Larry Emme Chemeketa Community College

1. Unsaturated Hydrocarbons Chapter 20 Larry Emme Chemeketa Community College

2. Unsaturated hydrocarbons enhance our lives in many ways: • Polyethylene plastic bags and bottles • Polystyrene Styrofoam cups • Plastic wraps • Essential oils in plants contain multiple bonds between carbon atoms. • Cosmetics, medicines, flavorings, perfumes • 5. Hydrocarbons also form rings of carbon atoms (aromatics) • Detergents, insecticides, and dyes

3. Bonding in Unsaturated Hydrocarbons

4. The unsaturated hydrocarbons consist of three families of homologous compounds that contain multiple bonds between carbon atoms. • Alkenes contain carbon-carbon double bonds. • Alkynes contain carbon-carbon triple bonds. • Aromatic compounds contain benzene rings.

5. The four orbitals available for bonding in alkenes are three sp2 orbitals and one p orbital. Schematic hybridization of 2s22px12py1 orbitals of carbon to form three sp2electron orbitals and one p electron orbital

6. (a) A single sp2 electron orbital and (b) a side view of three sp2 orbitals all lying in the same plane with a p orbital perpendicular to the three sp2orbitals.

7. The carbon-carbon pi () bond is much weaker and, as a consequence, much more reactive than the carbon-carbon sigma () bond.

10. The formation of a triple bond between carbon atoms, as in acetylene, CHCH, may be visualized as shown below. • These pi bond electrons are not as tightly held by the carbon nuclei as the sigma bond electrons. Acetylene, consequently, is a very reactive substance.

12. Nomenclature of Alkenes

13. The general formula for alkenes is: • CnH2n

14. 1. Select the longest continuous carbon-carbon chain that contains the double bond. IUPAC Rules for Naming Alkenes 2. Name this parent compound as you would an alkane, but change the –ane ending to –ene. CH3CH2CH3 propane CH3CH=CH2 propene

15. IUPAC Rules for Naming Alkenes 3. Number the carbon chain of the parent compound starting with the end nearer to the double bond. Use the smaller of the two numbers on the double-bonded carbon atoms to indicate the position of the double bond. Place this number in front of the alkene name. CH3CH=CHCH3 2- butene CH3CH2CH2CH=CH2 1-pentene

16. IUPAC Rules for Naming Alkenes 4. Branch chains and other groups are treated as in naming alkanes, by numbering and assigning them to the carbon atom to which they are bonded.

17. How would we write the structural formula for 4-methyl-2-pentene? • The name indicates: • Five carbons in the longest chain • A double bond between carbons 2 and 3 • A methyl group on carbon 4

18. Write a structural formula for: 7-methyl-2-octene • Octene indicates an 8-carbon chain • The chain contains a C=C between carbons 2 and 3. • There is a –CH3 group on carbon 7

19. Name this compound: • Longest chain containing C=C is 5 carbons • 2-ethyl-1-pentene

20. Geometric Isomerism in Alkenes

21. Geometric Isomerism in Alkenes • Compounds containing a carbon-carbon double bond (pi bond) have restricted rotation about that double bond. • This restricted rotation in a molecule gives rise to a type of isomerism known as geometric isomerism. • Isomers that differ from each other only in the geometry of their molecules and not in the order of their atoms are known as geometric isomers. • They are also called cis-trans isomers.

22. Trans is a Latin noun or prefix, meaning “across”, “beyond” or “on the opposite side”. Cis is a Latin prefix, meaning “on the same side [as]” , “on this side [of]”, or “near side [of]”.

23. Geometric Isomers in Alkenes

24. An alkene shows cis-trans isomerism when each carbon atom of the double bond has two different kinds of groups attached to it.

25. An alkene does not show cis-trans isomerism if even one carbon of the double bond has two identical groups attached to it.

26. Draw a structure for cis-5-chloro-2-hexene • The compound contains 6 carbons with a C=C between carbons 2 and 3, and a Cl atom on carbon 5.

27. Is the compound below the cis or trans isomer? trans-3-methyl-2-pentene

28. Cycloalkenes

29. Cycloalkenes • As the name implies, cycloalkenes are cyclic compounds that contain a C=C in the ring. • The carbons of the double bond are assigned numbers 1 and 2.

30. Cycloalkenes

31. Preparation and Physical Properties of Alkenes

32. Preparation of Alkenes • Cracking • Dehydration of Alcohols

33. Cracking • Cracking, or pyrolysis, is the process in which saturated hydrocarbons are heated to very high temperatures in the presence of a catalyst (usually silica-alumina): Alkane (CnH2n+2)  Mixture of alkenes + Alkanes + H2 (g) 2CH3CH2CH3  CH3CH=CH2 + CH2=CH2 + CH4 + H2 Heat catalyst ~500°C

34. Dehydration of Alcohols • Dehydration involves the elimination of a molecule of water from a reactant molecule.

35. Physical Properties of Alkenes • Alkenes have physical properties very similar to the corresponding alkanes.

36. Chemical Properties of Alkenes

37. Addition Reactions of Alkenes • Addition at the C=C bond is the most common reaction of alkenes. • H2 • Br2 and Cl2 • HBr, HCl • H2O

38. Addition of H2 • Hydrogenation

39. Addition of X2 Bromination (Br2) or Chlorination (Cl2) Bromine changes from a red-orange (flask on the left) to colorless when added to an alkene as shown in the flask on the right.

40. Addition of HX • Hydrobromination (HBr) or Hydrochlorination (HCl)

41. Addition of H2O

42. Addition of HX to an Unsymmetrical Alkene Why????

43. Markovnikov’s Rule Vladimir Markovnikov • When an unsymmetrical molecule such as HX (HCl) adds to a carbon-carbon double bond, the hydrogen from HX goes to the carbon atom that has the greater number of hydrogen atoms.

44. Markovnikov’s Rule This reaction proceeds via the formation of the most stable carbocation intermediate (2°).

45. Write formulas for the organic products formed when 2-methyl-1-butene reacts with: • H2, Pt/25°C • Cl2 • HCl • H20, H+

46. 2-methyl-1-butene + H2, Pt/25 °C

47. 2-methyl-1-butene + Cl2

48. 2-methyl-1-butene + HCl

49. 2-methyl-1-butene + H2O

50. Oxidation