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Chapter 7 Alkenes: Structure and Reactivity

Chapter 7 Alkenes: Structure and Reactivity. 7.2 Calculating Degree of Unsaturation. Relates molecular formula to possible structures Degree of unsaturation : number of multiple bonds or rings Formula for a saturated acyclic compound is C n H 2n+2

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Chapter 7 Alkenes: Structure and Reactivity

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  1. Chapter 7Alkenes: Structure and Reactivity

  2. 7.2 Calculating Degree of Unsaturation • Relates molecular formula to possible structures • Degree of unsaturation: number of multiple bonds or rings • Formula for a saturated acyclic compound is CnH2n+2 • Alkene has fewer hydrogens than an alkane with the same number of carbons —CnH2n because of double bond • Each ring or multiple bond replaces 2 H's

  3. Example: C6H10 • Saturated is C6H14 • therefore 4 H's are not present • This has two degrees of unsaturation • Two double bonds? • or triple bond? • or two rings? • or ring and double bond?

  4. Degree of Unsaturation With Other Elements • Organohalogens (X: F, Cl, Br, I) • Halogen replaces hydrogen • C4H6Br2 and C4H8 have one degree of unsaturation

  5. Degree of Unsaturation (Continued) • Organoxygen compounds (C,H,O) – Oxygen forms 2 bonds • these don't affect the formula of equivalent hydrocarbons • May be ignored in calculating degrees of unsaturation

  6. Organonitrogen Compounds • Nitrogen has three bonds • So if it connects where H was, it adds a connection point • Subtract one H for equivalent degree of unsaturation in hydrocarbon

  7. Summary - Degree of Unsaturation • Count pairs of H's below CnH2n+2 • Add number of halogens to number of H's (X equivalent to H) • Ignore oxygens (oxygen links H) • Subtract N's - they have two connections

  8. 7.4 Cis-Trans Isomerism in Alkenes • Carbon atoms in a double bond are sp2-hybridized • Three equivalent orbitals at 120º separation in plane • Fourth orbital is atomic p orbital • Combination of electrons in two sp2 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

  9. Cis-Trans Isomerism in Alkenes (Continued) • Rotation of  bond is prohibitive • This prevents rotation about a carbon-carbon double bond (unlike a carbon-carbon single bond). • Creates possible alternative structures

  10. Cis-Trans Isomerism in Alkenes (Continued) • the presence of a carbon-carbon double bond 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 occur

  11. Cis-Trans Isomerism in Alkenes (Continued) • Cis-Trans Isomerization requires that end groups differ in pairs • Bottom pair cannot be superposed without breaking C=C

  12. 7.5 Alkene Stereochemistry and the E,Z Designation • Cis-Trans naming system discussed thus far only works with disubstituted alkenes • Tri- and Tetra substituted double bonds require more general method • Method referred to as the E,Z system

  13. Alkene Stereochemistry and the E,Z Designation (Continued): E,Z Stereochemical Nomenclature • Priority rules of Cahn, Ingold, and Prelog • Compare where higher priority groups are with respect to bond and designate as prefix • E -entgegen, opposite sides • Z - zusammen, together on the same side

  14. Alkene Stereochemistry and the E,Z Designation (Continued): Cahn-Ingold-Prelog Rules RULE 1 • Must rank atoms that are connected at comparison point • Higher atomic number gets higher priority • Br > Cl > S > P > O > N > C > H

  15. Alkene Stereochemistry and the E,Z Designation (Continued): Cahn-Ingold-Prelog Rules RULE 2 • If atomic numbers are the same, compare at next connection point at same distance • Compare until something has higher atomic number • Do not combine – always compare

  16. Alkene Stereochemistry and the E,Z Designation (Continued): Cahn-Ingold-Prelog Rules RULE 3 • Multiple-bonded atoms are equivalent to the same number of single-bonded atoms • Substituent is drawn with connections shown and no double or triple bonds • Added atoms are valued with 0 ligands themselves

  17. 7.6 Stability of Alkenes • Cis alkenes are less stable than trans alkenes • Compare heat given off on hydrogenation: Ho • Less stable isomer is higher in energy • And gives off more heat • hyperconjugation of R groups stabilize the pi bond

  18. Effects of Increasing Alkene Substitution • Increasing substitution on alkene carbon atoms increases stability. • This will become very important when we begin to look at possible products resulting from elimination reactions.

  19. Stability of Alkenes (Continued): Hyperconjugation • Electrons in neighboring filled  orbital stabilize vacant antibonding  orbital – net positive interaction • Alkyl groups are better than H

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