CAP-6-SISTEMAS CONJUGADOS INSATURADOS [Modo de Compatibilidade]

CAP-6-SISTEMAS CONJUGADOS INSATURADOS [Modo de Compatibilidade]

Chapter 06 Conjugated Unsaturated Systems

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Introduction

Conjugated unsaturated systems have a porbital on a carbon adjacent to a double bond

The porbital can come from another double or triple bond

The porbital may be the empty porbital of a carbocation or a porbital with a single electron in it (a radical)

Conjugation affords special stability to the molecule Conjugated molecules can be detected using UV spectroscopy

Allylic Substitution and the Allylic Radical

Reaction of propene with bromine varies depending on reaction conditions

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

conditions

At low temperature the halogen adds across the double bond

At high temperature or at very low concentration of halogen an allylic substitution occurs

Allylic Chlorination (High Temperature)

Allylic chlorination can be performed at high temperature in the gas phase

The reaction is a free radical chain reaction

In the initiation step a low concentration of chlorine radical is produced In the first propagation step an allyl radical is formed In the second propagation step the allyl radical reacts with molecular chlorine

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Allylic radicals form readily because they are more stable than ordinary primary, secondary, tertiary, or vinyl radicals This trend is reflected in their respective C-H bond dissociation energies

4 The relative stability of some carbon radicals is as follows:

Allylic Bromination with N-Bromosuccinimide

Propene undergoes allylic bromination with N-bromosuccinimide

(NBS) in the presence of light or peroxides

NBS provides a continuous low concentration of bromine for the radical reaction A low bromine concentration favors allylic substitution over alkene addition

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The radical reaction is initiated by a small amount of bromine radical formed by exposure of NBS to light or peroxides

The propagation steps for allylic bromination with NBS are:

A bromine radical reacts with propene to produce an allylic radical and HBr HBr reacts with NBS to produce a bromine molecule

A molecule of bromine reacts with a propene radical to regenerate a bromine radical

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The Stability of the Allyl Radical

Both molecular orbital theory and resonance theory can explain the stability of allyl radicals

Molecular Orbital Description of the Allyl Radical

When an allylic hydrogen is abstracted to form an allyl radical, the developing porbital on the sp2 carbon overlaps with the p orbitals of the alkene

The new porbital is conjugated with the double bond p orbitals

The radical electron and the pi pipi pielectrons of the alkene are delocalized over the entire conjugated system

Delocalization of charge and electron density leads to increased stability

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The three porbitals of the allylic system combine to form three molecular orbitals

The bonding molecular orbital contains two spin-paired electrons and this orbital increases bonding between the carbons

The nonbonding orbital contains a lone electron which is located at carbons 1 and 3 only

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Resonance Description of the Allyl Radical

The allyl radical has two contributing resonance forms

These resonance forms can be interconverted using single-barbed arrows The resonance structures are equivalent

Recall that equivalent resonance structures lead to much greater stability of the molecule than either structure alone would suggest

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The true structure of the allyl radical as suggested by resonance theory is as follows

The Allyl Cation

The allyl cation is intermediate in stability between a tertiary and secondary carbocation

The molecular orbital description of the allyl cation is very similar to the allyl radical except it contains one fewer electrons Stability arises from the delocalization of the positive charge over C1 and C3

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Resonance theory predicts that the allyl cation is a hybrid of equivalent structures D and E

Both molecular orbital theory and resonance theory suggest that structure F (below) is the best representation for the allyl cation

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Summary of Rules for Resonance

Rules for Writing Resonance Structures

Individual resonance structures are not a true representation of the real structure of a molecule A hybrid of all major resonance structures gives an indication of the true structure

Only electrons may be moved in resonance structures, not atoms

Onlypi pipi piand nonbonding electrons are moved

All resonance structures must be proper Lewis structures

All resonance structures must have the same number of paired and unpaired electrons

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

and unpaired electrons

All atoms that are part of the delocalized pi pipi pi-electron system must lie in a plane or be nearly planar

The molecule on the next slide does not behave like a conjugated diene because the large tert-butyl groups twist the structure and prevent the diene from being planar

The energy of the actual molecule is lower than the energy calculated for any one contributing resonance structure Allyl cation has much lower energy than either contributing structures 4 or 5

A system with equivalent resonance structures is particularly

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

A system with equivalent resonance structures is particularly stable

The allyl cation has two equivalent resonance structures and is therefore particularly stable

