Explain how to make the butadiene and other conjugated systems.
Predict which Diels-Alder reaction will give a specific synthetic product.
The color of a system.
Double bonds can communicate with each other if they are separated by one bond.
The difference between the two is that penta-1,3-diene has double bonds and penta-1,4-diene has isolated double bonds.
The interaction between the double bonds makes systems with conju gated double bonds more stable than systems with isolated double bonds.
In this chapter, we consider the unique properties of conjugated systems, the theoretical reasons for this extra stability, and some of the characteristic reactions of molecule containing double bonds.
The structures of conjugated systems can be determined with the help of ultraviolet spectroscopy.
12 kJ>mol is more stable than the monosubstituted double bond.
The heat of hydrogenation is close to the sum of the heats of hydrogenation for the individual double bonds.
The heat of hydrogenation of penta-1,4-diene is more than twice that of pent-1-ene.
The heat of hydrogenation is less than the sum for the individual double bonds.
There is a monosubstituted double bond and a disubstituted double bond inpenta-1,3-diene.
The conjugate diene has about 15 kJ>mol of extra stability.
There are double bonds in penta-1,2-diene.
The value of the heat of hydrogenation is larger than any of the others.
We conclude that the double bonds of allenes are less stable than the single bonds of allenes because of the larger heat of hydrogenation.
The relative energy of dienes compared with alkynes was based on the amount of hydrogenation.
In order to increase heat of hydrogenation, rank each group of compounds.
Explain why this rearrangement is favorable.
The two ends of allene are parallel.
There are two enantiomers of penta-2,3-diene.
A model can be helpful.
The compound with double bonds is more stable than a compound with isolated double bonds.
The simplest conjugated diene is buta-1,3-diene.
The heat of hydrogenation of buta-1,3-diene is less than that of but-1-ene, which has a resonance energy of 15 kJ>mol.
A carbon-carbon single bond in an alkane is 1.54 A shorter than a C3 bond in buta-1,3-diene.
The most important cause of this short bond is its pi bonding overlap and partial double-bond character.
The normal length of a single bond and that of a double bond is intermediate.
To represent the bonding accurately, we must consider the entire system, not just one bond at a time.
The green and blue color of the lobes will be used to emphasize the phase difference.
The combination of two atomic orbitals must be perfect.
Constructive overlap is an important feature of bonding.
There are two electrons in the ground state of ethylene.
Stable Molecules tend to have filled bonding and empty antibonding MOs.
The number of molecular orbitals is the same as the number of atomic Stable molecules.
Half of them are antibonding.
The buta-1,3-diene is ready to be constructed.
Two of the MOs are bonding and the other two are antibonding.
The straight-line representation of buta-1,3-diene makes it easier to draw and visualize the orbitals.
The p1 bonding MO is buta-1,3-diene.
Bonding interac tions are always part of the lowest-energy molecular orbital.
There are three bonding interactions, and the electrons are delocalized over four nuclei, which makes this lowest-energy orbital extremely stable.
The conjugated system is more stable than two isolated double bonds.
The second molecule of butadiene has a single vertical point in the center.
The classic picture of a diene is represented by this MO.
The C1:C2 and C3:C4 bonds have bonding and antibonding interactions.
There are two bonding interactions and one antibonding interaction in the p2 orbital.
It is not as strong as the all-bonding p1 orbital.
Adding and subtracting bonding and antibonding interactions is not a reliable method for calculating the energies of orbitals.
Predicting whether a given orbital is bonding or antibonding is useful.
The third butadiene MO has two.
The p2 bonding is made of buta-1,3-diene.
The antibonding orbital is vacant in the ground state.
The fourth, and last, molecule of buta-1,3-diene is antibonding.
The molecule's ground state is empty and it has the highest energy.
Most systems have antibonding interactions between all pairs of adjacent atoms.
Butadiene has four pi electrons, two of which are in the double bonds in the Lewis structure.
The lowest-energy MOs are filled first.
Four pi electrons go into p1 and p2.
