1. Chapt 21 Hydrocarbons [Selected]
21.1 Introduction to Hydrocarbons
21.2 Alkanes [Straight-Chain Only]
21.4 Hydrocarbon Isomers [also with O]

2. Section 21.1 Introduction to Hydrocarbons
Hydrocarbons are carbon-containing organic compounds that provide a source of energy and raw materials.
Explain the terms organic compound and organic chemistry.
Identify hydrocarbons
Recognize the different ways that hydrocarbon molecules may be represented (molecular formula, structural formula, ball-and-stick model, etc.) and convert a molecular formula into a valid structural formula and vice versa,
Distinguish between saturated and unsaturated hydrocarbons.

3. Section 21.1 Introduction to Hydrocarbons
Key Concepts
Organic compounds contain the element carbon
Hydrocarbons are organic substances composed of carbon and hydrogen.
The major sources of hydrocarbons are petroleum and natural gas.

4. Organic Compounds
Name used because living organisms known to contain/produce them
Term applied to all carbon-containing compounds except for a small number of compounds considered to be inorganic
Because huge number exist, an entire branch of chemistry – organic chemistry – devoted to their study

5. Essential Organic Chemistry
Bond – force that hold atoms together in compounds
Carbon atom (C) always forms 4 bonds with other atoms; bond represented by a line
C forms covalent bonds (a particular type of bond)
Hydrogen can only form one bond

6. Organic Compounds - Hydrocarbons
Simplest organic compounds; consist of only the elements carbon (C) and hydrogen (H)
In hydrocarbon, C either attached (bonded) to another C or to a hydrogen
Are thousands of hydrocarbons; can be in chain, branched chain, ring, and cage-like structures
Major source of hydrocarbons – petroleum and natural gas (mostly CH4 – methane)

7. Bond to some unspecified atom
Hydrocarbons
Carbon atoms bond to each other by single, double, & triple bonds (always 4 total bonds)
Saturated hydrocarbons contain only single bonds
Unsaturated hydrocarbons contain at least one double or triple bond
Bond to some unspecified atom
Single
Double
Triple

8. Simplest Hydrocarbon - Methane
Chemical (molecular) formula: CH4
Structural formula: C H
Chemical bond
Carbon atom with 4 bonds

9. Simple Hydrocarbons - Methane
One carbon atom attached to 4 hydrogens
Shape of carbon bonded to 4 other atoms is a tetrahedron – bond angles of 109.5
Hydrogens occupy corners of tetrahedron
109.5

10. Ways of Representing Compounds
Compounds may be represented by various types of formulas and graphical presentations
Variety of these shown on following slides
Chemists use form that best shows information they wish to highlight
Molecular formula most compact but no information about connections & geometry
Structural show connections but no 3D info
Most graphical forms can be generated and/or manipulated using online chemical software

11. Ways of Representing Methane
Chemical (molecular) formula CH4 C H
Structural formula
Structural formula with additional geometry information – solid wedge coming toward you, dashed one away
Ball & stick model
Space-filling model

12. Chapt 21 Hydrocarbons [Selected]
21.1 Introduction to Hydrocarbons
21.2 Alkanes
21.4 Hydrocarbon Isomers [also with O]

13. Section Alkanes
Alkanes are hydrocarbons that contain only single bonds.
Name a straight-chain alkane from its molecular formula or by examining its structure (up to octane).
Draw the structural formula or write the molecular formula of a straight-chain alkane when given its name (up to octane).

14. Section 21.2 Alkanes Key Concepts
Alkanes contain only single bonds between carbon atoms.
Alkanes and other organic compounds are best represented by structural formulas and can be named using systematic rules determined by the International Union of Pure and Applied Chemistry (IUPAC).
Alkanes that contain hydrocarbon rings are called cyclic alkanes.

