinylic ethers. Equilibrium favors the formation of the C=O bond because it is more stable than a C=C bond.

UV-Visible Spectroscopy

Compounds with conjugated pi systems absorb UV or visible light.