Both bonding and antibonding MOs are empty.
This arrangement of filled bonding and vacant antibonding orbitals is found in most stable molecules.
The partial double-bond character between C2 and C3 in buta-1,3-diene explains why the molecule is most stable.
overlap between C2 and C3 can be achieved with two planar conformations.
There is interference between the two hydrogen atoms.
The rotation of a double bond in an alkene takes about 250 kJ>mol and the -cis takes about 29 kJ>mol.
Trans conformers of butadiene are easy to convert at room temperature.
Some of the reactions involve intermediates that retain some of the resonance stabilization of the conjugated system.
allylic cations and radicals are common intermediates.
Delocalization is the key to stabilizing Allylic cations and radicals.
First, we consider some reactions involving allylic cations and radicals, and then we derive a picture of their bonding.
This terminology is used by many common names.
The positive charge over two carbon atoms is delocalized by resonance with the adjacent double bond.
The positive charge is shared by another carbon atom and each of the substituted allylic cations shown in the preceding figure.
Do you think your second resonance form is more important than the first structure?
Two products are formed when 3-bromo-1-methylcyclohexene undergoes solvolysis.
There is a mechanism that accounts for these products.
We can represent a delocalized ion by either resonance forms or a combined structure.
The combined structure tries to convey all the information implied by two or more resonance forms, which can be confusing.
The allyl cation is stable because of its resonance stabilization.
Most substituted allylic cations have at least one secondary carbon atom.
The rates of reactions that involve carbocation intermediates are enhanced by resonance stabilization.
allylic cations are usually used as intermediates inphilic additions.
An allylic cation can react with a nucleophile at either of its positive centers.
There is a mixture of two constitutional isomers with the addition of buta-1,3-diene.
The second product adds the bromide and protons at the end of the conjugated system to carbon atoms.
The mechanism is similar to others.
The most stable carbocation is given by the addition of the alkene and the protons.
The allylic cation is stable by resonance delocalization of the positive charge over two carbon atoms.
The two carbon atoms sharing the positive charge can be attacked by bromide.
Attack at the primary carbon gives 1,4-addition.
A resonance-stabilized allylic cation is formed by the destruction of one of the double bonds.
The key to forming these two products is the presence of a double bond.
resonance-stabilized intermediates are likely to react with double bonds.
Ionizing can be promoted by the treatment of an alkyl halide with AgNO3 in alcohol.
Two isomeric ethers are formed when 4-chloro-2-methylhex-2-ene reacts with AgNO3 Suggest structures and come up with a mechanism for their formation.
Show how the observed mixture of products are formed by proposing a mechanism for each reaction.
The reaction of buta-1,3-diene with HBr has an interesting effect on the products.
The 1,2-addition product dominates if the reagents are allowed to react briefly.
The composition favors the 1,4-addition product if this reaction mixture is later allowed to warm to 40 degC, or if the original reaction is carried out at 40 degC.
The most stable product is not always the major product.
The 1,4-product is expected to be more stable because it has a more substituted double bond.
This prediction is supported by the fact that the reaction mixture is allowed to equilibrate when it is warmed to 40 degC.
The diagram shows why one product is favored at low temperatures and another at higher temperatures.
The diagram has the allylic cation in the center, which can give the left or right product.
The initial product depends on where bromide attacks the allylic cation.
The two carbon atoms share a positive charge.
Attack at the primary carbon gives 1,4-addition and attack at the secondary carbon gives 1,4-addition.
The more substituted secondary carbon bears more of the positive charge because it is better stable than the primary carbon.
Attack by bromide on the allylic cation is a strongly exothermic process, so the reverse reaction has a large activation energy.
The rate of the reverse reaction is almost zero at -80 degC.
The product that is formed faster is the one that dominates.
There is enough energy in a lot of the collisions to make a reverse reaction.
The reverse of the 1,4-addition has less activation energy than the reverse of the 1,2-addition.
The 1,4-product is formed faster than the 1,2-product.
The concentration of each species is determined by the relative energy of each species.