2010SpringB_[Compatibility_Mode] - 1 Alkene - Hydrocarbon...

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Unformatted text preview: 1 Alkene - Hydrocarbon With C-C Double Bond Also called an olefin but alkene is better Includes many naturally occurring material Includes many naturally occurring materials Flavors, fragrances, vitamins Important industrial products These are feedstocks for industrial processes 1 Degree of Unsaturation Relates molecular formula to possible structures Degree of unsaturation: number of multiple bonds or rings Formula for saturated a acyclic compound is C n H 2n+2 Each ring or multiple bond replaces 2 H's 2 2 Example: C 6 H 10 Saturated is C H C C H H H H C 6 H 14 Therefore 4 H's are not present This has two degrees of unsaturation H 3 C C C C C CH 3 H H H H 3 Two double bonds? or triple bond? or two rings or ring and double bond Degree of Unsaturation With Other Elements Degree of Unsaturation = (C+1) (H + X N) / 2 4 3 Naming of Alkenes Find longest continuous carbon chain containing the double bond Number carbons in chain so that double bond carbons have lowest possible numbers Rings have cyclo prefix Rings have cyclo prefix 5 Many Alkenes Are Known by Common Names Ethylene = ethene Propylene = propene Propylene propene Isobutylene = 2- methylpropene Isoprene = 2-methyl-1,3- butadiene 6 4 Practice 7 2,3,3-trimethyl-1,4,6-octatriene 4-tert-butyl-2-methyl-2-heptene Electronic Structure of Alkenes Carbon atoms in a double bond are sp 2-hybridized Three equivalent orbitals at 120 separation in plane Fourth orbital is atomic p orbital Combination of electrons in two sp 2 orbitals of two atoms forms bond between them Additive interaction of p orbitals creates a bonding orbital Subtractive interaction creates a anti-bonding orbital Occupied orbital prevents rotation about -bond Rotation prevented by bond - high barrier, about 268 kJ/mole in ethylene This prevents rotation about a carbon- carbon double bond (unlike a carbon- 8 carbon double bond (unlike a carbon carbon single bond). Creates possible alternative structures 5 Cis-Trans Isomerism in Alkenes The presence of a carbon- carbon double can create two possible structures cis isomer - two similar groups on same side of the double bond trans isomer similar groups on opposite sides Each carbon must have two different groups for these isomers to occu 9 isomers to occur Cis, Trans Isomers Require That End Groups Must Differ in Pairs 180rotation superposes Bottom pair cannot be superposed without breaking C=C X 10 X 6 Sequence Rules: The E,Z Designation Neither compound is clearly cis or trans Substituents on C1 are different than those on C2 We need to define similarity in a precise way to distinguish the two stereoisomers Cis, trans nomenclature only works for disubstituted double 11 works for disubstituted double bonds Ranking Priorities: Cahn-Ingold-Prelog Rules Higher atomic number gets higher priority...
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This note was uploaded on 04/28/2010 for the course ORGANIC organic taught by Professor Nikles during the Spring '10 term at University of Alabama at Birmingham.

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2010SpringB_[Compatibility_Mode] - 1 Alkene - Hydrocarbon...

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