Chem101 General Chemistry. Lecture 11 Unsaturated Hydrocarbons

Transcription

1 hem101 General hemistry Lecture 11 Unsaturated ydrocarbons

2 Unsaturated ydrocarbons ontain one or more double or triple carbon-carbon bond. University of Wisconsin-Eau laire hem101 - Lecture 11 2

3 Unsaturated ydrocarbons There are three classes of unsaturated hydrocarbons: - Alkenes have at least one carbon-carbon double bond ethylene University of Wisconsin-Eau laire hem101 - Lecture 11 3

4 Unsaturated ydrocarbons There are three classes of unsaturated hydrocarbons: - Alkynes have at least one carbon-carbon triple bond acetylene (common name) University of Wisconsin-Eau laire hem101 - Lecture 11 4

5 Unsaturated ydrocarbons University of Wisconsin-Eau laire hem101 - Lecture 11 5

6 Unsaturated ydrocarbons There are three classes of unsaturated hydrocarbons: - Aromatics are rings with alternating double and single bonds. University of Wisconsin-Eau laire hem101 - Lecture 11 5

7 Unsaturated ydrocarbons There are three classes of unsaturated hydrocarbons: - Aromatics are rings with alternating double and single bonds. benzene University of Wisconsin-Eau laire hem101 - Lecture 11 5

8 Unsaturated ydrocarbons There are three classes of unsaturated hydrocarbons: - Aromatics are rings with alternating double and single bonds. benzene University of Wisconsin-Eau laire hem101 - Lecture 11 5

9 Nomenclature of Alkenes Nomenclature means a systematic set of names. The presence of double bonds means that there is even a greater number of possible isomers than for alkanes. We will use the IUPA system for naming alkenes and alkynes. University of Wisconsin-Eau laire hem101 - Lecture 11 6

10 Nomenclature of Alkenes IUPA Rules: 1 ount the carbons in the longest chain that contains the double bond to determine the root name, 2 Use the ending -ene to designate the presence of a double bond. 3 Number the longest chain so that the carbons participating in the double bond have the lowest possible number. 4 In the prefix, locate the double bond by using the lower of the two carbon numbers. University of Wisconsin-Eau laire hem101 - Lecture 11 7

11 Nomenclature of Alkenes IUPA Rules (con d): 5 Also in the prefix, identify and locate any substituent groups that are attached to the longest chain containing the double bond. 6 ombine the prefix, root and ending names for the attached groups and the longest chain into the complete name. 7 If the molecule contains more than one double bond, the prefixes di-, tri-, tetra before the -ene ending. University of Wisconsin-Eau laire hem101 - Lecture 11 8

12 Exercise 12.1(p388) Select the compounds that can be correctly called unsaturated and classify each one as an alkene or an alkyne: a b. c. 3 d. 3 e. 3 2 f. University of Wisconsin-Eau laire hem101 - Lecture 11 9

13 Exercise 12.3 (p388) Give the IUPA name for the following compounds: a b c. University of Wisconsin-Eau laire hem101 - Lecture 11 10

14 sp 2 ybridization The orbitals containing the valence electrons in alkenes also hybridize But it is done differently than for alkanes University of Wisconsin-Eau laire hem101 - Lecture 11 11

15 sp 2 ybridization The 2s orbital mixes with two of the 2p orbitals to form three sp 2 hybrid orbitals This leaves the remaining 2p orbital unhybridized. University of Wisconsin-Eau laire hem101 - Lecture 11 12

16 sp 2 ybridization The three sp 2 hybrid orbitals are oriented in the same plane and separated by 120 from one another. The unhybridized 2p orbital is oriented at right angles to the sp 2 orbitals University of Wisconsin-Eau laire hem101 - Lecture 11 13

17 sp 2 ybridization University of Wisconsin-Eau laire hem101 - Lecture 11 14

18 sp 2 ybridization The three sp 2 hybrid each form and σ bond to another atom. The 2p orbital forms a second bond to one of the three atoms. - This second bond is called π (pi) bond. University of Wisconsin-Eau laire hem101 - Lecture 11 15

19 sp 2 ybridization University of Wisconsin-Eau laire hem101 - Lecture 11 16

20 σ and π Bonds s and p atomic orbitals: University of Wisconsin-Eau laire hem101 - Lecture 11 17

21 σ and π Bonds Two half-filled s orbitals overlapping to form a σ bond: University of Wisconsin-Eau laire hem101 - Lecture 11 18

22 σ and π Bonds A half-filled s orbital overlapping with a p orbital to form a σ bond: University of Wisconsin-Eau laire hem101 - Lecture 11 19

23 σ and π Bonds A half-filled p orbital overlapping with a p orbital to form a σ bond: University of Wisconsin-Eau laire hem101 - Lecture 11 20

24 σ and π Bonds A half-filled p orbital overlapping with a p orbital to form a π bond: University of Wisconsin-Eau laire hem101 - Lecture 11 21

