1 / 19

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

slade
Download Presentation

Chapter 7 Alkenes: Structure and Reactivity

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related