Depending on the conditions, we will see many additional reactions whose products can be determined by either kinetic control or thermodynamic control.
No equilibrium is established so reactions that do not reverse easily are controlled.
The product with the lowest-energy transition state dominates.
If something happens to prevent equilibrium from being attained, reactions that are easily reversible are controlled.
The lowest-energy product dominates.
The product mixture contains 60 percent of product A and 40 percent of product B when added to buta-1,3-diene.
The product ratio is 10% A and 90% B when the same reaction takes place.
There is a mechanism to account for the formation of A and B.
There is a mechanism to support your prediction.
The allylic radicals are stable by resonance delocalization.
The mechanism of free-radical bromination is shown in Mechanism 15-2.
The allylic position is where substitution occurs because of the resonance-stabilized allylic radical.
Each step forms another radical that leads to products.
The bromine radical has an allylic hydrogen.
The allylic radical reacts with a bromine molecule to form an allyl bromide and a new bromine atom.
The molecule of bromine used in the allylic bromination step is regenerated.
The allylic free radical is resonance-stabilized so abstraction is preferred.
The bond-dissociation enthalpies are used to generate free radicals.
The product of the second propagation step may be either of the radical carbons or Stability of free radicals.
The double bond can be seen at either of the positions it occupies in the allylic radical.
An allylic shift in a radical reaction is similar to the 1,4-addition of an philic reagent.
The free-radical allylic bromination of but-1-ene results in a mixture of products.
Two substitution products are formed when methylenecyclohexane is treated with a low concentration of bromine.
There is a mechanism to account for their formation.
The key to substitution is to have a low concentration of bromine with light or free radicals.
A small amount of radicals can produce a fast chain reaction.
Adding bromine might raise the concentration too high, resulting in ionic addition of bromine across the double bond.
A succinimide is a four-carbon diacid.
The low concentration of Br2 is due to the fact that it reacts with HBr liberated in the substitution.
The reaction removes the by-prod uct, which prevents it from adding across the double bond.
The reaction is carried out in a clever way.
The allylic compound is dissolved in carbon tetrachloride.
It sinks to the bottom of the CCl4 solution because it is denser than CCl4.
The reaction can be initiated using a sunlamp or a radical initiator.
It is less dense than CCl4.
The sunlamp is turned off, the solution is removed, and the CCl4 is evaporated to recover the product.
A sunlamp is shone on the mixture afterbromosuccinimide is added.
Give the structure of the products.
There is a mechanism that accounts for the formation of these three products.
The allyl radical is an example of the electronic structure of allylic systems.
The pi bond between C2 and C3 is shown in one resonance form.
There is a pi bond between C1 and C2.
There is half a pi bond between C1 and C2 and half a pi bond between C2 and C3 according to the two resonance forms.
Several important features of the butadiene system are shared by these three MOs.
The first is bonding and the second is not.
We expect half of the MOs to be bonding and the other half to overlap.
The bonding was divided in a stable system.
There are no electrons in the nonbonding MOs.
The only symmetrical position for p2 is in the center of the molecule, crossing C2.
C1 and C3 both have zero overlap with C2, so p2 must be nonbonding.
A nonbonding orbital is implied by the total being zero bonding.
The lowest-energy MO has antibonding and is completely bonding.
The center carbon atom of the unpaired electron has zero electron density.
The resonance picture shows the radical electron shared equally by C1 and C3 but not C2.
The resonance and MO pictures show that the radical will react at either of the end carbon atoms.
The allyl cation lacks the unpaired electron in p2, which has half of its electron density on C1 and half on C3.
We removed half an electron from each of C1 and C3 in order to keep C2 the same.
The resonance picture shows the positive charge shared by C1 and C3.
Figure 15-12 shows the electronic configuration of the allyl anion, which is different from the allyl radical in that it has an additional electron in p2.
The allyl anion has a negative charge and a lone pair of nonbonding electrons evenly divided between C1 and C3.
The formation of a Grignard reagent is caused by the addition of 1-bromobut-2-ene to magnesium metal in dry ether.
The mixture of but-1-ene and but-2-ene is given by adding water to this Grignard reagent.
Adding water to the Grignard reagent produces the same mixture of products in the same ratios.