25 σ and π Bonds A double-bond is a combination of a σ bond and a π bond: University of Wisconsin-Eau laire hem101 - Lecture 11 22

26 σ and π Bonds A double-bond is a combination of a σ bond and a π bond: University of Wisconsin-Eau laire hem101 - Lecture 11 23

27 Geometry of Alkenes This places the two carbons, and the four other atoms that these two carbons are bonded to, all in the same plane. - The bond angles between the three bonds are all approximately 120. University of Wisconsin-Eau laire hem101 - Lecture 11 24

28 Geometry of Alkenes Unlike single bonds, there is no free rotation about double bonds. This situation can produce additional isomers. - These isomers are called geometric isomers. University of Wisconsin-Eau laire hem101 - Lecture 11 25

29 Geometry of Alkenes The following strategy can be applied to determine if a double bond is producing a pair of stereo isomers A B D - If A and B are different, and and D are different, then there is a pair of geometric isomers. University of Wisconsin-Eau laire hem101 - Lecture 11 26

30 Geometry of Alkenes An example of this is provided by 2 butene 3 3 cis-2-butene 3 - These are two different molecules. - They have different physical properties: 3 trans-2-butene Isomer Melting Point { } Boiling Point { } Density {g/ml} cis-2-butene trans-2-butene University of Wisconsin-Eau laire hem101 - Lecture 11 27

31 Geometry of Alkenes An example of this is provided by 2 butene The cis prefix is used to indicate that the hydrocarbon chain enters and leaves on the same side of the double bond. Whereas the trans prefix is used to indicate that the hydrocarbon chain enters and leaves on opposite sides of the double bond. University of Wisconsin-Eau laire hem101 - Lecture 11 28

32 Seeing: The Difference Between cis and trans. University of Wisconsin-Eau laire hem101 - Lecture 11 29

33 Physical Properties of Alkenes The physical properties of alkenes are very similar to those of alkanes. University of Wisconsin-Eau laire hem101 - Lecture 11 30

34 hemical Properties of Alkenes We learned that alkanes are fairly un reactive. - Except combustion reactions, alkanes do not undergo very many types of reactions. Alkenes can also undergo combustion reactions. In addition, unlike the carbon-carbon single bonds found in alkanes, the double bonds found in alkenes are chemically reactive. University of Wisconsin-Eau laire hem101 - Lecture 11 31

35 hemical Properties of Alkenes They are particularly susceptible to a type of reaction called the addition reaction. A B + A B Addition reactions have two reactants and one product. University of Wisconsin-Eau laire hem101 - Lecture 11 32

36 hemical Properties of Alkenes A B + A B In this reaction A and B represent two atoms or two groups of atoms that are bonded to one another by a single bond. - A and B can be the same. University of Wisconsin-Eau laire hem101 - Lecture 11 33

37 hemical Properties of Alkenes We will look at four different examples of addition reactions: - alogenation - Addition of halogens F-F, l-l, Br-Br, I-I - ydrogenation - Addition of hydrogen - - Addition of ydrogen halides -F, -l, -Br, -I - ydration - Addition of water -O University of Wisconsin-Eau laire hem101 - Lecture 11 34

38 hemical Properties of Alkenes alogenation - Addition of halogens F-F, l-l, Br-Br, I-I + Br Br Br Br alkene bromine (halogen) haloalkane - This reaction is often used to test for the presence of alkenes Bromine is brown and the alkene and haloalkane are colorless The loss of bromine s brown color is a positive test for alkenes University of Wisconsin-Eau laire hem101 - Lecture 11 35

39 hemical Properties of Alkenes ydrogenation - Addition of hydrogen - - Pt, Pd or Ni + alkene hydrogen alkane The Pt, Pd, or Ni written above the arrow indicates that this reaction is catalyzed by platinum, palladium or nickel. - This commercially important reaction is used to convert polyunsaturated vegetable oils, which are liquid, to partially hydrogenated oils, which are solids. University of Wisconsin-Eau laire hem101 - Lecture 11 36

40 hemical Properties of Alkenes Addition of ydrogen halides - -F, -l, -Br, -I l + l alkene hydrogen halide haloalkane - Because the substituents being added are different, it is possible to obtain two different products for the reaction University of Wisconsin-Eau laire hem101 - Lecture 11 37

41 hemical Properties of Alkenes Addition of ydrogen halides - -F, -l, -Br, -I Br Br bromopropane or propene hydrogen bromide Br bromopropane University of Wisconsin-Eau laire hem101 - Lecture 11 38

42 hemical Properties of Alkenes Addition of ydrogen halides - -F, -l, -Br, -I When there are two possible products, one is usually favored greatly over the other. Markovnikov s Rule can be used to predict which product will be favored: - The hydrogen will attach itself to the carbon that has the greatest number of hydrogens. - The rich get richer University of Wisconsin-Eau laire hem101 - Lecture 11 39

