1. Alkenes and Cycloalkenes
Bettelheim, Brown Campbell and Farrell
Chapter 12

2. Alkenes and Alkynes
Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds
ethylene is the simplest alkene
Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds
acetylene is the simplest alkyne

3. Review of Hybrid Orbitals
Sigma bonds formed on axis between nuclei
pi bonds formed between overlapping p orbitals (above and below internuclear axis)

4. sp orbitals
Figure: 09-16

5. sp2 orbitals
Figure: 09-18

6. Figure: 09-19
sp3 orbitals

7. Double Bond 2 sp2 orbitals (one from each atom) overlap
The resulting bond is a sigma bond
Additionally, the unused p orbitals overlap to form pi bond (with two lobes—one above and one below the sigma bond)

8. pi bonds form between overlapping p orbitals
Two lobes formed--above and below internuclear axis
Figure: 09-22

9. Figure: 09-24

10. Alkene pi bond
Figure: 09-25

11. Figure: 09-27EX9.6

12. Alkyne has two pi bonds and one sigma bond (on axis between nuclei)
Figure: 09-26

13. Alkenes and Alkynes
Alkene: a hydrocarbon that contains one or more carbon-carbon double bonds
ethylene is the simplest alkene
Alkyne: a hydrocarbon that contains one or more carbon-carbon triple bonds
acetylene is the simplest alkyne

14. Alkenes Cis-trans isomerism
because of restricted rotation about a carbon-carbon double bond, an alkene with two different groups on each carbon of the double bond shows cis-trans isomerism

15. Alkenes
Structure:
VSEPR model predicts bond angles of 120° about each carbon of a double bond
Actual angles in ethylene are close to 120°
Angles about each carbon of the double bond in substituted alkenes may be greater than 120° because of repulsion of alkyl groups bonded to the double bond

16. Alkenes - IUPAC Names To name an alkene
Parent name is that of the longest chain that contains the C=C
Number the chain to give the lower numbers to the carbons of the C=C
Locate the C=C by the number of its first carbon
Add -ene ending to show the presence of the C=C
Branched-chain alkenes are named in a manner similar to alkanes. Name and locate substituents.

17. Alkenes - IUPAC Names
Examples

18. Alkynes - IUPAC Names
Use same rules as for alkenes, but use the ending -yne to show the presence of the triple bond

19. Name the following compound

20. Common Names
Common names are still used for some alkenes and alkynes, particularly those of low molecular weight

21. Cycloalkenes To name a cycloalkene
Number the carbon atoms in ring so that the double bond is between C1 and C2
Give lower number to the substituent encountered first
Number and list substituents in alphabetical order

22. Dienes, Trienes, Polyenes
Alkenes with more than one double bond are named as alkadienes, alkatrienes, etc.
Compounds with several double bonds are referred to more generally as polyenes (Greek: poly, many)

23. Name the following

24. Name the following

27. Physical Properties of Alkenes and Alkynes
Nonpolar compounds
London dispersion forces between molecules
Physical properties are similar to alkanes
Insoluble in water
Soluble in nonpolar organic liquids
Liquid or solid have low densities
Float on water
Density less than 1 g/mL

28. Terpenes
Terpene: a compound whose carbon skeleton can be divided into five-carbon units identical with the carbon skeleton of isoprene
Example of an important principle of the molecular logic of living systems
Small subunits are combined (and modified) to make larger molecules
In nature, reactions carried out by enzymes (catalysts)

29. Examples of Terpenes

30. Chemical Properties Addition Reactions
Add two atoms (or groups) to a double bond
Break double bond
One atom (or group) added to each carbon
May require catalyst

31. Chemical Properties
Addition Reactions

32. Reactions of Alkenes Most alkene addition reactions are exothermic
Products are more stable (lower in energy) than the reactants
Reaction rate depends on activation energy
Many alkene addition reactions require a catalyst
For hydrogenation (H2), Ni, Pt, or Pd catalyst used

33. Addition of H2 - Reduction
Hydrogenation
Requires metal catalyst (Pd, Pt, or Ni)
Used for converting polyunsaturated oils into margarine

34. Addition of Cl2 and Br2
Addition takes place readily at room temperature
Reaction is generally carried out using pure reagents, or mixing them in a nonreactive organic solvent
Br2 Test useful to determine C=C double bond
Br2 has a deep red color; dibromoalkanes are colorless

35. Addition of HX
Addition of HX (HCl, HBr, or HI) to an alkene gives a haloalkane
H adds to one carbon of the C=C and X to the other
Product is NOT symmetrical

36. Addition of HX, cont. If you start with a nonsymmetrical alkene:
Reaction is regioselective
Markovnikoff’s Rule: H adds to the less substituted carbon and X to the more substituted carbon (connected to more Cs)
H adds to carbon that has more H’s
Them what has, gets! (H)

37. Addition of HX
Chemists use reaction mechanism to show how a reaction proceeds in steps
Use curved arrows to show the movement of electron pairs
Tail of arrow shows where the electron pair is before the electrons move (lone pair or bond)
Head of the arrow shows its new position
Curved arrows show which bonds break and which new ones form

38. Addition of HCl to 2-Butene
Step 1:
Reaction of the carbon-carbon double bond with H+ gives a secondary carbocation intermediate
2o = Carbon connected to 2 other Carbons
Carbocation = Carbon with positive charge

39. Addition of HCl to 2-Butene
Step 2:
Reaction of the carbocation intermediate with chloride ion completes the addition

40. Addition of H2O Addition of water is called hydration
Hydration is acid catalyzed, most commonly by H2SO4
Hydration follows Markovnikov’s rule; H adds to the less substituted carbon and OH adds to the more substituted carbon

41. Another example of Hydration

42. Mechanism for Hydration
Step 1:
Step 2:
Step 3:

43. Step 3:

44. Polymerization Polymerization is a VERY important reaction of alkenes
polymer: Greek: poly, many and meros, part
monomer: Greek: mono, single and meros, part

45. Polymerization Use parentheses around the repeating monomer unit
Subscript, n, indicates that this unit repeats n times
Show that a polymer chain can be reproduced by repeating the enclosed structure in both directions
Example: section of polypropene (polypropylene)

48. Polyethylene Low-density polyethylene (LDPE)
Highly branched polymer, so chains do not pack well—weak London Force interactions
Softens and melts above 115°C
Primarily used for packaging for trash bags
High-density polyethylene (HDPE)
Little branching, so chains pack well--London dispersion forces between them are stronger
Higher melting point and stronger than LDPE Used for squeezable jugs and bottles

49. Codes for Plastics