Allylic Halides and Tosylates show enhanced reactivity toward nucleophilic displacement reactions by the SN2 mechanism.
This rate enhancement can be explained by allylic delocalization of electrons.
The lower the energy of the transition state, the higher the rate.
The enhanced reactivity of allylic halides and tosylates makes them attractive to the eye.
Unactivated halides work well with Grignard and organolithium reagents, but they don't work well with Allylic halides.
The reaction rate increases when the overlap lowers the transition state's energy.
The prize for their work was awarded in 1950.
The one-step mechanism of the Diels-Alder reaction is shown in Key Mechanism 15-3.
Three arrows are used to show the movement of three pairs of electrons.
The Diels-Alder is a concerted mechanism.
A cyclohexene ring is created when a diene reacts with an electron-poor alkene.
The Diels-Alder reaction is similar to a nucleophile reaction.
Both the diene and the dienophile are electron-poor.
Simple dienes such as buta-1,3-diene are effective for the Diels-Alder reaction.
The reactivity of the diene may be enhanced by groups.
Simple alkenes and alkynes such as ethene and ethyne are poor dienophiles.
The pi bond has electron density pulled away from it.
The groups enhance their Diels-Alder reactivity.
Predicting the products of the Diels-Alder reactions is always done by a Diels-Alder product.
OCH3 is a single bond or triple bond.
The Diels-Alder reaction moves six electrons in a row, four in the diene and two in the dienophile.
The geometry of the Diels-Alder transition state allows us to predict the stereochemistry of the products.
Thecis conformation is used to react.
The energy difference is not enough to prevent dienes from undergoing Diels-Alder reactions.
The Diels-Alder reaction has a concerted mechanism, with all the bond making and bond breaking occurring in a single step.
The ability to participate in Diels-Alder reactions is affected by thecission.
cis and butadiene react differently.
Trans conformation react faster than butadiene.
It is reac tive in the Diels-Alder reaction.
At room temperature, cyclopentadiene reacts with itself to form dicyclopentadiene.
Conjugated Systems, Orbital Symmetry, and Ultraviolet Spectroscopy can be regenerated by heating the dimer above 200 degC.
At this temperature, the Diels-Alder reaction reverses and the more volatile cyclopentadiene monomer distills over into a cold flask.
At dry-ice temperatures, the monomer can be stored indefinitely.
The Diels-Alder reaction is an addition to both the diene and the dienophile.
The diene adds to one face of the dienophile.
The transition state in Figure 15-15 shows that there is no chance for the substituents to change their stereochemical positions during the reaction.
Substituents that are on the same side of the diene will be on the newly formed ring.
The results of this syn addition are shown in the following examples.
Secondary overlap helps to keep the transition state stable.
The influence of secondary overlap was first observed in reactions using cyclopen tadiene.
Predicting the products of many types of Diels-Alder reactions, regardless of whether they use cyclopentadiene or not, is possible with the endo rule.
The examples show the use of the rule with other reactions.
The product of the following cycloaddition can be predicted using the endo rule.
The diene will be a substitute cyclopentadiene and the product will be formed.
The two inside hydrogens were replaced with the rest of the cyclopentadiene ring.
We drew the actual product after putting them back.
Predict the major product for the Diels-Alder reaction.
The Diels-Alder reaction usually gives a single product, rather than a random mixture, when the diene and dienophile are both unsymmetrically substituted.
The major product can be predicted by considering how the substituents polarize the diene and the dienophile in their charge-separated resonance forms.
We can usually predict the correct orientation if we arrange the reactants to connect the negatively charged carbon in the diene with the positively charged carbon in the dienophile.
An electron-donating substituent (D) on the diene and an electron-drawing substituent (W) on the dienophile usually show a relationship in the product.
We don't need to draw the charge-separated resonance forms to determine which orientation of the reactants is preferred.
The major products of unsymmetrical Diels-Alder reactions can be predicted by remembering that the electron-donating groups of the diene and the electron-withdrawing groups of the dienophile usually bear either a 1,2-relationship or a 1,4-relationship in the products.
Predict the Diels-Alder reactions.
The carbonyl group is drawing electrons from the dienophile, while the methyl group is weakly electron-donating to the diene.
There are two possible orientations for these groups.
The 1,4-relationship was selected for our predicted product.