William H. Brown Christopher S. Foote. Eric Anslyn. Chapter 2. William H. Brown Beloit College

Transcription

1 William. Brown Christopher S. Foote Brent L. Iverson Eric Anslyn Chapter 2 Alkanes and Cycloalkanes lk William. Brown Beloit College 2-1

2 Structure ydrocarbon: A compound composed only of carbon and hydrogen. Saturated hydrocarbon: A hydrocarbon containing only single bonds. Alkane: A saturated hydrocarbon whose carbons are arranged in an open chain. Aliphatic hydrocarbon: Another name for an alkane. 2-2

3 ydrocarbons ydrocarbons Saturated Unsaturated Class Alkanes (Chapter 2) Alkenes (Chapters 5-6) Alkynes (Chapter 7) Arenes (Chapter 21-22) Carbon- Ol Only carbon- One or more One or more One or more carbon carbon single carbon-carbon carbon-carbon benzenelike bonding bonds double bonds triple bonds rings Example -C-C- C C -C C- Name Ethane Ethene Ethyne Benzene (Ethylene) (Acetylene 2-3

4 Structure Shape tetrahedral about carbon all bond angles are approximately

5 Drawing Alkanes Line-angle formulas: an abbreviated way to draw structural formulas. each vertex and line ending represents a carbon. Ball-and- and stick model Line-angle formula Structu ral formula C 3 C 2 C 3 C 3 C 2 C 2 C 3 C 3 C 2 C 2 C 2 C Propane Butan e Pentan e 2-5

6 Constitutional Isomerism Constitutional isomers: Compounds with the same molecular formula but a different connectivity of their atoms. example: C 4 10 C 3 C 3 C 2 C 2 C 3 Butane (bp -0.5 C) C 3 CC 3 2-Methylpropane (bp C) 2-6

7 Constitutional Isomerism Question: Do these formulas represent constitutional isomers? C 3 C 3 C 3 C 3 CC 2 C and C 3 C 2 CCC 3 (each is C 7 16 ) C 3 C 3 Write the molecular formula of each. Find the longest carbon chain. Number each chain from the end nearest the first branch. Compare chain lengths as well as the identity and location of branches. They are constitutional isomers. 5 C 3 C 3 C C C 5 3 CC 2 C and 4 3 C 2 CCC C 3 C

8 Constitutional Isomerism The ability of carbon to form strong bonds with other carbon atoms results in a staggering number of constitutional isomers. Molecular Formula Constitutional Isomers C 4 C World population C is about C ,347 6,000,000,000 C ,797,588 C ,111,846,

9 Nomenclature - IUPAC Suffix -ane specifies an alkane, e.g. ethane. Prefix tells the number of carbon atoms. Number of Prefix carbons meth- 1 eth- 2 prop- 3 but- 4 pent- 5 hex- 6 hept- 7 oct- 8 non- 9 dec- 10 Number of Prefix carbons undec- 11 dodec- 12 tridec- 13 tetradec- 14 pentadec- 15 hexadec- 16 heptadec- 17 octadec- 18 nonadec- 19 eicos

10 Nomenclature - IUPAC Parent name: The longest carbon chain. Substituent: A group bonded to the parent chain. Alkyl group: A substituent derived by removal of a hydrogen from an alkane; given the symbol R-. Alkane Name Alkyl group Name C 4 Methane C 3 - Methyl group C 3 C 3 Ethane C 3 C 2 - Ethyl group 2-10

11 Nomenclature - IUPAC 1.The name of a saturated hydrocarbon with an unbranched chain consists of a prefix and suffix. 2. The parent chain is the longest chain of carbon atoms. 3. Each substituent is given a name and a number. Use a hyphen to connect the number to the name. C 3 CC C 3 2-Methylprop ane 4. If there is one substituent, number the chain from the end that gives it the lower number. C C C 2 C 2 CC 3 2-Methylpentane (not 4-methylpentane) 2-11

12 Nomenclature - IUPAC 5. If there are two or more identical substituents, number the chain from the end that gives the lower number to the substituent encountered first. Indicate the number of times the substituent appears by a prefix di-, tri-, tetra-, etc. Use commas to separate position numbers ,4-Dimethylhexane (not 3,5-dimethylhexane)

