1. Chapter 4 Unsaturated Hydrocarbons

2. Objectives  Bonding in Alkenes  Constitutional isomers in alkenes  Cis-trans stereoisomers in alkenes  Addition, Polymerization and Oxidation reactions of alkenes.  Structure of Alkynes and Aromatics  Constitutional isomers of aromatics  Reactions of aromatics

3. Bonding in unsaturated hydrocarbons Unsaturated hydrocarbon AlkeneAlkyneAromatic The unsaturated hydrocarbon families, the alkene, alkyne and the aromatic contain C-C multiple bonds. Alkene has double bond and alkyne has triple bond. Aromatic compound has alternative double and single bond.

4. Characteristics  Alkenes have fewer hydrogen compared to alkanes, therefore referred as unsaturated.  Presence of multiple bonds enhances the reactivity.  Aromatic compounds are generally stable compared to alkenes and alkynes  General formula for alkene is C n H 2n

5. Compounds having C-C double bond  Animal fats and vegetable oils, some proteins, nucleic acids and vitamins.  Cis-11 retinal, visual perception  Smoke – fused aromatic compounds  Sex hormone – aromatic ring structure  Spices contain aromatic compounds  Pesticides – DDT  Synthetic compounds (drugs, plastics, synthetic rubbers, dyes, soaps and cosmetics).

6. Structure of alkane, alkene and cycloalkane

7. Table 12.1

8. Bonding in Alkenes  The two bonds in the double bonds in alkene are not equivalent and have different bond strength.  One bond (sigma) is strong and other bond (pi) is weak.  The pi bond is responsible for the characteristic behavior of alkene.  The orbitals used to form the C-C double bonds are of sp 2 type.

9. Formation of sp 2 orbitals for carbon–carbon double bonds.

10. Double-bond formation in ethene

11. Pi-bond formation in ethene.

12. Trigonal bond angles of sp 2 carbons in the alkene double bond.

13. Ball-and-stick and space-filling models of ethene.

14. Constitutional isomers of alkenes  More number of constitutional isomers are formed for alkenes and other families compared to alkanes.  With alkenes different C skeleton are possible  With alkenes different placement of double bond is possible  Two isomers for C 4 H 10 but there are three C 4 H 8 isomers.  There are five C-5 alkenes but there are only three C-5 alkanes.

15. Structural isomers of C 4 H 10 and C 4 H 8

16. Drawing constitutional isomers

17. Naming alkenes  Identify the longest chain containing both C of the double bond.  Replace –ane to –ene.  The longest chain is numbered from the end nearest the double bond.  The compound having two double bonds ends in –adiene.  A cyclic compound is named as cycloalkene.  Numbering in cycloalkene starts from the carbon containing double bond.

18. Example

19. Exercise – IUPAC names

20. Common names  Ethene = Ethylene  Propene = Propylene  They are important industrial chemicals.  Plastic polyethylene and polypropylene are synthesized from these organic chemicals.

21. Importance of alkenes  Ethene produced as hormone in plants.  Ethene used in fruit industry to ripen the fruits.  Other alkenes distinguishes the odor in different plants

22. Geraniol is extracted from roses and used in perfume formulations.

23. Box 12.1 (b)

24. Cis-trans isomerism Concept  Geometric isomers are possible only when each carbon of the double bond is a stereocenters The C - C double bond has restricted rotation The C - C double bond has restricted rotation There are two 2-butene compounds, cis-2-butene and trans-2-butene. There are two 2-butene compounds, cis-2-butene and trans-2-butene. Substituents are placed on the same or opposite side of the double bond. Substituents are placed on the same or opposite side of the double bond. The pi bond presents a significant barrier to rotation. The pi bond presents a significant barrier to rotation. Two isomers have different physiological behavior. Two isomers have different physiological behavior.