15. Alkanes
Simple Alkanes – hydrocarbons with only single bonds and no ring structures
All have formula CnH2n+2 n = integer
All have names ending in “ane”
Simplest possible hydrocarbon = methane
Chemical (molecular) formula: CH4
Structural formula: C H
Chemical bond
Carbon atom with 4 bonds

16. Alkanes All have formula CnH2n+2 n = integer n = 2 ethane
Chemical (molecular) formula: C2H6
Structural formula:
n = 3 propane
Chemical (molecular) formula: C3H8 C H C H

17. “Constructing” Alkanes Stepwise
Can think of alkanes larger than methane as being built from smaller molecules by adding a methyl group: CH3
Process: 1) Remove H atom (leave bond)
2) Replace removed atom with CH3
If start with CH4, four possible choices for H to remove, but all choices result in exactly the same molecule, ethane = C2H6

18. Making Ethane (C2H6) From CH4
methane CH4
methyl group: CH3
ethane C2H6

19. Ethane Can write formula as C2H6 or as CH3CH3
As represented by skeletal formula
Can write formula as C2H6 or as CH3CH3
Latter method allows one to visualize and draw structure more easily
Additional very compact representation possible – skeletal (aka line-angle or bond line) formula
Bonds are lines (as before)
Carbon atoms present where line begins or ends or where 2 lines meet
H not shown unless attached to drawn atom

20. Ethane
Most alkanes rotate freely about the single bond between carbon atoms

21. Single Bond Free Rotation
Free rotation occurs about single bonds
Consequence of free rotation: 2 molecules that may appear different when drawn may in fact be identical because one molecule may be twisted about its single bonds to have the exact same shape as the 2nd molecule

22. Two Equivalent Butane Molecules
Right hand structure is twisted version of left hand structure 22

23. Single Bond Free Rotation
Free rotation occurs about single bonds
Because of free rotation all six hydrogen atoms in ethane are equivalent
If making new compound from ethane by replacing a hydrogen, doesn’t matter which one is chosen – result will be the same

24. Making Propane (C3H8) From Ethane
Ethane: C2H6
Note: “straight” chain shown in structural formula isn’t
methyl group: CH3
As represented by skeletal formula
Propane: C3H8 or CH3CH2CH3

25. Straight-Chain Alkanes
n = 3 propane
Propane: Molecular formula: C3H8
Structural formula:
For n > 3, it makes a difference which carbon the next methyl group is added
For straight-chain alkanes, next methyl always added to an end carbon – structural formula (untwisted) has all carbons in a line C H

26. Simple Hydrocarbons - Alkanes
Condensed formula helps to see structure
Butane C4 shown as straight-chain isomer
Type of Formula
Molecular Structural Ball-and-Stick Space Fill Condensed
CH3CH3
CH3CH2CH3
CH3CH2CH2CH3

27. Hydrocarbons – Straight-Chain Alkanes
Type of Formula
Name Molecular Condensed

28. Chapt 21 Hydrocarbons [Selected]
21.1 Introduction to Hydrocarbons
21.2 Alkanes [Straight-Chain Only]
21.4 Hydrocarbon Isomers [also with O]

29. Section 21.4 Hydrocarbon Isomers
Some hydrocarbons [and other compounds] have the same molecular formula but have different molecular structures.
Define the terms isomer, structural isomer, and stereoisomer.
Categorize molecular structures as being structural isomers, stereoisomers or as not being isomers.
Distinguish between geometric (diastereomers) and optical isomers (enantiomers)
Differentiate between geometric isomers with cis- and trans prefixes.
Describe describe the structural characteristics that are associated with optical isomers

30. Section 21.4 Hydrocarbon Isomers
Some hydrocarbons [and other compounds] have the same molecular formula but have different molecular structures.
Generate isomers of compounds containing oxygen in addition to carbon and hydrogen

31. Section 21.4 Hydrocarbon Isomers
Key Concepts
Isomers are two or more compounds with the same molecular formula but different molecular structures.
Structural isomers differ in the order in which atoms are bonded to each other.
Stereoisomers have all atoms bonded in the same order but arranged differently in space; stereoisomers can either be optical isomers (enantiomers) or not (diastereomers)
Stereoisomers which are non-superimposable mirror images of each other are called optical isomers
Some diastereomers are geometric isomers; these are associated with carbon compounds containing double bonds

32. Section 21.4 Hydrocarbon Isomers
Key Concepts
When oxygen is present in a compound with carbon and hydrogen, isomers can involve hydroxy (-OH), ether (-O-) and carbonyl (C=O) groupings

33. Formulas for Compounds - Isomers
Isomers – different compounds which have the same chemical formula
2 main categories: structural (aka constitutional) isomers and stereoisomers (aka configurational)
Structural isomer - atoms bonded in different order
Stereoisomer – atoms bonded in same order but differ in spatial orientation
# of isomers increase as # of atoms increase
Good web resources at:

