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Organic Compounds That Conduct Electricity

Conjugated Systems

• 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.

Conjugated and Nonconjugated Dienes

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 sp2–sp2 Bond Is Stronger Than an sp2–sp3 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

• Typically by eliminaAon in allylic halide • Specific industrial processes for large scale producAon of commodiAes by catalyAc dehydrogenaAon and dehydraAon

Stability of Conjugated Dienes: Molecular Orbital Theory

• 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

Molecular Orbital DescripAon of 1,3-‐Butadiene

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

• Review: addiAon of electrophile to C=C – Markovnikov regiochemistry via more stable carbocaAon

Electrophilic AddiAons to Conjugated Dienes

Mechanism

Double Bonds Can Have Different ReacAviAes

ReacAons of Conjugated Dienes

1,2-‐AddiAon and 1,4-‐AddiAon

• AddiAon of H+ leads to delocalized secondary allylic carbocaAon

CarbocaAons from Conjugated Dienes

• Nucleophile can add to either caAonic site • The transiAon states for the two possible products are not equal

in energy

Products of AddiAon to Delocalized CarbocaAon

Mechanism for the ReacAon of a Conjugated Diene

Which Carbon Gets the Proton?

Protonate the end that forms the more stable carbocaAon:

• 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 vs. Thermodynamic Control of ReacAons

• 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

KineAc and Thermodynamic Control Example

If the ReacAon is Irreversible, the KineAc Product Predominates

If the ReacAon is Reversible, the Thermodynamic Product Predominates

KineAc Control

Thermodynamic Control

DCl was used instead of HCl, so the 1,2-‐ and 1,4-‐products would be different.

The 1,2-‐Product is Always The KineAc Product

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

• 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 Diels-‐Alder CycloaddiAon ReacAon

The Mechanism

a pericyclic reacAon takes place by a cyclic shic of electrons

a [4+2] cycloaddiAon 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

View of the Diels-‐Alder ReacAon

Faster if There is an Electron Withdrawing Group on the Dienophile

• 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

CharacterisAcs of the Diels-‐Alder ReacAon

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?

• 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

ConformaAons of Dienes 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

• The reacAon is stereospecific, maintaining relaAve relaAonships from reactant to product – There is a one-‐to-‐one relaAonship between stereoisomeric reactants and products

Stereospecificity 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

Regiochemistry of the Diels-‐Alder ReacAon

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

• 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

Diene Polymers: Natural and SyntheAc Rubbers

• 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)

Natural 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

SyntheAc Rubber

• 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)

VulcanizaAon

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