Organic Chemistry Chapter 16: Conjugated Pi Systems and Pericyclic Reactions

What does it mean when a Pi Bond is conjugated?

There is a single single-bond between it and another Pi Bond.

What does it mean when a Pi Bond is Cumulated?

Two Pi Bonds are side by side

What does it mean when a Pi Bond is Isolated?

There is more then a single bond between two pi bonds.

How long are C=C bonds and C-C bonds compared to a C-C bond adjacent to a C=C bond?

C=C bonds are short, C-C bonds adjacent to a C=C bond are shorter then a C-C bond.

Rank the types of Dienes in terms in increasing stability

Isolated < Cumulated < Conjugated

Upon hydrogenation, what type of diene will release the LEAST heat?

The most stable diene, a Conjugated system.

Upon hydrogenation, what type of diene will release the MOST heat?

The least stable diene, an isolated system.

How do you prepare a conjugated Diene?

You can make an alkene via elimination, use NBS and UV light to add a bromine to an allylic position, and do another E2 reaction to get a conjugated diene.

Why are c-c bonds adjacent to c=c bonds shorter then normal c-c bonds?

Because the hybridization of the Carbon in the C=C bond causes electrons to be pulled in closer

Why are c-c bonds between two alkenes MUCH shorter then other c-c bonds?

Resonance and Molecular Orbitals give double-bond character to the C-C bond.

How can you determine how many electrons are involved in a Pi System?

Each Double Bond = 2 electrons

What are the LUMO and HOMO?

Lowest Unoccupied Molecular Orbital and Highest Occupied Molecular Orbital.

How does Molecular Orbital theory explain why c-c bonds in dienes tend to be shorter than normal?

The lower energy orbitals lack nodes and bond with each other, creating double-bond character.

Is a diene where the orbitals aren't aligned still a conjugated Diene?

No, the orbitals must be aligned for the sake of stability.

The more stable the alkene/diene, the _______ its energy is.

Lower.

How do you name a diene?

Same as an alkene, just replace "ene" with "diene"

A terminal diene is ______ as full of energy as a terminal alkene

twice

Is (E)-1,3-pentadiene twice as full of energy as other alkenes? Why or why not?

No, because of the additional stability from the molecular orbitals and resonance creating double bond character.

Are conjugated dienes or isolated dienes more stable?

Conjugated dienes, due to the double bond character of the conjugated dienes

Conjugation adds about ___

~15 kJs of energy, otherwise known as Resonance Energy

What is the easiest way to make a conjugated diene?

Elimination on a bromo-alkene

If you use a bulky base for an elimination reaction on a bromo-alkene, it will create the Zaitsev product and not the hoffmann product even if it's given the oppurtunity. Why is this?

The stability of the Conjugated diene influences it

What are conformational isomers?

Isomers that aren't actually isomers, but vary due to positioning.

What is S conformation?

The way to determine the conformation of a diene, it works similarly to E-Z conformation but focused on the sigma bond between the C=C bonds instead of a C=C bond.

How much energy is required for a diene to change conformation?

~12 kJ

What is more stable, a S-Trans Diene or an S-Cis Diene?

S-Trans Diene

Can a double bond ever act as a nucleophile?

Yes

What occurs during addition of a halogenic atom on a diene?

Resonance can occur, creating a normal addition product as the major product but an anti-markovnikov addition product as a minor product.

What is a 1,2-adduct?

The addition product of a diene without resonance

What is a 1,4-adduct?

The addition product of a diene with resonance

What determines whether or not you get 1,2 adducts or 1,4 adducts?

The temperature, if you have low temperatures then you get 1,2 adduct and vice versa with high temperatures.

Why does temperature affect the product of an addiction reaction involving a diene?

Because 1,4 adduct is thermodynamically preferred, so lower temperatures means that the less thermodynamically controlled product moves faster.

Is 1,2 adduct or 1,4 adduct thermodynamically preferred?