13 Nomenclature - IUPAC 6. If there are two or more different substituents, list them in alphabetical order. number from the end of the chain that gives the substituent encountered first the lower number Ethyl-5-methylheptane (not 3-methyl-5-ethylheptane) ethylheptane) 2-13

14 Nomenclature - IUPAC 7. The prefixes di-, tri-, tetra-, etc. are not included in alphabetization!!. Alphabetize the names of substituents t first and then insert these prefixes Ethyl-2,2-dimethylhexane y (not 2,2-dimethyl-4-ethylhexane)

15 Nomenclature - IUPAC Alkyl groups Name methyl ethyl propyl 1-methylethyl (isopropyl) Condensed Structural Formula -C 3 -C 2 C 3 -C 2 C 2 C 3 -CC 3 C 3 Name Condensed Structural Formula butyl -C 2 C 2 C 2 C 3 2-methylpropyl yp py -C 2 CC 3 (isobutyl) C 3 1-methylpropyl (sec- butyl) 1,1-dimethylethyl (tert- butyl) -CC 2 C 3 C 3 C 3 -CC 3 C

16 Nomenclature - Common The number of carbons in the alkane determines the name. All alkanes with four carbons are butanes, those with five carbons are pentanes, etc. iso- indicates the chain terminates in -C(C 3 ) 2 ; neothat it terminates in -C(C 3 ) 3. C 3 C 3 C 2 C 2 C 3 C 3 CC 3 Butane Isobutane C 3 C 3 C 3 C 2 C 2 C 2 C 3 C 3 C 2 CC 3 C 3 CC 3 C 3 Pentane Isopentane Neopentane 2-16

17 Classification of C & Primary (1 ) C: A carbon bonded to one other carbon. 1 : a hydrogen bonded to a 1 carbon Secondary (2 ) C: A carbon bonded to two other carbons. 2 : a hydrogen bonded to a 2 carbon Tertiary (3 ) )C C: A carbon bonded d to three other carbons. 3 : a hydrogen bonded to a 3 carbon Quaternary (4 ) C: A carbon bonded to four other carbons. 2-17

18 IUPAC - General prefix-infix-suffix Prefix:Tells the number of carbon atoms in the parent chain. Infix : Tells the nature of the carbon-carbon bonds in the parent chain. Suffix : Tells the class of the compound. Infix -an- -en- -yn- Nature of Carbon-Carbon Suffix Class Bonds in the Parent Chain -e hydrocarbon all single bonds -ol alcohol one or more double bonds -al aldehyde one or more triple bonds -amine amine -one ketone -oic acid carboxylic acid 2-18

19 IUPAC - General prop-en-e = propene eth-an-ol = ethanol C 3 C=C 2 C 3 C 2 O but-an-one = butanone O C 3 CC 2 C 3 O but-an-al = butanal C 3 C 2 C 2 C O pent-an-oic acid = pentanoic acid C 3 C 2 C 2 C 2 CO cyclohex-an-ol = cyclohexanol O eth-yn-e = ethyne eth-an-amine = ethanamine C C C 3 C 2 N

20 Cycloalkanes General formula C n 2n Five- and six-membered rings are the most common. Structure and nomenclature Add the prefix cyclo- to the name of the open-chain alkane containing i the same number of carbons. If only one substituent, no need to give it a number. If two substituents, number from the substituent of lower alphabetical order. If three or more substituents, number to give them the lowest set of numbers and then list substituents in alphabetical order

21 Cycloalkanes Line-angle drawings Each line represents a C-C bond. Each vertices and line ending represents a C. C C C C C 2 3 C 2 C C C C C C C 2 C C2 C 3 C

22 Cycloalkanes Example: name these cycloalkanes. (a) (b) (c) (d) a) Isopropyl cyclopentane b) 1-tertbutyl-4methylcyclohexane c) 1-ethyl-2,5-dimethylcyclohexane d) 1,1-ethylmethylcyclopropane 1 lc clopropane