34. Structural (Constitutional)
Types of Isomers
All Isomers
Structural (Constitutional)
Stereoisomers
(Configurational)
Diastereomers
Enantiomers
(optical)
Geometric
(Cis-trans)
Other diastereomers
(>1 chiral centers)

35. Butane – Structural Isomers
Butane, C4H10 - smallest alkane to have isomers (has two)
Unlike construction of ethane and propane, choice of which H in propane to replace with a methyl group makes a difference
Two possible choices generate two structural isomers – carbons connected to each other differently
Structural isomers differ in physical and chemical properties

36. Making Butane (C4H10) From Propaneor
propane (C3H8)

37. Two Equivalent n-Butane Molecules
Right hand structure is twisted version of left hand molecule 37

38. Butane - Bond Rotations (not isomers)

39. Butane C4H10. =
Not isomers – carbons connected in same way and forms can convert from one to the other by rotating around a bond =
Structural isomers – carbons connected in different way; bond must be broken to convert one form into the other

40. Structural Isomers of C4H10
Butane, BP = 0°C
Isobutane, BP = -12°C 40

41. Butane (C4H10) Skeletal Formula Structural Formula Other n-butane
straight chain
CH3CH2CH2CH3
Butane (C4H10)
iso-butane
branched
CH3CH(CH3)CH3

42. n-Butane CH3CH2CH2CH3

43. Making Pentane (C5H12) From Butane
At first glance, appear to be 4 isomers - but
3 isomers remain

44. The 3 Structural Isomers of Pentane

45. Structural Isomers of C5H12 (Pentane)
n-pentane
isopentane
neopentane
Longest continuous carbon chain:
pentane 5 isopentane 4 neopentane 3

46. Alkane Isomers – Alternate Strategy
Rather than building new isomers by adding a methyl group to known isomers of a particular alkane, the following slides illustrate an alternative approach
Draw straight chain version of alkane of interest
Break one or more carbon-carbon bonds and rearrange the pieces
Check that new molecules don’t repeat existing ones (reflection, rotation)

47. Structural Isomers of Hexane (C6H14)
Start: connect carbons in a line
Break bonds & rearrange to get other isomers 47

48. Hexane (C6H14) Isomers Showing H Atoms48

49. Hexane (C6H14) Isomers In Skeleton Form49

50. Structural Isomers of Hexane (C6H14)2 3 1 4 5 2 3 1 2 3 5 4 4 5

51. Boiling Points - Hexane Isomers

52. Structural Isomers of Hexane (C6H14) Site has rotatable models with display options

53. # of Alkane (CnH2n+2) Structural Isomers
Molecular Formula
Possible # Isomers
C4H10 2
C11H24
159
C5H12 3
C12H32
355
C6H14 5
C15H32
4,347
C7H16 9
C20H42
366,319
C8H18 18
C30H62
4,111,846,763
C9H20 35
C40H82
62,481,801,147,341
C10H22 75

54. Structural (Constitutional)
Types of Isomers
All Isomers
Structural (Constitutional)
Stereoisomers
(Configurational)
Diastereomers
Enantiomers
(optical)
Geometric
(Cis-trans)
Other diastereomers
(>1 chiral centers)

55. Enantiomers - Optical Isomers
Tied to concept of non superimposable mirror images
Familiar example – hands: left & right hands are mirror images but do not superimpose

56. Superimposable Mirror Images
Some molecules are like socks - two socks from pair are mirror images that are superimposable (sock and its mirror image are identical)
Molecule/object that is superimposable on its mirror image is achiral; it is chiral if it is not superimposable on its mirror image

57. Superimposable Mirror Images
All molecules have a mirror image – but for many molecules it is the same molecule (achiral) 57

58. Achiral Examples

59. Nonsuperimposable Mirror Images
Mirror image cannot be rotated so all its atoms align with same atoms of original molecule – i.e., mirror image is not superimposable on original 59

60. Non-Superimposable Mirror Images
Only when C attached to 4 different groups
When this occurs, C variously referred to as a chiral center, asymmetric carbon, stereogenic center, or stereocenter 60

61. Identifying Chiral Centers
Examine each tetrahedral carbon atom and look at four groups (not the four atoms) bonded to it
If groups all different, have a chiral (stereogenic) center

62. Chiral Molecules - Entantiomers
A pair of nonsuperimposable mirror images are called a pair of enantiomers – these molecules will be optical isomers of each other 62