43 hemical Properties of Alkenes Addition of ydrogen halides - -F, -l, -Br, -I - For the previous example, 2-bromopropane will be the major product of the reaction Br bromopropane >> Br bromopropane University of Wisconsin-Eau laire hem101 - Lecture 11 40

44 hemical Properties of Alkenes ydration - Addition of water - -O 2 SO 4 + O O alkene water alcohol The 2 SO 4 above the arrow is sulfuric acid. It is used as a catalyst in the hydration reaction - This is biochemically important reaction used in the breakdown of food for obtaining energy. University of Wisconsin-Eau laire hem101 - Lecture 11 41

45 hemical Properties of Alkenes ydration - Addition of water - -O O O O fumaric acid O + O water fumarse O O OO malic acid O - fumarase is an enzyme catalyst, which is able to carry out the reaction under the mild conditions found in a living cell. - Malic acid is an α-hydroxy acid and is what makes apples sour. (The genus name for apple is Malus) University of Wisconsin-Eau laire hem101 - Lecture 11 42

46 hemical Properties of Alkenes Summary of alkene addition reactions: University of Wisconsin-Eau laire hem101 - Lecture 11 43

47 Addition Polymers A polymer is a chain of monomer units - Like beads on a string. Addition polymers are commercially very important: - Polyethylene - Polypropylene - Polystyrene - Polyvinylchloride (PV) - Polytetrafluoroethylene (Teflon) University of Wisconsin-Eau laire hem101 - Lecture 11 44

48 Addition Polymers Though important commercially, addition polymerization is not commonly used in biological systems. We will encounter other types of polymerization when we discuss proteins and nucleic acids. University of Wisconsin-Eau laire hem101 - Lecture 11 45

49 Alkynes Alkynes contain carbon-carbon triple bonds. To form a triple bond requires sp hybridization University of Wisconsin-Eau laire hem101 - Lecture 11 46

50 Alkynes University of Wisconsin-Eau laire hem101 - Lecture 11 47

51 Alkynes University of Wisconsin-Eau laire hem101 - Lecture 11 48

52 Alkynes Alkynes are very reactive and therefore not found in biological system. An example of an alkyne is acetylene, which is used in oxy-acetylene welding torches. acetylene The IUPA system for naming alkynes is the same as for alkenes, but uses the -yne ending. The IUPA name for acetylene is ethyne. University of Wisconsin-Eau laire hem101 - Lecture 11 49

53 Aromatic compounds Often are very fragrant, hence the name aromatic. It was discovered that these compounds are highly unsaturated hydrocarbons. owever, there low reactivity is more like that of alkanes, rather than alkenes or alkynes. Benzene, which is representative of the aromatic compounds, has a molecular formula of 6 6. University of Wisconsin-Eau laire hem101 - Lecture 11 50

54 Aromatic compounds Friedrich Kekulé proposed a structure for benzene in e proposed a rich structure with alternating double and single bonds. KekulŽ Structures for Benzene University of Wisconsin-Eau laire hem101 - Lecture 11 51

55 Aromatic compounds Friedrich Kekulé proposed a structure for benzene in e proposed a rich structure with alternating double and single bonds. KekulŽ Structures for Benzene University of Wisconsin-Eau laire hem101 - Lecture 11 51

56 Aromatic compounds All of the carbons in the benzene ring are sp 2 hybridized. It is now recognized that the electrons in the unhybridized p orbitals can travel completely around the ring. - They are said to be delocalized. University of Wisconsin-Eau laire hem101 - Lecture 11 52

57 Aromatic ompounds The electrons in the p orbitals are delocalized around the ring: University of Wisconsin-Eau laire hem101 - Lecture 11 53

58 Aromatic ompounds When drawing structures that contain aromatic rings, we emphasize the delocalization by representing the π-bonds by a circle: University of Wisconsin-Eau laire hem101 - Lecture 11 54

59 Aromatic ompounds University of Wisconsin-Eau laire hem101 - Lecture 11 55

60 Aromatic ompounds University of Wisconsin-Eau laire hem101 - Lecture 11 56

61 Aromatic ompounds IUPA names - When a single hydrogen is replaced the compounds is named as a derivative of benzene. Br 2 3 Bromobenzene Ethylbenzene University of Wisconsin-Eau laire hem101 - Lecture 11 57

62 Aromatic ompounds ommon names - Some aromatic compounds have common names that have stuck: 3 O Toluene Phenol Note: Eventhough the hydrogens and carbons are not shown, it is understood that they are present. University of Wisconsin-Eau laire hem101 - Lecture 11 58

63 Aromatic ompounds The physical properties of aromatic compounds are similar to other hydrocarbons. The chemical properties of aromatic compounds are more similar to those of the alkanes than to the alkenes. - This is because the delocalization of the p orbital electrons make the double bonds in aromatic compounds more resistant to addition reactions than the double bonds in alkenes University of Wisconsin-Eau laire hem101 - Lecture 11 59