1. Physical and Chemical Properties and Reactions of Alkenes and Alkynes CHAPTER SEVEN TERRENCE P. SHERLOCK BURLINGTON COUNTY COLLEGE 2004 CHE-240 Unit 3

2. Chapter 72 Orbital Description Sigma bonds around C are sp 2 hybridized. Angles are approximately 120 degrees. No nonbonding electrons. Molecule is planar around the double bond. Pi bond is formed by the sideways overlap of parallel p orbitals perpendicular to the plane of the molecule. =>

3. Chapter 73 Pi Bond Sideways overlap of parallel p orbitals. No rotation is possible without breaking the pi bond (63 kcal/mole). Cis isomer cannot become trans without a chemical reaction occurring. =>

4. Chapter 74 Elements of Unsaturation A saturated hydrocarbon: C n H 2n+2 Each pi bond (and each ring) decreases the number of H’s by two. Each of these is an element of unsaturation. To calculate: find number of H’s if it were saturated, subtract the actual number of H’s, then divide by 2. =>

5. Chapter 75 Propose a Structure: First calculate the number of elements of unsaturation. Remember:  A double bond is one element of unsaturation.  A ring is one element of unsaturation.  A triple bond is two elements of unsaturation. => for C 5 H 8

6. Chapter 76 Heteroatoms Halogens take the place of hydrogens, so add their number to the number of H’s. Oxygen doesn’t change the C:H ratio, so ignore oxygen in the formula. Nitrogen is trivalent, so it acts like half a carbon. C H H C H H NC H HH =>

7. Chapter 77 Structure for C 6 H 7 N? Since nitrogen counts as half a carbon, the number of H’s if saturated is 2(6.5) + 2 = 15. Number of missing H’s is 15 – 7 = 8. Elements of unsaturation is 8 ÷ 2 = 4. =>

8. Chapter 78 IUPAC Nomenclature Parent is longest chain containing the double bond. -ane changes to -ene. (or -diene, -triene) Number the chain so that the double bond has the lowest possible number. In a ring, the double bond is assumed to be between carbon 1 and carbon 2. =>

9. Chapter 79 Name These Alkenes 1-butene 2-methyl-2-butene 3-methylcyclopentene 2-sec-butyl-1,3-cyclohexadiene 3-n-propyl-1-heptene =>

10. Chapter 710 Name these: trans-2-pentene cis-1,2-dibromoethene =>

11. Chapter 711 E-Z Nomenclature Use the Cahn-Ingold-Prelog rules to assign priorities to groups attached to each carbon in the double bond. If high priority groups are on the same side, the name is Z (for zusammen). If high priority groups are on opposite sides, the name is E (for entgegen). =>

12. Chapter 712 Example, E-Z 1 2 1 2 2Z2Z 2 1 1 2 5E5E (2Z, 5E)-3,7-dichloro-2,5-octadiene =>

13. Chapter 713 Commercial Uses: Ethylene =>

14. Chapter 714 Commercial Uses: Propylene =>

15. Chapter 715 Other Polymers =>

16. Chapter 716 Substituent Effects More substituted alkenes are more stable. H 2 C=CH 2 < R-CH=CH 2 < R-CH=CH-R < R-CH=CR 2 < R 2 C=CR 2 unsub. < monosub. < disub. < trisub. < tetra sub. Alkyl group stabilizes the double bond. Alkene less sterically hindered. =>

17. Chapter 717 Cycloalkene Stability Cis isomer more stable than trans. Small rings have additional ring strain. Must have at least 8 carbons to form a stable trans double bond. For cyclodecene (and larger) trans double bond is almost as stable as the cis. =>

18. Chapter 718 Alkene Synthesis Overview E2 dehydrohalogenation (-HX) E1 dehydrohalogenation (-HX) Dehalogenation of vicinal dibromides (-X 2 ) Dehydration of alcohols (-H 2 O) =>

19. Chapter 719 Removing HX via E2 Strong base abstracts H + as X - leaves from the adjacent carbon. Tertiary and hindered secondary alkyl halides give good yields. Use a bulky base if the alkyl halide usually forms substitution products. =>

20. Chapter 720 Some Bulky Bases (CH 3 CH 2 ) 3 N : triethylamine =>

21. Chapter 721 Hofmann Product Bulky bases abstract the least hindered H + Least substituted alkene is major product. =>

22. Chapter 722 E2: Cyclohexanes Leaving groups must be trans diaxial. =>

23. Chapter 723 E2: Vicinal Dibromides Remove Br 2 from adjacent carbons. Bromines must be anti-coplanar (E2). Use NaI in acetone, or Zn in acetic acid. Br CH 3 H Br CH 3 H =>

24. Chapter 724 Removing HX via E1 Secondary or tertiary halides Formation of carbocation intermediate Weak nucleophile Usually have substitution products too =>

25. Chapter 725 Dehydration of Alcohols Reversible reaction Use concentrated sulfuric or phosphoric acid, remove low-boiling alkene as it forms. Protonation of OH converts it to a good leaving group, HOH Carbocation intermediate, like E1 Protic solvent removes adjacent H + =>

26. Chapter 726 Dehydration Mechanism =>

27. Chapter 727 POWER POINT IMAGES FROM “ORGANIC CHEMISTRY, 5 TH EDITION” L.G. WADE ALL MATERIALS USED WITH PERMISSION OF AUTHOR PRESENTATION ADAPTED FOR BURLINGTON COUNTY COLLEGE ORGANIC CHEMISTRY COURSE BY: ANNALICIA POEHLER STEFANIE LAYMAN CALY MARTIN