1.Chapter 11The Unsaturated Hydrocarbons:Alkenes, Alkynes, and Aromatics

2. 1. StructureAlkenes are hydrocarbons with a double bond. CnH2n 3. 1. StructureAlkynes are hydrocarbons with a triple bond.CnH2n-2 4. 1. StructureAlkenes and alkynes are unsaturated (dont have themaximum number of hydrogens bonded to each carbon). Tegrity lecture video 5. 1. Comparison 6. 1. Geometry 7. 1. Geometry [3.4 Lewis structures] Four groups of electrons Ethane tetrahedral extend toward the corners of a regular tetrahedron bond angle = 109.5o 8. 1. Geometry [3.4 Lewis structures] 8 9. 1. Geometry [3.4 Lewis structures] Three groups of electrons Ethene All in the same plane Trigonal planar Bond angle = 120o 10. 1. Geometry [3.4 Lewis structures] 10 11. 1. Geometry [3.4 Lewis structures] Two groups of electrons Ethyne Linear Bond angle = 180o 12. 1. Geometry [3.4 Lewis structures] 12 13. 1. Physical propertiesNameMelting point Boiling point ethene-160.1oC -103.7oCpropene-185.0oC-47.6oC1-butene -185.0oC -6.1oCmethylpropene-140.0oC -6.6oC ethyne -81.8oC-84.0oCpropyne-101.5oC-23.2oC1-butyne -125.9oC8.1oC2-butyne-32.3oC 27.0oC 14. 1. Physical properties In each case, the alkyne has a higher boiling point than thealkene. Its structure is more linear. The molecules pack together more efficiently. Intermolecular forces are stronger.Tegrity lecture video 15. 2. NomenclatureThe root name is based on the longest chain thatincludes both carbons of the multiple bond.The ane ending is changed to ene for double bondsand yne for triple bonds. ethyne ethene propynepropene 16. 2. NomenclatureThe chain is numbered from the end nearest the multiplebond.2-pentyne1-butene[not 3-pentyne][not 3-butene]The position of the multiple bond is indicated with thelower-numbered carbon in the bond. 17. 2. NomenclatureDetermine the name and number of each substituentand add in front of the name of the parent compound.5-chloro-4-methyl-2-hexene2,6-dimethyl-3-octene 5-bromo-4-ethyl-2-heptene 18. 2. NomenclatureAlkenes with more than one double bond are called alkadienes (2 double bonds) alkatrienes (3 double bonds) etcEach double bond is designated by its lower-numberedcarbon.2,4-hexadiene 19. 2. NomenclatureCycloalkenes must be numbered so the double bond isbetween carbons one and two. 3-chloro-cyclopentene 4-ethyl-5-methylcyclooctene 20. 2. NomenclatureName the following compounds.CH3CH=C(CH2CH3)2H2C=C-CH2-CH=CH2pencast 21. 2. NomenclatureName the following compounds.pencast 22. 2. NomenclatureWrite a structural formula for each of the followingcompounds. 1-hexene1,3-dicholoro-2-butene4-methyl-2-hexyne pencast1,4-cyclohexadiene 23. 2. NomenclatureDraw a structural formula for each of the followingcompounds:1-bromo-3-hexyne2-butyne pencast dichloroethyne 9-iodo-1-nonyne 24. 3. Geometric isomers Rotation around a double bond is restricted, in much thesame was as rotation is restricted for the cycloalkanes. In the alkenes, geometric isomers occur when there aretwo different groups on each of the double-bonded carbonatoms. 1,2-dichloroethene 25. 3. Cis-trans isomers If both constituents are on the same side of the doublebond, the isomer is cis-. cis-1,2-dichloroethene If the constituents are on opposite sides of the double bond,the isomer is trans-.trans-1,2-dichloroethene 26. 3. Cis-trans isomers Alkenes without substituents also may exhibit cis-transisomerism. cis-4-octenetrans-4-octene 27. 3. Cis-trans isomers In order for cis and trans isomers to exist, neither double-bonded carbon may have two identical substituents.2-methyl-2-buteneno cis/trans isomerism1-buteneno cis/trans isomerism 28. 3. Cis-trans isomers 28 29. 3. Cis-trans isomers Which of the following compounds can exist as geometricisomers? 1-bromo-1-chloro-2,2-dimethylpropene 1,1-dichloroethene 1,2-dibromoethene 3-ethyl-2-methyl-2-hexene 30. 5. Reactions of alkenes and alkynes The most common reactions of alkenes and alkynes areaddition reactions. Hydrogenation: addition of H2 Halogenation: addition of X2 Hydration: addition of H2O Hydrohalogenation: addition of HX 31. 5. General addition reaction A double bond consists of a sigma bond: two electronsconcentrated on a line between the twoconnected atoms; a pi bond: two electrons concentrated inplanes above and below the sigma bond. 32. 5. General addition reaction 32 33. 5. General addition reaction In an addition reaction, the pi bond is lost and its electronsbecome part of the single bonds to A and B. 34. 5. General addition reaction For hydrogenation, halogenation, hydration, andhydrohalogenation, identify the A and B portions of what isbeing added to the double bond. hydrogenation, H2 halogenation, X2 (where X = F, Cl, Br, or I) hydration, H2O hydrohalogenation, HX (where X = F, Cl, Br, or I) 35. 5. Hydrogenation In hydrogenation of an alkene, one molecule of hydrogen(H2) adds to one mole of double bonds. Reaction conditions: platinum, palladium, or nickel catalyst [sometimes] heat and/or pressure 36. 5. Hydrogenation In hydrogenation of an alkyne, two molecules of hydrogen(H2) add to one mole of triple bonds. Reaction conditions: same as for alkenes. 37. 5. Hydrogenation Compare the products resulting from the hydrogenation oftrans-2-pentene and cis-2-pentene.pencast 38. 