The more stable a resonance structure is, the more important it is and the more it contributes to the hybrid

Structure 6 is a more stable tertiary carbocation and contributes more than structure 7

Estimating the Relative Stability of Resonance Structures

Structures with more covalent bonds are more important Structure 8 is more important than 9 or 10

Structures in which all atoms have complete octets are more important

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

important Structure 12 is more important than structure 1

Separation of charge decreases stability Structure 13 is more important because it does not have a separation of charge

Alkadienes and Polyunsaturated Hydrocarbons

Alkadienes contain two double bonds

These are often referred to simply as dienes

Alkadiynes contain 2 triple bonds and alkenynes contain a double and a triple bond

Polyunsaturated compounds can be classified as being cumulated, conjugated or isolated

Conjugated dienes affect each other when they react, isolated double bonds react separately and do not affect each other

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

1,3-Butadiene: Electron Delocalization

Bond Lengths of 1,3-Butadiene

The double bonds of 1,3-butadiene have the expected length of regular double bonds

The central bond is much shorter than a regular carbon-carbon single bond Ethane has a carbon-carbon bond length of 1.54 Å

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The central bond in 1,3-butadiene is shorter than that in ethene for two reasons

The σ σσ σbond between C2 and C3 is made from sp2 - sp2 overlap

There is significant overlap between the C2-C3 porbitals

Conformations of 1,3-Butadiene

There are two possible planar conformations of 1,3-butadiene called s-cis and s-trans s Indicates the conformations originate from rotation around a single bond s-Trans is more stable because it is less sterically hindered

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Molecular Orbitals of 1,3-Butadiene

The first (lowest energy) pi pipi pibonding molecular orbital in 1,3- butadiene shows significant overlap of the porbitals between C2 and C3

The second pi pipi pibonding molecular orbital in 1,3-butadiene is the highest occupied molecular orbital (HOMO) and shows no overlap between C2 and C3

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The Stability of Conjugated Dienes

1,3-butadiene has a lower heat of hydrogenation by 15 kJ mol - 1 than two molecules of 1-butene

A lower heat of hydrogenation means 1,3-butadiene is more stable

These molecules can be compared directly because upon hydrogenation they lead to the same product

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Ultraviolet- Visible Spectroscopy

Conjugated compounds absorb energy in the ultraviolet (UV) and visible (Vis) regions on the electromagnetic spectrum

The wavelength of radiation absorbed and the intensity of the absorption depend on the structure of the molecule

UV- Vis Spectrophoto meters

A UV-Vis spectrum is typically measured from 200-800 nm, spanning the near UV and visible regions

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The wavelength of maximum absorption (λ λλ λ max ) is reported in units of nanometers (nm)

Molar absorptivity (ε) ε)ε) ε)is also reported ε εε εIs the intensity of the absorption

A is the observed absorbance, C is the molar concentration of the sample andlis length of the sample cell in centimeters

Example: UV absorption spectrum of 2,5-dimethyl-2,4-hexadiene in methanol at a concentration of 5.95 x 10 - 5 M in a 1.0 cm cell

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Absorption Maxima for Nonconjugated and Conjugated Dienes

In UV-Vis spectroscopy the electrons are excited from lower energy levels to higher ones

The electron is generally excited from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO)

Alkenes and nonconjugated dienes have absorptions below 200 nm because the energy difference between the HOMO and LUMO is large

In conjugated dienes these energy levels are much closer together and the wavelengths of absorption are longer than 200 nm

Ethene has λ λλ λ max at 171 nm and 1,3-butadiene has λ λλ λ max at 217 nm

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The longer the conjugated system, the smaller the energy difference between the HOMO and the LUMO

A smaller energy gap results in longer λ λλ λ max in the ultraviolet -visible spectrum β ββ β-Carotene has 1 conjugated double bonds and an absorbance maximum at 497 nm which is in the blue-green region of the visible spectrum β ββ β-Carotene is perceived as red-orange, the complementary color of blue-green

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Carbonyl compounds also absorb light in the UV region

An unshared (n) electron on oxygen is promoted to a pi pipi pi* orbital

Electrophilic Attack on Conjugated Dienes: 1,4

Addition

When 1,3-butadiene reacts with one equivalent of HCl at room temperature 78% of the 1,2 addition product and 2% of the 1,4 addition product are obtained

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

In step 1 hydrogen chloride reacts to add hydrogen to a terminal carbon which gives a stable allyl cation intermediate

