Ch 13- Conjugated Unsaturated Systems

Ch 13- Conjugated Unsaturated Systems

Introduction In chapter 8, we saw how important the pi bond was to the chemistry of unsaturated compounds In this chapter, we study a special group of unsaturated compounds, and again, we will find that the pi bond is an important part of the molecule Here, we shall examine species that have a p orbital on an atom adjacent to a double bond

The p orbital The p orbital adjacent to a double bond may be:
One that contains a single electron as in the allyl radical One that is vacant as in the allyl cation One that is part of another double bond

The p orbital Having a p orbital on an atom adjacent to a double bond allows for the formation of an extended pi bond, that is, one that encompasses more than two nuclei Molecules with delocalized pi bonds are said to be conjugated unsaturated systems

Conjugation Conjugation gives molecules special properties such as:
Increased stability Special energy absorption (UV-Vis) Unusual reactions

Allylic Substitution and the Allyl Radical
We have seen the typical reaction of Chlorine and Bromine with a double bond at low temperatures Ex This is a typical addition reaction.

Allylic Substitution and the Allyl Radical
However, when propene or any other compound with a double bond reacts with bromine or chlorine at high temperatures, we get a substitution reaction! Ex In this substitution, the halogen replaces one of the Hydrogens on the carbon adjacent to the double bond

Allylic Position The carbon adjacent to the double bond is called the Allylic position. This reaction can be referred to as an allylic substitution.

Allylic Chlorination Ex.
The mechanism is the same type that we saw with the other radical reactions: Other possible steps not seen: Vinylic radicals

Allylic/Vinylic Radicals
So now we have to add both the allylic and the vinylic radicals to our order of reactivity series It turns out that they go at opposite ends of the series: More on why later!

Allylic/Vinylic Radicals
The terms allylic and vinylic take presendence over primary, secondary, and tertiary So in our example, an allylic radical was formed, not a primary. Actually, it was a primary allylic radical.

Allylic Bromination Allylic bromination can be achieved only by using NBS If Br2 is used, the addition reaction occurs faster than the substitution Examples The mechanism is the same:

More example of allylic halogenation

Stability of Allylic Radical
The stability of the allylic radicals can be explained in two ways Actually, it’s the same way, just from two different perspectives, or theories The first is using the molecular orbital theory When the allylic hydrogen is abstracted, the carbon it is bonded to converts from an sp3 hybridization to a sp2

The unpaired electron is left in the unhybridized p orbital This p orbital that houses the unpaired electron can overlap with the p orbital of the central carbon Because of the overlap, the new p orbital is said to be conjugated with the p orbital of the double bond

The unpaired electron and the two electrons of the double bond are therefore delocalized over 3 carbons instead of just one. The central carbon is always overlapping with one of the end carbons so its electron is never available for bonding Although some delocalization occurs in primary, secondary, and tertiary radicals, the delocalization is not as effective because it occurs only through hyperconjugation with sigma bonds

The second way to describe the stability is through resonance theory Recall that structures are stabilized through resonance because the charge or radical is delocalized through space Resonance structures of propene radical: Overall hybrid structure:

The dashed lines indicate that both the double bonds are partial double bonds and that there is a pi bond encompassing all three carbon atoms. We also see the “partial” radicals are only on the terminal carbons, not the center one.

The Allylic Cation By the same reasoning used to describe the stability of the allylic radical, we can explain the stability of the allylic carboncation The new order for stability of carbocations is:

Alkadienes and Polyunsaturated hydrocarbons
Compounds with two double bonds are alkadienes Compounds with three double bonds are alkatrienes Often these types of compounds are collectively called dienes or trienes A compound with 2 triple bonds is an alkadiyne A compound with 1 double bond and one triple bond is an alkenyne.

