Conjugated Systems Organic Compounds That Conduct Electricity
Conjugated and Nonconjugated Dienes Compounds can have more than one double or triple bond If they are separated by only one single bond they are conjugated and their orbitals interact The conjugated diene 1,3- butadiene has properaes that are very different from those of the nonconjugated diene, 1,4- pentadiene.
The Smaller the Heat of HydrogenaAon, the More Stable the Compound Conjugated dienes are more stable than nonconjugated based on heats of hydrogenaaon
Conjugated Dienes Have Delocalized Electrons
An sp 2 sp 2 Bond Is Stronger Than an sp 2 sp 3 Bond
Carbon- Carbon Bond Length Depends on HybridizaAon
Cumulated Double Bonds
Allyl and Benzyl Allylic and Benzylic
Resonance Contributors for an Allylic CaAon Resonance Contributors for a Benzylic CaAon
RelaAve StabiliAes of CarbocaAons
Stability of Conjugated Dienes: Molecular Orbital Theory Typically by eliminaaon in allylic halide Specific industrial processes for large scale producaon of commodiaes by catalyac dehydrogenaaon and dehydraaon
Molecular Orbital DescripAon of 1,3- Butadiene The single bond between the conjugated double bonds is shorter and stronger than normal The bonding π- orbitals are made from 4 p orbitals that provide greater delocalizaaon and lower energy than in isolated C=C The 4 molecular orbitals include fewer total nodes than in the isolated case In addiaon, the single bond between the two double bonds is strengthened by overlap of p orbitals In summary, we say electrons in 1,3- butadiene are delocalized over the π bond system DelocalizaAon leads to stabilizaaon
An MO Diagram for Ethene
An MO Diagram for 1,3- Butadiene
Symmetric and AnAsymmetric Molecular Orbitals
MO Diagrams for 1,4- Pentadiene and 1,3- Butadiene
Electrophilic AddiAons to Conjugated Dienes Review: addiaon of electrophile to C=C Markovnikov regiochemistry via more stable carbocaaon
Mechanism
Double Bonds Can Have Different ReacAviAes
ReacAons of Conjugated Dienes
1,2- AddiAon and 1,4- AddiAon
CarbocaAons from Conjugated Dienes AddiAon of H + leads to delocalized secondary allylic carbocaaon
Products of AddiAon to Delocalized CarbocaAon Nucleophile can add to either caaonic site The transiaon states for the two possible products are not equal in energy
Mechanism for the ReacAon of a Conjugated Diene
Which Carbon Gets the Proton? Protonate the end that forms the more stable carbocaaon:
KineAc vs. Thermodynamic Control of ReacAons At compleaon, all reacaons are at equilibrium and the relaave concentraaons are controlled by the differences in free energies of reactants and products (Thermodynamic Control) If a reacaon is irreversible or if a reacaon is far from equilibrium, then the relaave concentraaons of products depends on how fast each forms, which is controlled by the relaave free energies of the transiaon states leading to each (Kine5c Control)
KineAc and Thermodynamic Control Example AddiAon to a conjugated diene at or below room temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct At higher temperature, the product raao changes and the 1,4 adduct predominates
If the ReacAon is Irreversible, the KineAc Product Predominates If the ReacAon is Reversible, the Thermodynamic Product Predominates
Why?
KineAc Control
Thermodynamic Control
The 1,2- Product is Always The KineAc Product DCl was used instead of HCl, so the 1,2- and 1,4- products would be different.
The Proximity Effect The proximity effect causes the 1,2- product to be formed faster.
KineAc and Thermodynamic Products Although the 1,2- product is always the kineac product, do not assume that the 1,4- product is always the thermodynamic product.
KineAc and Thermodynamic Products
The Diels- Alder CycloaddiAon ReacAon Conjugate dienes can combine with alkenes to form six- membered cyclic compounds The formaaon of the ring involves no intermediate (concerted formaaon of two bonds) Discovered by O_o Paul Hermann Diels and Kurt Alder in Germany in the 1930 s
The Mechanism a pericyclic reacaon takes place by a cyclic shic of electrons a [4+2] cycloaddiaon reacaon
View of the Diels- Alder ReacAon Woodward and Hoffman showed this reacaon to be an example of the general class of pericyclic reac3ons Involves orbital overlap, change of hybridizaaon and electron delocalizaaon in transiaon state The reacaon is called a cycloaddi3on
Faster if There is an Electron Withdrawing Group on the Dienophile
CharacterisAcs of the Diels- Alder ReacAon The alkene component is called a dienophile C=C is conjugated to an electron withdrawing group, such as C=O or C N Alkynes can also be dienophiles
The Electron Withdrawing Group Makes the Electrophile a Be_er Electrophile
Another Diels Alder ReacAon
Alkynes Can Also Be Dienophiles The cyclic product has two double bonds.
Both Reactants are Not Symmetric Two products are possible.
The Reactants Can Be Aligned in Two Ways
Which Alignment Gives The Major Product?
ConformaAons of Dienes in the Diels- Alder ReacAon The relaave posiaons of the two double bonds in the diene are cis or trans to each other about the single bond (being in a plane maximizes overlap) These conformaaons are called s- cis and s- trans ( s stands for single bond ) Dienes react in the s- cis conforma3on in the Diels- Alder reacaon
s- Cis and s- Trans ConformaAons
The Diene Must Be in an s- Cis ConformaAon
Locked in an s- Cis ConformaAon
Cis Forms Cis Trans Forms Trans
Stereospecificity of the Diels- Alder ReacAon The reacaon is stereospecific, maintaining relaave relaaonships from reactant to product There is a one- to- one relaaonship between stereoisomeric reactants and products
Regiochemistry of the Diels- Alder ReacAon Reactants align to produce endo (rather than exo) product endo and exo indicate relaave stereochemistry in bicyclic structures SubsAtuent on one bridge is exo if it is an3 (trans) to the larger of the other two bridges and endo if it is syn (cis) to the larger of the other two bridges
Endo and Exo
Bridged Bicyclic Rings and Fused Bicyclic Rings
If the Diels Alder ReacAon Creates an Asymmetric Center A racemic mixture
How to Determine the Reactants of a Diels Alder ReacAon
Diene Polymers: Natural and SyntheAc Rubbers Conjugated dienes can be polymerized The iniaator for the reacaon can be a radical, or an acid PolymerizaAon: 1,4 addiaon of growing chain to conjugated diene monomer
Natural Rubber A material from latex, in plant sap In rubber repeaang unit has 5 carbons and Z stereochemistry of all C=C Gu_a- Percha is natural material with E in all C=C Looks as if it is the head- to- tail polymer of isoprene (2- methyl- 1,3- butadiene)
SyntheAc Rubber Chemical polymerizaaon of isoprene does not produce rubber (stereochemistry is not controlled) SyntheAc alternaaves include neoprene, polymer of 2- chloro- 1,3- butadiene This resists weathering be_er than rubber
VulcanizaAon Natural and syntheac rubbers are too soc to be used in products Charles Goodyear discovered heaang with small amount of sulfur produces strong material Sulfur forms bridges between hydrocarbon chains (cross- links)