5. Hydrogenation Compare the products resulting from the hydrogenation of1-butene and cis-2-butene. pencast 39. 5. Vegetable oil and margarine Why does hydrogenation make oils more solid?MP = 13-14oC MP = 69.6oC MP = 62.9oC 40. 5. Halogenation In halogenation of an alkene, one mole of a halogen (Cl2,Br2, I2) adds to one mole of double bonds. Since halogens are more reactive than hydrogen, nocatalyst is needed. 41. 5. Halogenation In halogenation of an alkyne, two moles of a halogen (Cl2,Br2, I2) add to one mole of double bonds. 42. 5. Halogenation Draw the structure and write a balanced equation for thehalogenation of each of the following compounds. 3-methyl-1,4-hexadiene 4-bromo-1,3-pentadiene 3-chloro-2,4-hexadiene pencast 43. 5. Halogenation A solution of bromine in waterhas a reddish-orange color. A simple test for the presenceof an alkene or alkane is toadd bromine water. If a double or triple bond ispresent, the bromine will beused up in a halogenationTest of cyclohexanereaction and the color will and cyclohexenedisappear. 44. 5. Hydration In hydration, one mole of water (H2O) is added to one moleof double bonds. A trace of acid is required as a catalyst. 45. 5. Hydration Unlike hydrogenation and halogenation, hydration is not asymmetric addition to a double bond. If the double bond is not symmetrically located in themolecule, there are two possible hydration products. 46. 5. Hydration The predominant product is determined by Markovnikovsrule: The rich get richer. OR: The carbon that already has more hydrogens will getthe hydrogen from the water. Hydration of propene: + H 2O 47. 5. Hydration Write a balanced equation for the hydration of each of thefollowing compounds: 2-butene 2-ethyl-3-hexene pencast 2,3-dimethylcyclohexene Alkynes undergo a much more complicated hydration that you dont needto remember at this time! 48. 5. Hydrohalogenation Like hydration, hydrohalogenation is an asymmetric additionto a double bond. Hydrohalogenation also follows Markovnikovs rule. 49. 5. Hydrohalogenation 2-butene + HBr ? 3-methyl-2-hexene + HCl ? cyclopentene + HI ?pencast 50. 6. Aromatic compounds Consider the following molecular formulas for unsaturatedhydrocarbons: Hexane (all single bonds): C6H14 Cyclohexane (one ring): C6H12 Hexene (one double bond): C6H12 Hexadiene (two double bonds): C6H10 Cyclohexene (one ring, one double bond): C6H10 Hexatriene (three double bonds): C6H8 Cyclohexadiene (one ring, two double bonds): C6H8 51. 6. Aromatic compounds The molecular formula for benzene is C6H6. The structure must be highly unsaturated. One ring, three double bonds? Reactions of benzene: Benzene does not decolorize bromine solutions. Benzene does not undergo typical addition reactions. Benzene reacts mainly by substitution. The first three items are opposite from what is expectedfrom unsaturated compounds. The last item is identical to what is expected for alkanes. 52. 6. Benzene structure The benzene ring consists of: six carbon atoms joined in a planar hexagonal arrangement with each carbon bonded to one hydrogen atom. Two equivalent structures proposed by Kekul arerecognized today as resonance structures. The real benzene molecule is a hybrid with each resonancestructure contributing equally to the true structure. 53. 6. Benzene structure Sigma and pi bonding in benzene: The sharing of six electrons over the entire ring gives thebenzene structure extra stability. Removing any one of the six electrons would destroy thatstability. 54. 6. Nomenclature Most single-substituent compounds are named asderivatives of benzene. Bromobenzene Ethylbenzene 55. 6. Nomenclature A few common names have been adopted as IUPACnomenclature. toluene phenol aniline xylene (any benzene ring with two methyl groups) 56. 6. Nomenclature There are three ways for the methyl groups on xylene to bearranged. 1,2 [ortho-xylene] 1,3 [meta-xylene] 1,4 [para-xylene] 57. 6. Nomenclature The substituent created by removing one hydrogen from thebenzene ring is called phenyl-. 2-phenylhexane 3-phenylcyclopentene 58. 6. Nomenclature The substituent consisting of a CH2 attached to a benzenering is called benzyl-. Benzyl chloride 59. 6. Polynuclear aromatic hydrocarbons These consist of rings joined along one side. Good news! You dont have to memorize these names! 60. 6. Reactions of benzene Because of the stability of benzenes ring structure, onlysubstitution reactions are characteristic. Halogenation: substitution of one or more halogen atoms forhydrogen atoms. Cl2 requires FeCl3 catalyst. Br2 requires FeBr3 catalyst. Nitration: substitution of one or more nitro- (-NO2) groups forhydrogen atoms. Requires nitric acid and concentration sulfuric acid. Sulfonation: substitution of one sulfonic acid (-SO3H) group for ahydrogen atom. SO3 reactant and concentration sulfuric acid. 61. 7. Heterocyclic aromatic compounds Heterocyclic aromatic compounds have at least one non-carbon atom incorporated in an aromatic ring or polynucleararomatic compound. Many of these compounds are biologically important. Components of DNA and RNA Components of hemoglobin and chlorophyll Pharmaceuticalspyridine