23 Bicycloalkanes Bicycloalkane: An alkane that contains two rings that share two carbons. We will refer to the most common of these by their common rather than their systematic (IUPAC) names. Bridgehead carbons A one-carbon bridge (ydrindane) (Decalin) (Norbornane) Bicyclo[4.3.0]nonane Bicyclo[4.4.0]decane Bicyclo[2.2.1]heptane

24 Conformations Conformation: Any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond

25 Chem Activity 6. Part A: Energy Diagrams Split quickly in groups of 4 Work on the critical thinking gq questions (1-10) You have 10 min We will discuss the questions in 5 min

26 Conformations Staggered conformation: A conformation about a carbon-carbon single bond in which the atoms or groups on one carbon are as far apart as possible from the atoms or groups on an adjacent carbon. Newman projection: A way to view a molecule by looking along a carbon-carbon single bond

27 Conformations Eclipsed conformation: A conformation about a carbon-carbon single bond in which the atoms or groups of atoms on one carbon are as close as possible to the atoms or groups of atoms on an adjacent carbon

28 Conformations Torsional strain strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation. also called eclipsed interaction strain. the torsional strain between eclipsed and staggered ethane is approximately 12.6 kj (3.0 kcal)/mol kj/mol

29 Conformations Dihedral angle Θ (Greek theta): The angle created by two intersecting planes

30 Conformations Figure 2.8 The energy of ethane as a function of dihedral angle. 2-30

31 Conformations - Check this link:

32 Conformations The origin of torsional strain in ethane: Originally thought to be caused by repulsion between eclipsed hydrogen nuclei. Alternatively, caused by repulsion between electron clouds of eclipsed C- bonds. Theoretical molecular orbital calculations suggest that the energy difference is not caused by destabilization of the eclipsed conformation but rather by stabilization of the staggered conformation. This stabilization arises from the small donor-acceptor interaction between a C- bonding MO of one carbon and the C- antibonding MO on an adjacent carbon. Stabilization ti is lost when a staggered conformation is converted to an eclipsed conformation. 2-32

33 Conformations Strain energy is the increase in energy resulting from distortion of bond angles and bond lengths from their optimal values. Steric strain (nonbonded interaction strain): The strain thatt arises when nonbonded datoms separated by four or more bonds are forced closer to each other than their atomic (contact) radii will allow. Angle strain: Strain that arises when a bond angle is either compressed or expanded compared to its optimal value. 2-33

34 Conformations Anti conformation: A conformation about a single bond in which the groups on adjacent carbons lie at a dihedral angle of 180 C 3 C

35 Anti Butane Energy-minimized anti conformation (computed) the C-C-C bond angle is and all -C- bond angles are between and the calculated strain is 9.2 kj (2.2 kcal)/mol. 2-35

36 Eclipsed Butane The calculated energy difference between (a) the nonenergy-minimized and (b) the energy-minimized eclipsed conformations is 5.6 kj(0.86 kcal)/mol. Decrease steric strain Increase angle strain 2-36

37 Gauche Butane Gauche conformation: A conformation about a single bond of an alkane in which two groups on adjacent carbons lie at a dihedral angle of 60. ere is one of two energy-minimized gauche conformations of butane. The second one has equal energy. 2-37

38 Conformations The energy of butane as a function of the dihedral angle about the bond between carbons 2 and

39 Conformations - Check this link: At 20 o C % anti conformation is 70% 2-39

40 Cycloalkenes Check this link: OS/cycloalkanes.html 2-40

41 Cyclopropane Angle strain: The C-C-C bond angles are compressed from to 60. Torsional strain: There are 6 sets of eclipsed hydrogen interactions. Strain energy is about 116 kj (27.7 kcal)/mol. 2-41

42 Cyclobutane Puckering from planar cyclobutane reduces torsional strain but increases angle strain. The conformation of minimum energy is a puckered butterfly conformation. Strain energy is about 110 kj (26.3 kcal)/mol. 2-42

43 Cyclopentane Puckering from planar cyclopentane reduces torsional strain, but increases angle strain. The conformation of minimum energy is a puckered envelope conformation. Strain energy is about 42 kj (6.5 kcal)/mol. 2-43

44 ChemActivity 6. Part A (Model 2) and Part B: Cyclohexane Conformations Read hand out Work on the critical thinking questions (11-19) 19) Skip questions 15, 18 and 19c You have min We will discuss the questions in 5 min 2-44

