Conjugated Systems Nomenclature, Properties and Applications Dr. Sapna Gupta Locations of Double Bonds: Definitions • Conjugated • double bonds are separated by one single bond. • 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 alkene which is closest to the terminal carbon and in the longest chain. • Two double bonds: diene; three: triene; four: tetraene etc. • Where applicable you should use the cis/trans or E/Z configuration. Energy m ax Name Structural Formula (nm) [kJ (kcal)/mol] Ethylene 165 724 (173) 1,3-Butadiene 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 Alkenes 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 Polymers: Natural and Synthetic Rubbers • Conjugated dienes can be polymerized • The initiator for the reaction can be a radical, 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 polymer 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 hydrocarbon chains (cross-links) Conjugated System - Intro 13 Conjugation, Color, and the Chemistry of Vision • Visible region is about 400 to 800 nm • Extended systems of conjugation absorb in visible region • b-Carotene, 11 double bonds in conjugation, max = 455 nm • Visual pigments are responsible for absorbing light 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.