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Chapter 11

Chapter 11. The Unsaturated Hydrocarbons: Alkenes, Alkynes, and Aromatics. 1. Structure. Alkenes are hydrocarbons with a double bond. C n H 2n Alkynes are hydrocarbons with a triple bond. C n H 2n-2

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Chapter 11

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

  2. 1. Structure • Alkenesare hydrocarbons with a double bond. • CnH2n • Alkynesare hydrocarbons with a triple bond. • CnH2n-2 • Alkenes and alkynes are unsaturated(don’t have the maximum number of hydrogens bonded to each carbon). ♫

  3. 1. Comparison

  4. 1. Geometry

  5. 1. Geometry [3.4 Lewis structures] • Four groups of electrons • Ethane • tetrahedral • extend toward the corners of a regular tetrahedron • bond angle = 109.5o

  6. 1. Geometry [3.4 Lewis structures] • Three groups of electrons • Ethene • All in the same plane • Trigonal planar • Bond angle = 120o

  7. 1. Geometry [3.4 Lewis structures] • Two groups of electrons • Ethyne • Linear • Bond angle = 180o

  8. 1. Physical properties

  9. 1. Physical properties • In each case, the alkyne has a higher boiling point than the alkene. • Its structure is more linear. • The molecules pack together more efficiently. • Intermolecular forces are stronger.

  10. 2. Nomenclature • The root name is based on the longest chain that includes bothcarbons of the multiple bond. • The –ane ending is changed to –ene for double bonds and –yne for triple bonds. ethyne ethene propyne propene

  11. 2. Nomenclature The chain is numbered fromthe end nearest the multiple bond. The position of the multiple bond is indicated with the lower-numbered carbon in the bond. 2-pentyne [not 3-pentyne] 1-butene [not 3-butene]

  12. 2. Nomenclature Determine the name and number of each substituent and add in front of the name of the parent compound. 5-chloro-4-methyl-2-hexene 2,6-dimethyl-3-octene 5-bromo-4-ethyl-2-heptene

  13. 2. Nomenclature • Alkenes with more than one double bond are called • alkadienes (2 double bonds) • alkatrienes (3 double bonds) • etc… • Each double bond is designated by its lower-numbered carbon. 2,4-hexadiene

  14. 2. Nomenclature Cycloalkenes must be numbered so the double bond is between carbons one and two. 3-chloro-cyclopentene 4-ethyl-5-methylcyclooctene

  15. 2. Nomenclature Name the following compounds. CH3CH=C(CH2CH3)2 H2C=C-CH2-CH=CH2

  16. 2. Nomenclature Name the following compounds.

  17. 2. Nomenclature Write a structural formula for each of the following compounds. 1-hexene 1,3-dicholoro-2-butene 4-methyl-2-hexyne 1,4-cyclohexadiene

  18. 2. Nomenclature • Draw a structural formula for each of the following compounds: • 1-bromo-3-hexyne • 2-butyne • dichloroethyne • 9-iodo-1-nonyne

  19. 3. Geometric isomers 1,2-dichloroethene Rotation around a double bond is restricted, in much the same was as rotation is restricted for the cycloalkanes. In the alkenes, geometric isomers occur when there are two different groups on each of the double-bonded carbon atoms.

  20. 3. Geometric isomers Time for the first Chapter 11 Journal question! [Use tag “difference”] In your own words, explain how constitutional isomers and geometric isomers are different. Be sure to consider BOTH their differences and their similarities! You might want to use examples of actual molecules.

  21. 3. Cis-trans isomers cis-1,2-dichloroethene trans-1,2-dichloroethene • If both constituents are on the same side of the double bond, the isomer is cis-. • If the constituents are on opposite sides of the double bond, the isomer istrans-.

  22. 3. Cis-trans isomers cis-4-octene trans-4-octene Alkenes without substituents also may exhibit cis-trans isomerism.

  23. 3. Cis-trans isomers 2-methyl-2-butene no cis/trans isomerism 1-butene no cis/trans isomerism In order for cis and trans isomers to exist, neither double-bonded carbon may have two identical substituents.