45 Cyclohexane Chair conformation: The most stable puckered conformation of a cyclohexane ring. all bond C-C-C C C bond angles are all bonds on adjacent carbons are staggered. 2-45

46 Cyclohexane In a chair conformation, six are equatorial and six are axial. 2-46

47 Cyclohexane For cyclohexane, there are two equivalent chair conformations. all C- bonds equatorial in one chair are axial in the alternative chair and vice versa. 2-47

48 Cyclohexane Boat conformation: A puckered conformation of a cyclohexane ring in which carbons 1 and 4 are bent toward each other. there are four sets of eclipsed C- interactions and one flagpole interaction (flagpole green). a boat conformation is less stable than a chair conformation by 27 kj (6.5 kcal)/mol. 2-48

49 Cyclohexane Twist-boat conformation approximately 20.9 kj (5.5 kcal)/mol less stable than a chair conformation. approximately 6.3 kj (1.5 kcal)/mol more stable than a boat conformation. 2-49

50 Cyclohexane Energy diagram for the interconversion of chair, twist-boat and boat conformations of cyclohexane. 2-50

51 ow to Draw Alternative Chairs Conformations of Cyclohexane Axial bonds Note alternated up and down bonds Equatorial bonds 2-51

52 Alternative Chairs Conformations of Cyclohexene with Substituents Up (above) Down (below) Axial becomes equatorial; equatorial becomes axial Above goes above and below goes below 2-52

53 Methylcyclohexane Equatorial and axial methyl conformations 2-53

54 ΔG 0 axial ---> equatorial Given the difference in strain energy between axial and equatorial conformations, it is possible to calculate the ratio of alternative conformations using the following relationship. ΔG 0 = -RT ln K eq axial equatorial ΔG ΔG Group (kj/mol) Group (kj/mol) As the size of the C N 0.8 N alkyl substituent F 1.0 COO 5.9 increases, the C C 1.7 C= C preference for I 1.9 C conformations with Cl 2.2 C 2 C the group in the eq. Br C( C ) position increases O 3.9 C( C 3 ) 2 C( C 3 )

55 Cis,Trans Isomerism Stereoisomers: Compounds that have the same molecular formula. the same connectivity. a different orientation of their atoms in space. Cis,trans isomers: Stereoisomers that are the result of the presence of either a ring (this chapter) or a carbon-carbon double bond (Chapter 5). 2-55

56 Isomers Relationships among isomers. 2-56

57 Cis,Trans Isomers 1,2-Dimethylcyclopentane 3 C C 3 C 3 C 3 3 C C 3 3 C C 3 cis -1,2-Dimethylcyclopentane trans -1,2-Dimethylcyclopentane 2-57

58 Convert planar cyclohexenes to chair cyclohexenes Chair? Up or above goes above and down or below goes below 4 5 C C 3 Axial becomes equatorial; equatorial becomes axial 2-58

59 Cis-Trans Isomers Stereocenter: An atom, most commonly carbon, about which exchange of two groups produces a different stereoisomer. example: 1,2-dimethylcyclobutane has two stereocenters. Configuration: Refers to the arrangement of atoms about stereocenter. 2-59

60 Cis,Trans Isomerism 1,4-Dimethylcyclohexane 3 C C 3 3 C C 3 C 3 C 3 3 C 3 C trans -1,4-Dimethylcyclohexane cis-1,4-dimethylcyclohexane 3 C C 3 C 3 C 3 The more stable chair conformation of each isomer (alternative chairs in blue) 2-60

61 Cis,Trans Isomerism trans-1,4-dimethylcyclohexane the diequatorial-methyl chair conformation is more stable by approximately 2 x (7.28) = kj/mol C 3 (less stable) C 3 3 C 3 (more stable) C

62 Cis,Trans Isomerism cis-1,4-dimethylcyclohexane C 3 3 C C 3 C conformation s are of equal s tability

63 Cis,Trans Isomerism The decalins A B trans-decalin A B cis-decalin

64 Steroids The steroid nucleus C D A B Cholestanol C 3 C 3 O O Cholestanol 2-64

65 Bicycloalkanes Norbornane drawn from three different perspectives one carbon brid ge between carbons 1 and 4 of the six ix-membered ring (a) (b) (c) Camp hor 4 O 2-65