If no other functional group is present, the double bond has priority and gets the lowest number. Remember to use E/Z or Cis/Trans when applicable for each double bond. Examples:

The double bonds of polyunsaturated compounds are classified as being cumulated, conjugated or isolated Cumulated double bonds have one carbon, the central one, that participates in two double bonds simultaneously Example:

Conjugated double bonds are one in which the double bonds and single bonds alternate, providing a continuous chain of p orbitals for the pi bonds Isolated double bonds are double bonds that contain at least one saturated, sp3, carbon between them that prevents continuous overlap of the p orbitals.

In general, cumulated dienes are less stable than isolated and conjugated dienes Isolated dienes behave just like typical double bonds They undergo all of the reactions of alkenes with the only exception being that each compound can react twice

Conjugated dienes are far more interesting because it has been found that their double bonds interact with one another This interaction leads to unexpected properties and reactions.

Properties of Conjugated Dienes
Bond lengths- 1,3-butadiene is the simplest example of a conjugated diene We see that the double bonds are of typical length when compared to the length of other double bonds However, the single bond between C2-C3 is considerably shorter than a typical single bond

Conformations- There are two possible planar conformations of 1,3-butadiene Examples: These are not true cis and trans forms since they can be interconverted through rotation about the single bond, hence the prefix s- The s-trans conformation is the predominant one at room temperature We shall see that the s-cis conformation is necessary for certain reactions

Molecular Orbitals- The central carbon atom of 1,3-butadiene are close enough for overlap to occur between the p orbitals of C2 and C3 These obitals do over lap, which gives the C2-C3 bond some double bond character, but they do not overlap as much as the C1-C2 and C3-C4 orbitals

Stability of Conjugated Dienes- Just know that conjugated dienes are thermodynamically more stable than the isolated diene forms Often times in reactions, double bonds will migrate to achieve conjugation Also, when forming double bonds, the conjugated product will always form over the unconjugated product example

Electrophilic Attack on Conjugated Dienes, 1,4-addition
Not only are conjugated dienes somewhat more stable than nonconjugated dienes, they also display special behavior when they react with electrophilic reagents For example, 1,3-butadiene reacts with 1 equivalent of HCl to produce two products:

The first product is not surprising, it is the simple addition of HCl to one of the double bonds It the second product that is unusual The double bond has moved to between C2-C3 and the HCl added to C1 and C4

The 1,4 addition and 1,2 addition products can be accounted for by examining the mechanism The first step is the typical Markovnikov addition of the hydrogen to one of the double bonds to form a carboncation: This is an allylic carbocation and therefore has a resonance structure

In the second step, the Chloride ion can attack C2 to form the 1,2 addition product or attack C4 to give the 1,4 addition product: The hydrogen must add to one of the primary carbons to form the allylic carbocation If the hydrogen were to add to one of the inner carbons, a regular primary carboncation with no resonance stability would result

1,3-butadiene show 1,4-addition with electrophilic reagents other than HCl Examples: Reaction of this type are quite general with other conjugated dienes as well and things like 1,6-addition are also possible. Example:

Take home quiz Provide a mechanism for the addition of bromine to 1,3,5-cyclooctatriene.

Kinetic Control vs Thermodynamic Control
The addition of HBr to 1,3-butadiene is interesting in another aspect The relative amounts of 1,2-addition and 1,4-addition products is dependent on the temperature at which we carry out the reaction When the reaction is done at low temperature (-80oC) we get 80% 1,2-product and 20% 1,4-product

At higher temperatures (40oC), the results are reversed with 80% 1,4-product and 20% 1,2-product formation Furthermore, when the mixture formed a low temperature is brought to the higher temperature, the 1,2-product is converted to the 1,4-product until an equilibrium is reached with amounts of each equal to that seen in the higher temperature reaction

Finally, it has also been shown that at higher temperatures and in the presence of HBr, the 1,2-addition product rearranges to the 1,4-product and that an equilibrium exists Because this equilibrium favors the 1,4-product, that product must be more stable

This reaction is an example of how the outcome of a chemical reaction can be determined in one instance by relative rates of competing reactions and in another by the relative stabilities of the final products At lower temperatures, the relative amounts of products are determined by the relative rates at which the two product occur.