25. Ball-and-stick and space-filling models of 2- butene.

26. Cis-trans isomerism of 11-retinal is responsible for vision.

27. Vitamin A

28. Box 12.2 (b)

29. Bombykol’s pheromone action as a sex attractant for the silkworm moth depends on geometric isomerism.

31. Addition Reactions  Addition reaction is the most characteristic reaction in alkene family.  Pi bond is responsible for high reactivity in alkenes.  Pi bond undergo reactions with several chemical reagents.  Hydrogen (H 2 )  Halogen (F 2, Cl 2, Br 2 )  Hydrogen halide (HCl, HBr, HI)  Water (H 2 O)

32. Addition of Symmetric Reagent  Hydrogen and Halogens are symmetric reagents. H H  CH 2 = CH 2 + H H H 2 C CH 2 Pt or Ni Pt or Ni  This reaction is catalytic hydrogenation.  Hydrogenation is a reduction reaction. There is an increase in the number of hydrogen atoms.

33. Halogenation  Addition of halogen is Halogenation.  Does not require any catalyst, takes place in ambient temperature.  Halogenation in alkanes is of substitution type, whereas, in alkenes it is addition type of reaction.  CH 2 = CH 2 + Br- Br Br Br H 2 C CH 2 H 2 C CH 2

34. The deep red color of Br 2 is decolorized as it reacts with the double bond of 1-hexene.

35. Addition of Asymmetric Reagents  Hydrogen halides and H 2 O are asymmetric reagents.  Addition of hydrogen halide is called hydrohalogenation.  Hydrogen is added to one carbon of the double bond and halogen is added to the other carbon.  CH 2 = CH 2 + H-Cl H Cl  H 2 C CH 2

36. Hydration  Addition of water is Hydration.  Requires strong acid catalyst for the reaction to occur. (H 2 SO 4 )  Product of hydration is alcohol.  Industrially an important reaction  In the living system, this reaction is important in metabolism. H OH  H 2 C=CH 2 + H-OH H+ H 2 C CH 2

37. Symmetric Vs Asymmetric alkene  Symmetric alkenes and Asymmetric alkenes.  Only one product is possible by the addition of symmetric or asymmetric reagent.  Two products are formed by the addition of asymmetric reagent to the asymmetric alkene.

39. Major and Minor Products

40. Markovnikov rule  Non-selective Vs selective type of reaction  Major and minor products  Asymmetric reagent and asymmetric alkene follows selective type of reaction  Markovnikov rule The major product is the one formed when the hydrogen from the asymmetric reagent adds to the carbon of the double bond that already had more hydrogens prior to the reaction. The major product is the one formed when the hydrogen from the asymmetric reagent adds to the carbon of the double bond that already had more hydrogens prior to the reaction.

41. Mechanism of alkene addition reaction HCl interacts with the electron pair of the weak pi bond of ethene. The H-Cl bond breaks to form H + and Cl -, and the H + adds to ethene.

42. Mechanism of addition reaction Bond formation between the positive carbocation and the negative chloride ion yields the final product.

43. Tertiary, secondary and primary carbocations

44. Carbocation stability and Markovnikov rule

45. Addition Polymerization  Addition polymerization is a self – addition reaction, in which thousands of alkene molecules add to one another to form polymer molecule.  Natural polymers – Carbohydrates, proteins, nucleic acids  Synthetic polymers – Plastics, rubber and fiber.  CH 2 = CH 2 + CH 2 = CH 2 +  -----CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 ----

46. Important industrial dienes

47. Commercial products created through addition polymerization. Commercial products created through addition polymerization.

48. Commercial products created through addition polymerization.

53. Table 12.3

54. Oxidation of alkenes  Alkenes oxidize with sufficient oxygen to produce CO 2 and H 2 O. (Combustion)  Selective Oxidation – Only the carbon of the double bond receives oxygen.  Oxidizing reagents like (MnO 4 - ) and dichromate (Cr 2 O 7 2- ) ions, Oxygen and Ozone from the atmosphere.  The products are aldehydes, ketones, alcohols and carboxylic acids.