66 Bicycloalkanes Adamantane add three axial bonds join the three axial bonds to a C group Cyclohexane Adamantane N 2 Amantadine (antiviral agent) 2-66

67 Physical Properties of Alkanes Boiling points: Theboilingpointsofalkanesarelowerthan those of almost any other type of compound of the same molecular weight. In general, both boiling and melting points of alkanes increase with increasing mw. 2-67

68 Physical Properties of Alkanes Intermolecular forces of attraction (example) ion-ion (Na + and Cl - in NaCl) ion-dipole (Na +.. O2 and Cl -.. -O- solvated in aqueous solution) dipole-dipole and hydrogen bonding dispersion forces (very weak electrostatic attraction between temporary dipoles) alkanes, Ne, X

69 Physical Properties Low-molecular-weight alkanes (methane to butane) are gases at room temperature. igher molecular-weight alkanes (pentane, decane, gasoline, and kerosene) are liquids at room temperature. t igh-molecular-weight alkanes (paraffin wax) are semisolids orsolids at room temperature. t 2-69

70 Physical Properties Constitutional isomers have different physical properties. Name mp ( C) bp ( C) Density (g/ml) hexane methylpentane methylpentane ,3-dimethylbutane t ,2-dimethylbutane Most tbranched isomer usually lowest boiling point Branching increases, shape becomes more compact, less surface area, contact among adjacent molecules decreases, strength of dispersion forces decreases, so does the bp exane 2,2-Dimethylbutane 2-70

71 Oxidation of Alkanes Oxidation is the basis for the use of alkanes as energy sources for heat and power. heat of combustion: The heat released when one mole of a substance in its standard state is oxidized to carbon dioxide and water. Δ 0 kj (kcal)/mol C 4 + 2O 2 CO O (-212.8) Methane (-530.6) C 3 C 2 C 3 5O 2 3CO O Propane 2-71

72 eat of Combustion eat of combustion for constitutional isomers ydrocarbon Octane Structural formula Δ 0 [kj(kcal)/mol] ( ) 2-Methylheptane 2,2-D imeth ylhexan e 2,2,3,3-Tetramethylbutane ( ) (1304.6) (1303.0) All signs negative 2-72

73 eats of Combustion For constitutional isomers [kj (kcal)/mol] All signs negative ( ) ( ) (1304.6) (1303.0) 8CO O Branching increases the stability of an alkane 2-73

74 eat of Combustion strain in cycloalkane rings as determined by heats of combustion. 2-74

75 Sources of Alkanes Natural gas 90-95% methane Petroleum (thick, viscous, liquid mixture of thousand of compounds, mostly hydrocarbons, product of the decomposition of marine plants and animals) gases (bp below 20 C) naphthas, including gasoline (bp C) kerosene (bp C) fuel oil (bp C) lubricating oils (bp above 350 C) asphalt (residue after distillation) Coal Production of methanol and acetic acid 2-75

76 Petroleum Fractional distillation of petroleum. 2-76

77 Gasoline Octane rating: Of a particular gasoline is the percent 2,2,4-trimethylpentane (isooctane) in a mixture of isooctane and heptane that t has antiknock k properties equivalent to that of the test gasoline. eptane 2,2,4-Trimethylpentane (octane rating 0) (octane rating 100) Very ypoor Very ygood Regardless of anti-knocking anti-knocking properties properties octane rating antiknock agents can be added E.g.: octane rating of 2-methylhexane is

78 Synthesis Gas A mixture of carbon monoxide and hydrogen in varying proportions which depend on how it is produced. C + 2 O Coal heat CO catalyst C 4 + O CO Methane 2-78

79 Synthesis Gas Synthesis gas is a feedstock for the industrial production of methanol and acetic acid. catalyst CO C 3 O Methanol C O + CO catalyst O 3 C 3 CO Methanol Acetic acid It is likely that industrial routes to other organic chemicals from coal via methanol will also be developed. 2-79

80 Alkanes and Cycloalkanes End Chapter