  24. 3. Cis-trans isomers ♫ • Which of the following compounds can exist as geometric isomers? • 1-bromo-1-chloro-2-methylpropene • 1,1-dichloroethene • 1,2-dibromoethene • 3-ethyl-2-methyl-2-hexene

  25. 4. Alkenes in nature • Ethene (ethylene) and ripening • Ripening agents • Ripening bowl

  26. 5. Reactions of alkenes and alkynes • The most common reactions of alkenes and alkynes are addition reactions. • Hydrogenation: addition of H2 • Halogenation: addition of X2 • Hydration: addition of H2O • Hydrohalogenation: addition of HX

  27. 5. General addition reaction • A double bond consists of • a sigma bond: two electrons concentrated on a line between the two connected atoms; • a pi bond: two electrons concentrated in planes above and below the sigma bond.

  28. 5. General addition reaction In an addition reaction, the pi bond is lost and its electrons become part of the single bonds to A and B.

  29. 5. General addition reaction ♫ • For hydrogenation, halogenation, hydration, and hydrohalogenation, identify the A and B portions of what is being 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)

  30. 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

  31. 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.

  32. 5. Hydrogenation Compare the products resulting from the hydrogenation of trans-2-pentene and cis-2-pentene.

  33. 5. Hydrogenation Compare the products resulting from the hydrogenation of 1-butene and cis-2-butene.

  34. 5. Vegetable oil and margarine MP = 13-14oC MP = 69.6oC ♫ MP = 62.9oC Why does hydrogenation make oils more solid?

  35. 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, no catalyst is needed.

  36. 5. Halogenation In halogenation of an alkyne, two moles of a halogen (Cl2, Br2, I2) add to one mole of double bonds.

  37. 5. Halogenation • Draw the structure and write a balanced equation for the halogenation of each of the following compounds. • 3-methyl-1,4-hexadiene • 4-bromo-1,3-pentadiene • 3-chloro-2,4-hexadiene

  38. 5. Halogenation • A solution of bromine in water has a reddish-orange color. • A simple test for the presence of an alkene or alkane is to add bromine water. • If a double or triple bond is present, the bromine will be used up in a halogenation reaction and the color will disappear.

  39. 5. Hydration In hydration, one mole of water (H2O) is added to one mole of double bonds. A trace of acid is required as a catalyst.

  40. 5. Hydration Unlike hydrogenation and halogenation, hydration is not a symmetric addition to a double bond. If the double bond is not symmetrically located in the molecule, there are two possible hydration products.

  41. 5. Hydration + H2O  The predominant product is determined by Markovnikov’s rule: The rich get richer. OR: The carbon that already has more hydrogens will get the hydrogen from the water. Hydration of propene:

  42. 5. Hydration • Write a balanced equation for the hydration of each of the following compounds: • 2-butene • 2-ethyl-3-hexene • 2,3-dimethylcyclohexene Alkynes undergo a much more complicated hydration that you don’t need to remember at this time!

  43. 5. Hydrohalogenation • Like hydration, hydrohalogenation is an asymmetric addition to a double bond. • Hydrohalogenation also follows Markovnikov’s rule.

  44. 5. Hydrohalogenation 2-butene + HBr  ? 3-methyl-2-hexene + HCl  ? cyclopentene + HI  ?

  45. 5. Hydrohalogenation Here’s your second Journal question! [Use tag “addition”] Explain how hydrogenation and halogenation are different from hydration and hydrohalogenation as addition reactions. [Hint: There’s a rule involved!]

  46. 6. Aromatic compounds • Consider the following molecular formulas for unsaturated hydrocarbons: • 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

  47. 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 expected from unsaturated compounds. • The last item is identical to what is expected for alkanes.

  48. 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 Kekulé are recognized today as resonance structures. • The real benzene molecule is a hybrid with each resonance structure contributing equally to the true structure.

  49. 6. Benzene structure Sigma and pi bonding in benzene: The sharing of six electrons over the entire ring gives the benzene structure extra stability. Removing any one of the six electrons would destroy that stability.

  50. 6. Nomenclature • Most single-substituent compounds are named as derivatives of benzene. • Bromobenzene • Ethylbenzene

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