1,4 adduct.

Is 1,2 adduct or 1,4 adduct kinetically preferred?

1,2 adduct.

Are adducts reversible reactions?

only if the temperature is high enough.

At low temperature, the major product of adduct reactions are determined by:

Whichever reaction is faster.

At high temperature, the major product of adduct reactions are determined by:

Whichever reaction is most stable.

Why is the formation of 1,2 adduct faster then the formation of 1,4 adduct?

Because the halogenic atom is closer to the 1,2 spot at the time of deprotonation.

What is a dienophile?

A molecule that wants to react with a diene.

What is the Diels-Alder Reaction?

A concerted reaction that features 4,2 cyclocaddition

What is a pericyclic reaction?

A reaction with a cyclic transition state.

What is 4,2 cycloaddition?

It means that the reaction is using two different Pi systems, one with four carbons and one with carbon atoms.

Why is the Diels-Adler Reaction so useful?

It makes C-C Bonds and Alkenes, and makes a chirality carbon

Why is the Diels-Alder reaction favorable?

It's enthalpically favorable because it breaks 2 pi bonds and creates 1 sigma bond.

Is the Diels-Alder reaction temperature dependent?

Yes, because entropy is positive.

Is it possible to turn a cyclohexene into a dienophile and diene?

Potentially yes, with a ton of heat.

What conformation must a diene be in for the Diels-Alder reaction, and why?

S-Cis, because the molecular orbitals of the diene must overlap with the orbitals of the dienophile

What is cyclopentadiene?

A diene that will never leave s-cis conformation, and creates bicyclic cyclohexanes

Can cyclopentadiene react with itself?

Yes

What kind of diene is needed for a Diels-Alder reaction?

A conjugated, s-cis, 1,3 diene

What kind of dienophile is best for a Diels-Alder reaction?

Electron-Poor Dienophiles

How can you make a dienophile electron-poor?

Attach a group that causes inductive effects and resonance effects.

What makes a dienophile electron-rich?

Things like halogenic atoms.

What happens when you use an Alkyne in a Diels-Alder reaction?

You create a 1,4-cyclohexadiene

What happens when the dienophile in a Diels-Alder reaction has substituents that are cis?

Those substituents will stay cis to each other (Pointing in the same direction)

What happens when the dienophile in a diels-alder reaction has substituents that are trans?

Those substituents will stay trans to each other (Pointing in different directions)

What does it mean when it's said that the Diels-Alder reaction is stereospecific?

Different conformations of the dienophile create different stereoisomers

What does it mean when it's said that the Diels-Alder reaction is stereoselective?

It can create specific stereoisomers with certain reagents

How many Chiral centers are created from the reaction between Cyclopentadiene?

4

What is the Endo-product of a diels-alder reaction?

If the electron withdrawing group on the dienophile is underneath the diene, then it will create the endo product.

What is the exo-product of a diels-alder reaction?

If the electron withdrawing group on the dienophile is above the Diene, then it will create the exo product.

Which product is preferred in a Diels-Alder Reaction?

Whichever is the most stable, via molecular orbital interactions with the diene and substituent.

What is the "why" behind the exo-endo products of a diels-alder reaction?

The HOMO of the diene interacts with the LUMO of the dienophile, and the HOMO of the dienophile interacts with the LUMO of the Dienophile.

What is a secondary orbital reaction?

The process of a diels-alder reaction wherein the orbitals of the two molecules overlap so well that the transitory state is stabilized.

Does a 4+2 adduct happen thermally or kinetically?

Thermically.

How does a 2+2 adduct occur?

Photochemically

What is a 2+2 adduct?

The reaction where two carbon atoms are added to two carbon atoms/two molecular orbitals interact with two other molecular orbitals

How does a 2+2 adduct work?

UV light excites the electrons and makes them jump up to a higher energy level, causing orbitals to overlap and the reaction to occur.