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Conjugated Systems Nomenclature, Properties and Applications Dr. Sapna Gupta Locations of Double Bonds: Definitions

• Conjugated • double bonds are separated by one . • Example: 1,3-pentadiene. • Isolated • double bonds are separated by two or more single bonds. • Example: 1,4-pentadiene. • Cumulated • double bonds are on adjacent carbons. • Example: 1,2-pentadiene.

Conjugated System - Intro 2 Nomenclature

• Start the numbering from the which is closest to the terminal carbon and in the longest chain. • Two double bonds: ; three: triene; four: tetraene etc. • Where applicable you should use the cis/trans or E/Z configuration.

Energy m ax Name (nm) [kJ (kcal)/mol]

Ethylene 165 724 (173) 1,3- 217 552 (132) (3E)-1,3,5-Hexatriene 268 448 (107) (3E,5E)-1,3,5,7-Octatetraene 290 385 (92)

Conjugated System - Intro 3 Stability of

Depends on location of double bonds. Conjugated are most stable and cumulated are least.

Conjugated System - Intro 4 Constructing Molecular Orbitals

• Pi molecular orbitals are the sideways overlap of p orbitals. • p orbitals have 2 lobes. Plus (+) and minus (-) indicate the + opposite phases of the wave function, not electrical charge. -

• When lobes overlap constructively, (+ and +, or - and -) a bonding MO is formed. + +

- -

• When + and - lobes overlap, waves cancel out and a node forms; antibonding MO. - +

nodes - +

Conjugated System - Intro 5 Ethylene Pi MO’s

• The combination of two p orbitals must give two molecular orbitals. • Constructive overlap is a bonding MO. • Destructive overlap is an anti-bonding MO.

Conjugated System - Intro 6 MO Diagram of 1,3-Butadiene

• Most stable conformation is planar. • Single bond is shorter than 1.54 Å. • Electrons are delocalized over molecule

Conjugated System - Intro 7 4* MO for 1,3-Butadiene • All antibonding interactions. • Highest energy. • Vacant at ground state.

3* MO for 1,3-Butadiene • Antibonding MO. • Empty at ground state. • Two nodes.

2 MO for 1,3-Butadiene • 2 bonding interactions. • 1 antibonding interaction. • A bonding MO.

1 MO for 1,3-Butadiene • Lowest energy. • All bonding interactions. • Electrons are delocalized over four nuclei. Conjugated System - Intro 8 MO Energy Diagram

The average energy of electrons is lower in the conjugated compound.

Conjugated System - Intro 9 Diene : Natural and Synthetic Rubbers

• Conjugated can be polymerized • The initiator for the reaction can be a , or an acid • Polymerization: 1,4 addition of growing chain to conjugated diene monomer

Conjugated System - Intro 10 Natural Rubber • A material from latex, in plant sap • In rubber repeating unit has 5 carbons and Z stereochemistry of all C=C • Gutta-Percha is natural material with E in all C=C • Looks as if it is the head-to-tail of isoprene • Isoprene: 5 carbon fragment with two double bonds (2-methyl-1,3- butadiene) – but the 5 C skeleton is the key for isoprene.

Conjugated System - Intro 11 Synthetic Rubber

• Chemical polymerization of isoprene does not produce rubber (stereochemistry is not controlled) • Synthetic alternatives include neoprene, polymer of 2-chloro-1,3- butadiene • This resists weathering better than rubber

Conjugated System - Intro 12 Vulcanization • Natural and synthetic rubbers are too soft to be used in products • Charles Goodyear discovered heating with small amount of sulfur produces strong material • Sulfur forms bridges between chains (cross-links)

Conjugated System - Intro 13 Conjugation, Color, and the of Vision • Visible region is about 400 to 800 nm • Extended systems of conjugation absorb in visible region

• b-, 11 double bonds in conjugation, max = 455 nm • Visual are responsible for absorbing in eye and triggering nerves to send signal to brain

Conjugated System - Intro 14 Key Concepts

• Conjugated Systems • Stability of alkenes • MO orbitals

Conjugated System - Intro 15