63. Chirality – Optical Isomers
Louis Pasteur discovered 2 forms of crystallized tartaric acid; forms were mirror images of each other called right and left-handed forms

64. Non-Superimposable Mirror Images
Chiral molecule – mirror images are enantiomers (optical isomers)

65. Non-Superimposable Mirror Images
Chiral molecule – mirror images are enantiomers (optical isomers) 65

66. Non-Superimposable Mirror Images
2-chlorobutane

67. Non-Superimposable Mirror Images
butan-2-ol

68. Return to Heptane – Optical Isomers
Earlier in presentation, structural isomers of heptane (C7H14) were determined
Some of these isomers are optically active (following slide)

69. Structural Isomers of C7H16 (Heptane)* *
Isomers marked with * have asymmetric carbons (have enantiomers – optical isomers)

70. Optical Isomers of 3-methylhexane
Tro, Chemistry: A Molecular Approach 70 70

71. Properties of Optical Isomers
Optical isomers have the same physical* and chemical properties except in chemical reactions where chirality is important
Chiral molecules often react differently with other chiral molecules; similar to idea that right hand does not fit a left handed glove – molecule must be correct shape to fit molecule it is reacting with
* Except for their rotation of polarized light

72. Impact of Chirality
Many natural molecules are chiral and most natural reactions are affected by optical isomerism; most amino acids are chiral
Many drugs are optically active, with only one enantiomer having the beneficial effect
For some drugs, other enantiomer can even be harmful, e.g. thalidomide

73. Structural (Constitutional)
Types of Isomers
All Isomers
Structural (Constitutional)
Stereoisomers
(Configurational)
Diastereomers
Enantiomers
(optical)
Geometric
(Cis-trans)
Other diastereomers
(>1 chiral centers)

74. Structural (Constitutional)
Types of Isomers
All Isomers
Structural (Constitutional)
Stereoisomers
(Configurational)
Diastereomers
Enantiomers
(optical)
Geometric
(Cis-trans)
Other diastereomers
(>1 chiral centers)

75. Stereoisomers – No Chiral Centers
Single covalent bonds can easily rotate - what appears to be a different structure is not
All structures above are the same (not isomers) because C-C bonds have free rotation

76. Stereoisomers – No Chiral Centers
A double bond between carbon atoms prevents free rotation – structure is locked into place

77. Stereoisomers – No Chiral Centers
Groups can on same or opposite sides of double bond – diastereomers (spatially different but not mirror images)

78. Geometrical Isomers
When double bonds involved, diastereomers are referred to as geometrical isomers
Although cis and trans modifiers to names are traditional, official naming system uses E and Z modifiers

79. Isomers with CHO Compounds
With carbon, hydrogen and oxygen (CHO) present, isomers become more varied
Rule: O in these compounds forms 2 bonds
For simplicity, limit scope to single oxygen
O found in form of alcohol (COH), ether (COC), or carbonyl (C=O) group
Look at: C3H8O, C4H10O, C5H12O, C3H6O
Latter departs from other examples – rings and double bonds present

80. Isomers of C3H8O 2 structural isomers in form of alcohol
1 structural isomer in form of ether
ethyl methyl ether
propan-1-ol
propan-2-ol

81. Isomers of C4H10O * 4 structural isomers in form of alcohol butan-1-ol
2-methylpropan-1-ol
butan-2-ol
2-methylpropan-2-ol *
* chiral center

82. Isomers of C4H10O 3 structural isomers in form of ether
1-methoxypropane
diethyl ether
2-methoxypropane

83. 1 chiral center  2 optical isomers
Isomers of C4H10O
1 chiral center  2 optical isomers
(2S)-butan-2-ol
butan-2-ol *
(2R)-butan-2-ol

84. Isomers of C4H10O
8 total isomers

85. Summary – Isomers of C4H10O
4 structural isomers in form of alcohol
1 alcohol has a chiral center  2 enantiomers (optical isomers)
5 total isomers in form of alcohol
3 structural isomers in form of ether
0 chiral centers
3 total isomers in form of ether
8 total isomers (7 structural)

86. # of CnH2n+2O Structural Isomers
Molecular Formula
Possible # Isomers
C2H6O 2
C9H20O
405
C3H8O 3
C10H22O
989
C4H10O 7
C11H24O
~2430
C5H12O 14
C12H32O
~6070
C6H14O 32
C7H16O 72
C8H18O
171

87. Summary – Isomers
Structural Isomers