Addition of hydrogen to in internal carbon leads to an unstable 1o carbocation

In step 2 chloride can react at either end of the allyl cation This leads to either 1,2 or 1,4 product

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Other electrophilic reagents add to conjugated dienes in similar fashion

Kinetic Control versus Thermodynamic Control of a Chemical Reaction

When HBr adds to 1,3-butadiene the temperature of reaction greatly affects the distribution of 1,2 and 1,4 products

Low temperature (e.g., -80o C) favors 1,2-addition product

High temperature (e.g., 40o C) favors 1,4-addition product

When the mixture of products formed at low temperature is heated, the product ratios change to favor 1,4-addition product

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Heating the 1,2-addition product leads to an equilibrium which favors the 1,4-addition product

Because equilibrium conditions favor the 1,4-addition product it must be the most stable

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

At lower temperaturesthe proportion of products is determined by the relative rates of formation of product

1,2-addition product is formed faster and is the major product at low temperatures

∆G‡ for formation of 1,2-addition product is lower than for 1,4-addition product

At low temperatures fewer molecules have enough energy to overcome the higher

∆G‡ for formation of the 1,4-addition product

The reaction is said to be under kinetic control

At higher temperatureswhen an equilibrium is established, the most stable product predominates

Enough energy is available to overcome ∆ ∆∆

∆G‡ barriers for formation of 1,2-and 1,4-

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Enough energy is available to overcome ∆ ∆∆

∆G‡ barriers for formation of 1,2-and 1,4- addition products andfor the reverse reactions

An equilibrium situation exists and the most stable product is the major one 1,4-addition product is more stable and is the major product at high temperatures The reaction is said to be under thermodynamic control

The 1,4 product is most stable because it leads to a disubstituted double bond 1,2-addition product has a less stable monosubstituted double bond

The 1,2-addition product is formed faster because the allyl cation has more δ δδ δ+ charge density at the 2o rather than the 1o carbon

The Diels-Alder Reaction: A 1,4-Cycloaddition

Reaction of Dienes

Heating 1,3-butadiene and maleic anhydride gives a 6-membered ring product in 100% yield

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The general Diels-Alder reaction forms a cylohexene product

Overall, two new σ σσ σbonds are formed at the expense of two pi pipi pi bonds

The conjugated diene is a 4pi pipi pi-electron system

The dienophile (“diene lover”) is a 2pi pipi pi-electron system

The product is called an adduct

Factors Favoring the Diels-Alder Reaction

The simplest possible example of a Diels-Alder reaction goes at very low yield and requires high temperatures

To proceed in good yield and at low temperature the dienophile should have electron-withdrawing groups

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

should have electron-withdrawing groups

It also helps if the diene has electron-releasing groups

Dienes with electron-donating groups and dienophiles with electron-withdrawing group can also react well together

Stereochemistry of the Diels-Alder Reaction

The Diels-Alder reaction is stereospecific i.e. the reaction is a syn addition, and the configuration of the dienophile is retained in the product

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

The diene must be in the s-cis conformation to react s-Trans conformation would lead to formation of a highly unstable trans bond in a 6-membered ring

Cyclic dienes which must be in the s-cis conformation are highly reactive

Cyclopentadiene is so reactive it spontaneously undergoes Diels-

Alder reaction with itself at room temperature

This dimer can be “cracked” (undergo retro-Diels-Alder reaction) by heating and the cyclopentadiene product isolated by distillation.

The Diels-Alder reaction occurs primarily in an endo rather than an exo fashion when the reaction is kinetically controlled A group that is exo in a bicyclic ring system is anti to the longest bridge

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

32 A group that is endo is on the same side as the longest bridge

Molecular Orbital Considerations that Favor an Endo Transition State

When maleic anhydride and cyclopentadiene react the major product is the endo product The major product has the anhydride linkage endo

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

When the molecules approach each other there are favorable interactions between the LUMO of maleic anhydride and the

HOMO of cyclopentadiene

In the endo orientation favorable secondary oribtal interactions between the LUMO of the carbonyl groups and the HOMO of the cyclopentadiene carbons at the C2 and C3 positions of the diene can also occur

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

Intramolecular Diels-Alder Reactions

Intramolecular reactions are those in which the reacting groups are part of the same molecule

Química Orgânica B -Reações em Sistemas ConjugadosProf. Dr. Sandro Rogério Giacomelli

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