1. Alkenes and alkynes The chemistry of unsaturation

2. Alkene structure Unsaturated hydrocarbons – those with one or more double or triple bonds. General formula is C n H 2n (for one double bond) Trigonal planar geometry, sp 2 hybridization Bond angles close to 120º No rotation about C=C

3. Alkyne structure One or more triple bonds Linear geometry 180º bond angles sp hybridization One sigma and two pi bonds H-C ≡ C-H Ethyne (acetylene)

4. Nomenclature Position of double bond is given by number of first doubly bonded carbon 1-butene CH 2 =CHCH 2 CH 3 2-butene CH 3 CH=CHCH 3 Longest chain always includes double/triple bonds

5. Cis-trans isomerism trans-2-butenecis-2-butene E-Z notation

6. E-Z Notation Cahn-Ingold sequence priority – priority is given to higher MW substituents Higher priority substituents on same side – Z isomer (zusammen, or together) Z-2-chloro-2-butene

7. Nomenclature Higher priority substituents on opposite sides – E isomer (entgegen, against) E-2-chloro-2-butene

8. Nomenclature Cycloalkenes – double bonded carbons are numbers 1 & 2 4-methylcyclohexene (A number for the double bond is not needed)

9. Nomenclature Dienes, trienes and polyenes Prefixes di=2, tri=3, etc. are used to indicate the number of double bonds Each double bond gets its own location number 3-chloro-1,3-pentadiene

10. Nomenclature 2-methyl-1,4-cyclohexadiene

11. Physical properties Physical properties of alkenes are similar to those of alkanes Alkene natural products: Terpenes Oligomers of isoprene (2-methyl-1,3- butadiene)

12. Terpenes Essential oils – two isoprene units – monoterpenes

13. Terpenes Sesquiterpenes – three isoprenes Tetraterpenes (8 isoprenes) b-carotene (precursor to vitamin A)

14. Reactions of alkenes Addition reactions Addition of hydrogen halides to form alkyl halides CH 2 =CH 2 + HBr  CH 3 CH 2 Br Mechanism –Hydrogen halides are polar –The positive end is attracted to the electrons in the double bond

15. Mechanism of hydrohalogenation protonation step nucleophile – double bond 1º (primary) carbocation electrophile – H of HCl (unfavorable) 

16. Mechanism of hydrohalogenation chloroethane Carbocation mechanism! 

17. Details of Carbocation Mechanisms motion of 1 e- motion of 2 e- Order of stability of carbocations: 3º>2º>1º Primary

18. Details of Carbocation Mechanisms Secondary Tertiary

19. Details of Carbocation Mechanisms More substituted carbocations are more stable because alkyl groups are slightly electron donating, and they stabilize the carbocation by diluting the positive charge. Most stable carbocation possible will be formed

20. Details of Carbocation Mechanisms Tertiary carbocation is always formed – never the secondary 2-methyl-2-butene: two possible sites for carbocation formation

21. Details of Carbocation Mechanisms Markovnikov’s rule – in hydrohalide addition, the hydrogen adds to the carbon that already has the most hydrogens bonded to it. The halogen adds to the carbon that has the most carbons attached (the location of the positive charge)

22. Addition of water Acid catalyzed addition of water to form alcohols

23. Addition of water Proceeds by a carbocation mechanism Follows Markovnikov’s rule (“gives the Markovnikov product”) Mechanism 

24. Addition of water  protonated ethanol

25. Addition of water ethanol

26. Addition of halogens Addition of halogens to form vicinal dihalides CH 2 =CH 2 + Br 2  BrCH 2 CH 2 Br Carried out in pure reagent, CCl 4 or other inert solvent Additions to cyclic alkenes always give the trans product (“anti” addition) Discoloration of bromine solutions is a test for alkenes

27. Addition of hydrogen Addition of hydrogen (reduction) to form alkanes – catalytic hydrogenation Powdered metal catalyst is used – usually Pd, Pt, Ni, Ru H 2 is used under high pressure

28. Polymerization reactions Polymers are long chains of identical units called monomers nCH 2 =CH 2 + initiator  ( ─ CH 2 CH 2 ─ ) n ethane polyethylene

29. Mechanism of free radical polymerization Free radicals are species with one unpaired electron Formed by heterolytic bond cleavage Initiation

30. Mechanism of free radical polymerization Propagation Termination

31. Mechanism of free radical polymerization

32. Polyethylenes Low density polyethylene (LDPE) Highly branched, clear, low melting –Made via radical mechanism –Used for packaging, trash bags High density polyethylene (HDPE) –Linear, opaque, high melting –Made via an ionic mechanism –Used for milk and water jugs, grocery bags, squeezable bottles