The 1,2-product occurs faster so the 1,2-product is the major product at low temperatures. At higher temperatures, the relative amounts of products are determined by the position of an equilibrium The 1,4-product is more stable, so it is the major product

The behavior of 1,3-butadiene and HBr can be more fully understood by examining the energy diagram The reaction is the same in the formation of the allylic cation The regioselectivity occurs in the step in which the Bromide ion attacks

We can see that there is a lower activation energy in forming the 1,2-product Because we are at lower temperatures, very few collisions will occur with enough energy to overcome the greater activation energy to form the 1,4-product Additionally, since we are at lower temperatures, there is not enough energy for the reverse reaction to occur so we say the products form irreversibly.

Since the 1,2-product forms faster, the 1,2-product is the major product and the reaction is said to be under Kinetic control (Rate control) At higher temperatures, the intermediates have enough energy to cross both barriers More importantly, at higher temperatures, both pathways are reversible

The 1,2-product still forms faster, but since it is less stable than the 1,4-product, it also is reversed faster. Under these conditions, higher temperatures, the relative amounts of products do not reflect the relative heights of the energy barriers Instead, they reflect the relative stability of the products

Since the 1,4-product is more stable, it is formed more because the overall change from 1,2-product to 1,4-product is energetically favored Such reactions are said to be under Thermodynamic Control (Equilibrium Control) This shows that prediction of relative reaction rates made on the basis of product stabilities alone can be wrong.

This is not always the case, however. For many reactions in which a common intermediate leads to two or more products, the most stable product is formed fastest.

The Diels-Alder Reaction
The Diels-Alder reactions is a 1,4-Cycloaddition reaction of Dienes example: In general terms, the reaction is one between a conjugated diene (a 4π electron system) and a compound containing a double bond (a 2π electron system) called a dienophile The product of a Diels-Alder reaction is often called an adduct

In the reaction, 2 new sigma bonds are formed at the expense of 2 pi bonds of the diene and dienophile The adduct contains a new six-membered ring with a double bond Since sigma bonds are usually stronger than pi bonds, formation of the adduct is usually favored energetically, but most Diels-Alder reactions are reversible

The following mechanism accounts for all of the bond changes in the reaction The mechanism is a concerted mechanism meaning that all bond making and bond breaking occur simultaneously The simplest example of the Diels-Alder is 1,3-butadiene and ethene: This reaction occurs much slower than the previous example

Another example is the use of the reaction is the preparation of the anti-cancer drug taxol:

Factors favoring the Diels-Alder Reaction
In general, the reaction is favored by: 1) the presence of electron withdrawing groups on the dienophile 2) the presence of electron donating groups on the diene Examples:

Recently, it has been shown that the location of the electron withdrawing and electron donating groups on the dienophile and diene can be reversed without reducing yields of the adducts Dienes with electron withdrawing groups have been found to react readily with dienophiles containing electron donating groups

Besides the use of dienes and dienophiles that have complementary electron donating and electron withdrawing groups, other factors found to enhance the rate of Diels-Alder reactions include high temperatures and high pressure Another widely used method is the use of Lewis Acid catalysis such as AlCl3, FeBr3, etc

Stereochemistry of the Diels-Alder Reaction
When we consider the stereochemical aspects of the Diels-Alder reaction, we see why the reaction is so extraordinarily useful in synthesis 1) the Diels-Alder reaction is highly stereospecific: The reaction is a syn addition, and the configuration of the dienophile is retained in the product. Examples:

Stereochemistry of the Diels-Alder Reaction
2) The diene, of necessity, only reacts in the s-cis conformation rather than the s-trans Cyclic dienes in which the double bonds are held in the s-cis conformation are usually highly reactive in the Diels-Alder Example Cyclopentadiene is so reactive, that on standing at room temperature, it slowly undergoes a Diels-Alder reaction with itself. The reaction is reversible upon distillation

Ch 13- Conjugated Unsaturated Systems

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